[PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Paul E. McKenney]

From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Provide a new command allowing processes to register their intent to use
the private expedited command.

This allows PowerPC to skip the full memory barrier in switch_mm(), and
only issue the barrier when scheduling into a task belonging to a
process that has registered to use expedited private.

Processes are now required to register before using
MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.

Changes since v1:
- Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
  powerpc membarrier_arch_sched_in(), given that we want to specifically
  check the next thread state.
- Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
- Use task_thread_info() to pass thread_info from task to
  *_ti_thread_flag().

Changes since v2:
- Move membarrier_arch_sched_in() call to finish_task_switch().
- Check for NULL t->mm in membarrier_arch_fork().
- Use membarrier_sched_in() in generic code, which invokes the
  arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
  build on PowerPC.
- Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
  allnoconfig build on PowerPC.
- Build and runtime tested on PowerPC.

Changes since v3:
- Simply rely on copy_mm() to copy the membarrier_private_expedited mm
  field on fork.
- powerpc: test thread flag instead of reading
  membarrier_private_expedited in membarrier_arch_fork().
- powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
  from kernel thread, since mmdrop() implies a full barrier.
- Set membarrier_private_expedited to 1 only after arch registration
  code, thus eliminating a race where concurrent commands could succeed
  when they should fail if issued concurrently with process
  registration.
- Use READ_ONCE() for membarrier_private_expedited field access in
  membarrier_private_expedited. Matches WRITE_ONCE() performed in
  process registration.

Changes since v4:
- Move powerpc hook from sched_in() to switch_mm(), based on feedback
  from Nicholas Piggin.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Boqun Feng <boqun.feng@gmail.com>
CC: Andrew Hunter <ahh@google.com>
CC: Maged Michael <maged.michael@gmail.com>
CC: gromer@google.com
CC: Avi Kivity <avi@scylladb.com>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Dave Watson <davejwatson@fb.com>
CC: Alan Stern <stern@rowland.harvard.edu>
CC: Will Deacon <will.deacon@arm.com>
CC: Andy Lutomirski <luto@kernel.org>
CC: Ingo Molnar <mingo@redhat.com>
CC: Alexander Viro <viro@zeniv.linux.org.uk>
CC: Nicholas Piggin <npiggin@gmail.com>
CC: linuxppc-dev@lists.ozlabs.org
CC: linux-arch@vger.kernel.org
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 MAINTAINERS                            |  2 ++
 arch/powerpc/Kconfig                   |  1 +
 arch/powerpc/include/asm/membarrier.h  | 43 +++++++++++++++++++++++++++++
 arch/powerpc/include/asm/thread_info.h |  3 ++
 arch/powerpc/kernel/Makefile           |  2 ++
 arch/powerpc/kernel/membarrier.c       | 45 ++++++++++++++++++++++++++++++
 arch/powerpc/mm/mmu_context.c          |  7 +++++
 fs/exec.c                              |  1 +
 include/linux/mm_types.h               |  3 ++
 include/linux/sched/mm.h               | 50 ++++++++++++++++++++++++++++++++++
 include/uapi/linux/membarrier.h        | 23 +++++++++++-----
 init/Kconfig                           |  3 ++
 kernel/fork.c                          |  2 ++
 kernel/sched/core.c                    | 10 -------
 kernel/sched/membarrier.c              | 25 ++++++++++++++---
 15 files changed, 199 insertions(+), 21 deletions(-)
 create mode 100644 arch/powerpc/include/asm/membarrier.h
 create mode 100644 arch/powerpc/kernel/membarrier.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 65b0c88d5ee0..c5296d7f447b 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -8822,6 +8822,8 @@ L:	linux-kernel@vger.kernel.org
 S:	Supported
 F:	kernel/sched/membarrier.c
 F:	include/uapi/linux/membarrier.h
+F:	arch/powerpc/kernel/membarrier.c
+F:	arch/powerpc/include/asm/membarrier.h
 
 MEMORY MANAGEMENT
 L:	linux-mm@kvack.org
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index 809c468edab1..6f44c5f74f71 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -138,6 +138,7 @@ config PPC
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_GCOV_PROFILE_ALL
+	select ARCH_HAS_MEMBARRIER_HOOKS
 	select ARCH_HAS_SCALED_CPUTIME		if VIRT_CPU_ACCOUNTING_NATIVE
 	select ARCH_HAS_SG_CHAIN
 	select ARCH_HAS_TICK_BROADCAST		if GENERIC_CLOCKEVENTS_BROADCAST
diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h
new file mode 100644
index 000000000000..61152a7a3cf9
--- /dev/null
+++ b/arch/powerpc/include/asm/membarrier.h
@@ -0,0 +1,43 @@
+#ifndef _ASM_POWERPC_MEMBARRIER_H
+#define _ASM_POWERPC_MEMBARRIER_H
+
+static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
+		struct mm_struct *next, struct task_struct *tsk)
+{
+	/*
+	 * Only need the full barrier when switching between processes.
+	 * Barrier when switching from kernel to userspace is not
+	 * required here, given that it is implied by mmdrop(). Barrier
+	 * when switching from userspace to kernel is not needed after
+	 * store to rq->curr.
+	 */
+	if (likely(!test_ti_thread_flag(task_thread_info(tsk),
+			TIF_MEMBARRIER_PRIVATE_EXPEDITED) || !prev))
+		return;
+
+	/*
+	 * The membarrier system call requires a full memory barrier
+	 * after storing to rq->curr, before going back to user-space.
+	 */
+	smp_mb();
+}
+static inline void membarrier_arch_fork(struct task_struct *t,
+		unsigned long clone_flags)
+{
+	/*
+	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
+	 * fork is protected by siglock. membarrier_arch_fork is called
+	 * with siglock held.
+	 */
+	if (test_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED))
+		set_ti_thread_flag(task_thread_info(t),
+				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
+}
+static inline void membarrier_arch_execve(struct task_struct *t)
+{
+	clear_ti_thread_flag(task_thread_info(t),
+			TIF_MEMBARRIER_PRIVATE_EXPEDITED);
+}
+void membarrier_arch_register_private_expedited(struct task_struct *t);
+
+#endif /* _ASM_POWERPC_MEMBARRIER_H */
diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
index a941cc6fc3e9..2a208487724b 100644
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h
@@ -100,6 +100,7 @@ static inline struct thread_info *current_thread_info(void)
 #if defined(CONFIG_PPC64)
 #define TIF_ELF2ABI		18	/* function descriptors must die! */
 #endif
+#define TIF_MEMBARRIER_PRIVATE_EXPEDITED	19	/* membarrier */
 
 /* as above, but as bit values */
 #define _TIF_SYSCALL_TRACE	(1<<TIF_SYSCALL_TRACE)
@@ -119,6 +120,8 @@ static inline struct thread_info *current_thread_info(void)
 #define _TIF_SYSCALL_TRACEPOINT	(1<<TIF_SYSCALL_TRACEPOINT)
 #define _TIF_EMULATE_STACK_STORE	(1<<TIF_EMULATE_STACK_STORE)
 #define _TIF_NOHZ		(1<<TIF_NOHZ)
+#define _TIF_MEMBARRIER_PRIVATE_EXPEDITED \
+				(1<<TIF_MEMBARRIER_PRIVATE_EXPEDITED)
 #define _TIF_SYSCALL_DOTRACE	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT | \
 				 _TIF_SECCOMP | _TIF_SYSCALL_TRACEPOINT | \
 				 _TIF_NOHZ)
diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
index 91960f83039c..2dd4b9e3313a 100644
--- a/arch/powerpc/kernel/Makefile
+++ b/arch/powerpc/kernel/Makefile
@@ -135,6 +135,8 @@ endif
 obj-$(CONFIG_EPAPR_PARAVIRT)	+= epapr_paravirt.o epapr_hcalls.o
 obj-$(CONFIG_KVM_GUEST)		+= kvm.o kvm_emul.o
 
+obj-$(CONFIG_MEMBARRIER)	+= membarrier.o
+
 # Disable GCOV & sanitizers in odd or sensitive code
 GCOV_PROFILE_prom_init.o := n
 UBSAN_SANITIZE_prom_init.o := n
diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
new file mode 100644
index 000000000000..b0d79a5f5981
--- /dev/null
+++ b/arch/powerpc/kernel/membarrier.c
@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * membarrier system call - PowerPC architecture code
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include <linux/sched/mm.h>
+#include <linux/sched/signal.h>
+#include <linux/thread_info.h>
+#include <linux/spinlock.h>
+#include <linux/rcupdate.h>
+
+void membarrier_arch_register_private_expedited(struct task_struct *p)
+{
+	struct task_struct *t;
+
+	if (get_nr_threads(p) == 1) {
+		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
+		return;
+	}
+	/*
+	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
+	 * fork is protected by siglock.
+	 */
+	spin_lock(&p->sighand->siglock);
+	for_each_thread(p, t)
+		set_ti_thread_flag(task_thread_info(t),
+				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
+	spin_unlock(&p->sighand->siglock);
+	/*
+	 * Ensure all future scheduler executions will observe the new
+	 * thread flag state for this process.
+	 */
+	synchronize_sched();
+}
diff --git a/arch/powerpc/mm/mmu_context.c b/arch/powerpc/mm/mmu_context.c
index 0f613bc63c50..22f5c91cdc38 100644
--- a/arch/powerpc/mm/mmu_context.c
+++ b/arch/powerpc/mm/mmu_context.c
@@ -12,6 +12,7 @@
 
 #include <linux/mm.h>
 #include <linux/cpu.h>
+#include <linux/sched/mm.h>
 
 #include <asm/mmu_context.h>
 
@@ -67,6 +68,10 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 		 *
 		 * On the read side the barrier is in pte_xchg(), which orders
 		 * the store to the PTE vs the load of mm_cpumask.
+		 *
+		 * This full barrier is needed by membarrier when switching
+		 * between processes after store to rq->curr, before user-space
+		 * memory accesses.
 		 */
 		smp_mb();
 
@@ -89,6 +94,8 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
 
 	if (new_on_cpu)
 		radix_kvm_prefetch_workaround(next);
+	else
+		membarrier_arch_switch_mm(prev, next, tsk);
 
 	/*
 	 * The actual HW switching method differs between the various
diff --git a/fs/exec.c b/fs/exec.c
index ac34d9724684..b2448f2731b3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1802,6 +1802,7 @@ static int do_execveat_common(int fd, struct filename *filename,
 	/* execve succeeded */
 	current->fs->in_exec = 0;
 	current->in_execve = 0;
+	membarrier_execve(current);
 	acct_update_integrals(current);
 	task_numa_free(current);
 	free_bprm(bprm);
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 46f4ecf5479a..5e0fe8ce053b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -445,6 +445,9 @@ struct mm_struct {
 	unsigned long flags; /* Must use atomic bitops to access the bits */
 
 	struct core_state *core_state; /* coredumping support */
+#ifdef CONFIG_MEMBARRIER
+	int membarrier_private_expedited;
+#endif
 #ifdef CONFIG_AIO
 	spinlock_t			ioctx_lock;
 	struct kioctx_table __rcu	*ioctx_table;
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 3a19c253bdb1..4af1b719c65f 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -205,4 +205,54 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
 	current->flags = (current->flags & ~PF_MEMALLOC) | flags;
 }
 
+#ifdef CONFIG_MEMBARRIER
+
+#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
+#include <asm/membarrier.h>
+#else
+static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
+		struct mm_struct *next, struct task_struct *tsk)
+{
+}
+static inline void membarrier_arch_fork(struct task_struct *t,
+		unsigned long clone_flags)
+{
+}
+static inline void membarrier_arch_execve(struct task_struct *t)
+{
+}
+static inline void membarrier_arch_register_private_expedited(
+		struct task_struct *p)
+{
+}
+#endif
+
+static inline void membarrier_fork(struct task_struct *t,
+		unsigned long clone_flags)
+{
+	/*
+	 * Prior copy_mm() copies the membarrier_private_expedited field
+	 * from current->mm to t->mm.
+	 */
+	membarrier_arch_fork(t, clone_flags);
+}
+static inline void membarrier_execve(struct task_struct *t)
+{
+	t->mm->membarrier_private_expedited = 0;
+	membarrier_arch_execve(t);
+}
+#else
+static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
+		struct mm_struct *next, struct task_struct *tsk)
+{
+}
+static inline void membarrier_fork(struct task_struct *t,
+		unsigned long clone_flags)
+{
+}
+static inline void membarrier_execve(struct task_struct *t)
+{
+}
+#endif
+
 #endif /* _LINUX_SCHED_MM_H */
diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
index 6d47b3249d8a..4e01ad7ffe98 100644
--- a/include/uapi/linux/membarrier.h
+++ b/include/uapi/linux/membarrier.h
@@ -52,21 +52,30 @@
  *                          (non-running threads are de facto in such a
  *                          state). This only covers threads from the
  *                          same processes as the caller thread. This
- *                          command returns 0. The "expedited" commands
- *                          complete faster than the non-expedited ones,
- *                          they never block, but have the downside of
- *                          causing extra overhead.
+ *                          command returns 0 on success. The
+ *                          "expedited" commands complete faster than
+ *                          the non-expedited ones, they never block,
+ *                          but have the downside of causing extra
+ *                          overhead. A process needs to register its
+ *                          intent to use the private expedited command
+ *                          prior to using it, otherwise this command
+ *                          returns -EPERM.
+ * @MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
+ *                          Register the process intent to use
+ *                          MEMBARRIER_CMD_PRIVATE_EXPEDITED. Always
+ *                          returns 0.
  *
  * Command to be passed to the membarrier system call. The commands need to
  * be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to
  * the value 0.
  */
 enum membarrier_cmd {
-	MEMBARRIER_CMD_QUERY			= 0,
-	MEMBARRIER_CMD_SHARED			= (1 << 0),
+	MEMBARRIER_CMD_QUERY				= 0,
+	MEMBARRIER_CMD_SHARED				= (1 << 0),
 	/* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */
 	/* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */
-	MEMBARRIER_CMD_PRIVATE_EXPEDITED	= (1 << 3),
+	MEMBARRIER_CMD_PRIVATE_EXPEDITED		= (1 << 3),
+	MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED	= (1 << 4),
 };
 
 #endif /* _UAPI_LINUX_MEMBARRIER_H */
diff --git a/init/Kconfig b/init/Kconfig
index 78cb2461012e..a3dc6a66f0d1 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1400,6 +1400,9 @@ config MEMBARRIER
 
 	  If unsure, say Y.
 
+config ARCH_HAS_MEMBARRIER_HOOKS
+	bool
+
 config EMBEDDED
 	bool "Embedded system"
 	option allnoconfig_y
diff --git a/kernel/fork.c b/kernel/fork.c
index 10646182440f..bd4a93915e08 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1840,6 +1840,8 @@ static __latent_entropy struct task_struct *copy_process(
 	 */
 	copy_seccomp(p);
 
+	membarrier_fork(p, clone_flags);
+
 	/*
 	 * Process group and session signals need to be delivered to just the
 	 * parent before the fork or both the parent and the child after the
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d17c5da523a0..f29525e453c4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2644,16 +2644,6 @@ static struct rq *finish_task_switch(struct task_struct *prev)
 	prev_state = prev->state;
 	vtime_task_switch(prev);
 	perf_event_task_sched_in(prev, current);
-	/*
-	 * The membarrier system call requires a full memory barrier
-	 * after storing to rq->curr, before going back to user-space.
-	 *
-	 * TODO: This smp_mb__after_unlock_lock can go away if PPC end
-	 * up adding a full barrier to switch_mm(), or we should figure
-	 * out if a smp_mb__after_unlock_lock is really the proper API
-	 * to use.
-	 */
-	smp_mb__after_unlock_lock();
 	finish_lock_switch(rq, prev);
 	finish_arch_post_lock_switch();
 
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index a92fddc22747..f06949a279ca 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -26,21 +26,25 @@
  * except MEMBARRIER_CMD_QUERY.
  */
 #define MEMBARRIER_CMD_BITMASK	\
-	(MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED)
+	(MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED	\
+	| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED)
 
 static void ipi_mb(void *info)
 {
 	smp_mb();	/* IPIs should be serializing but paranoid. */
 }
 
-static void membarrier_private_expedited(void)
+static int membarrier_private_expedited(void)
 {
 	int cpu;
 	bool fallback = false;
 	cpumask_var_t tmpmask;
 
+	if (!READ_ONCE(current->mm->membarrier_private_expedited))
+		return -EPERM;
+
 	if (num_online_cpus() == 1)
-		return;
+		return 0;
 
 	/*
 	 * Matches memory barriers around rq->curr modification in
@@ -94,6 +98,17 @@ static void membarrier_private_expedited(void)
 	 * rq->curr modification in scheduler.
 	 */
 	smp_mb();	/* exit from system call is not a mb */
+	return 0;
+}
+
+static void membarrier_register_private_expedited(void)
+{
+	struct task_struct *p = current;
+
+	if (READ_ONCE(p->mm->membarrier_private_expedited))
+		return;
+	membarrier_arch_register_private_expedited(p);
+	WRITE_ONCE(p->mm->membarrier_private_expedited, 1);
 }
 
 /**
@@ -144,7 +159,9 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
 			synchronize_sched();
 		return 0;
 	case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
-		membarrier_private_expedited();
+		return membarrier_private_expedited();
+	case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
+		membarrier_register_private_expedited();
 		return 0;
 	default:
 		return -EINVAL;
-- 
2.5.2

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Nicholas Piggin]

On Wed,  4 Oct 2017 14:37:53 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> 
> Provide a new command allowing processes to register their intent to use
> the private expedited command.
> 
> This allows PowerPC to skip the full memory barrier in switch_mm(), and
> only issue the barrier when scheduling into a task belonging to a
> process that has registered to use expedited private.
> 
> Processes are now required to register before using
> MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.
> 
> Changes since v1:
> - Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
>   powerpc membarrier_arch_sched_in(), given that we want to specifically
>   check the next thread state.
> - Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
> - Use task_thread_info() to pass thread_info from task to
>   *_ti_thread_flag().
> 
> Changes since v2:
> - Move membarrier_arch_sched_in() call to finish_task_switch().
> - Check for NULL t->mm in membarrier_arch_fork().
> - Use membarrier_sched_in() in generic code, which invokes the
>   arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
>   build on PowerPC.
> - Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
>   allnoconfig build on PowerPC.
> - Build and runtime tested on PowerPC.
> 
> Changes since v3:
> - Simply rely on copy_mm() to copy the membarrier_private_expedited mm
>   field on fork.
> - powerpc: test thread flag instead of reading
>   membarrier_private_expedited in membarrier_arch_fork().
> - powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
>   from kernel thread, since mmdrop() implies a full barrier.
> - Set membarrier_private_expedited to 1 only after arch registration
>   code, thus eliminating a race where concurrent commands could succeed
>   when they should fail if issued concurrently with process
>   registration.
> - Use READ_ONCE() for membarrier_private_expedited field access in
>   membarrier_private_expedited. Matches WRITE_ONCE() performed in
>   process registration.
> 
> Changes since v4:
> - Move powerpc hook from sched_in() to switch_mm(), based on feedback
>   from Nicholas Piggin.

For now, the powerpc approach is okay by me. I plan to test
others (e.g., taking runqueue locks) on larger systems, but that can
be sent as an incremental patch at a later time.

The main thing I would like is for people to review the userspace API.


> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 3a19c253bdb1..4af1b719c65f 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -205,4 +205,54 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
>  	current->flags = (current->flags & ~PF_MEMALLOC) | flags;
>  }
>  
> +#ifdef CONFIG_MEMBARRIER
> +
> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
> +#include <asm/membarrier.h>
> +#else
> +static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
> +		struct mm_struct *next, struct task_struct *tsk)
> +{
> +}

This is no longer required in architecture independent code, is it?

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Peter Zijlstra]

On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
> new file mode 100644
> index 000000000000..b0d79a5f5981
> --- /dev/null
> +++ b/arch/powerpc/kernel/membarrier.c
> @@ -0,0 +1,45 @@

> +void membarrier_arch_register_private_expedited(struct task_struct *p)
> +{
> +	struct task_struct *t;
> +
> +	if (get_nr_threads(p) == 1) {
> +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> +		return;
> +	}
> +	/*
> +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
> +	 * fork is protected by siglock.
> +	 */
> +	spin_lock(&p->sighand->siglock);
> +	for_each_thread(p, t)
> +		set_ti_thread_flag(task_thread_info(t),
> +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);

I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.

> +	spin_unlock(&p->sighand->siglock);
> +	/*
> +	 * Ensure all future scheduler executions will observe the new
> +	 * thread flag state for this process.
> +	 */
> +	synchronize_sched();

This relies on the flag being read inside rq->lock, right?

> +}

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Avi Kivity]

On 10/05/2017 07:23 AM, Nicholas Piggin wrote:
> On Wed,  4 Oct 2017 14:37:53 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
>> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>
>> Provide a new command allowing processes to register their intent to use
>> the private expedited command.
>>
>> This allows PowerPC to skip the full memory barrier in switch_mm(), and
>> only issue the barrier when scheduling into a task belonging to a
>> process that has registered to use expedited private.
>>
>> Processes are now required to register before using
>> MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.
>>
>> Changes since v1:
>> - Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
>>    powerpc membarrier_arch_sched_in(), given that we want to specifically
>>    check the next thread state.
>> - Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
>> - Use task_thread_info() to pass thread_info from task to
>>    *_ti_thread_flag().
>>
>> Changes since v2:
>> - Move membarrier_arch_sched_in() call to finish_task_switch().
>> - Check for NULL t->mm in membarrier_arch_fork().
>> - Use membarrier_sched_in() in generic code, which invokes the
>>    arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
>>    build on PowerPC.
>> - Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
>>    allnoconfig build on PowerPC.
>> - Build and runtime tested on PowerPC.
>>
>> Changes since v3:
>> - Simply rely on copy_mm() to copy the membarrier_private_expedited mm
>>    field on fork.
>> - powerpc: test thread flag instead of reading
>>    membarrier_private_expedited in membarrier_arch_fork().
>> - powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
>>    from kernel thread, since mmdrop() implies a full barrier.
>> - Set membarrier_private_expedited to 1 only after arch registration
>>    code, thus eliminating a race where concurrent commands could succeed
>>    when they should fail if issued concurrently with process
>>    registration.
>> - Use READ_ONCE() for membarrier_private_expedited field access in
>>    membarrier_private_expedited. Matches WRITE_ONCE() performed in
>>    process registration.
>>
>> Changes since v4:
>> - Move powerpc hook from sched_in() to switch_mm(), based on feedback
>>    from Nicholas Piggin.
> For now, the powerpc approach is okay by me. I plan to test
> others (e.g., taking runqueue locks) on larger systems, but that can
> be sent as an incremental patch at a later time.
>
> The main thing I would like is for people to review the userspace API.
>

As a future satisfied user of the expedited private membarrier syscall, 
I am happy with the change.

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Peter Zijlstra]

On Thu, Oct 05, 2017 at 02:12:50PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
> > diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
> > new file mode 100644
> > index 000000000000..b0d79a5f5981
> > --- /dev/null
> > +++ b/arch/powerpc/kernel/membarrier.c
> > @@ -0,0 +1,45 @@
> 
> > +void membarrier_arch_register_private_expedited(struct task_struct *p)
> > +{
> > +	struct task_struct *t;
> > +
> > +	if (get_nr_threads(p) == 1) {
> > +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> > +		return;
> > +	}
> > +	/*
> > +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
> > +	 * fork is protected by siglock.
> > +	 */
> > +	spin_lock(&p->sighand->siglock);
> > +	for_each_thread(p, t)
> > +		set_ti_thread_flag(task_thread_info(t),
> > +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> 
> I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.

Also, for easier reading I would suggest putting { } around the block.

> > +	spin_unlock(&p->sighand->siglock);
> > +	/*
> > +	 * Ensure all future scheduler executions will observe the new
> > +	 * thread flag state for this process.
> > +	 */
> > +	synchronize_sched();
> 
> This relies on the flag being read inside rq->lock, right?
> 
> > +}

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Mathieu Desnoyers]

----- On Oct 5, 2017, at 12:23 AM, Nicholas Piggin npiggin@gmail.com wrote:

> On Wed,  4 Oct 2017 14:37:53 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
>> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>> 
>> Provide a new command allowing processes to register their intent to use
>> the private expedited command.
>> 
>> This allows PowerPC to skip the full memory barrier in switch_mm(), and
>> only issue the barrier when scheduling into a task belonging to a
>> process that has registered to use expedited private.
>> 
>> Processes are now required to register before using
>> MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.
>> 
>> Changes since v1:
>> - Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
>>   powerpc membarrier_arch_sched_in(), given that we want to specifically
>>   check the next thread state.
>> - Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
>> - Use task_thread_info() to pass thread_info from task to
>>   *_ti_thread_flag().
>> 
>> Changes since v2:
>> - Move membarrier_arch_sched_in() call to finish_task_switch().
>> - Check for NULL t->mm in membarrier_arch_fork().
>> - Use membarrier_sched_in() in generic code, which invokes the
>>   arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
>>   build on PowerPC.
>> - Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
>>   allnoconfig build on PowerPC.
>> - Build and runtime tested on PowerPC.
>> 
>> Changes since v3:
>> - Simply rely on copy_mm() to copy the membarrier_private_expedited mm
>>   field on fork.
>> - powerpc: test thread flag instead of reading
>>   membarrier_private_expedited in membarrier_arch_fork().
>> - powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
>>   from kernel thread, since mmdrop() implies a full barrier.
>> - Set membarrier_private_expedited to 1 only after arch registration
>>   code, thus eliminating a race where concurrent commands could succeed
>>   when they should fail if issued concurrently with process
>>   registration.
>> - Use READ_ONCE() for membarrier_private_expedited field access in
>>   membarrier_private_expedited. Matches WRITE_ONCE() performed in
>>   process registration.
>> 
>> Changes since v4:
>> - Move powerpc hook from sched_in() to switch_mm(), based on feedback
>>   from Nicholas Piggin.
> 
> For now, the powerpc approach is okay by me. I plan to test
> others (e.g., taking runqueue locks) on larger systems, but that can
> be sent as an incremental patch at a later time.
> 
> The main thing I would like is for people to review the userspace API.
> 
> 
>> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
>> index 3a19c253bdb1..4af1b719c65f 100644
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -205,4 +205,54 @@ static inline void memalloc_noreclaim_restore(unsigned int
>> flags)
>>  	current->flags = (current->flags & ~PF_MEMALLOC) | flags;
>>  }
>>  
>> +#ifdef CONFIG_MEMBARRIER
>> +
>> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
>> +#include <asm/membarrier.h>
>> +#else
>> +static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
>> +		struct mm_struct *next, struct task_struct *tsk)
>> +{
>> +}
> 
> This is no longer required in architecture independent code, is it?

Yes, good point! I'll remove this unused code in a follow up patch.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Mathieu Desnoyers]

----- On Oct 5, 2017, at 8:22 AM, Avi Kivity avi@scylladb.com wrote:

> On 10/05/2017 07:23 AM, Nicholas Piggin wrote:
>> On Wed,  4 Oct 2017 14:37:53 -0700
>> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>>
>>> From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
>>>
>>> Provide a new command allowing processes to register their intent to use
>>> the private expedited command.
>>>
>>> This allows PowerPC to skip the full memory barrier in switch_mm(), and
>>> only issue the barrier when scheduling into a task belonging to a
>>> process that has registered to use expedited private.
>>>
>>> Processes are now required to register before using
>>> MEMBARRIER_CMD_PRIVATE_EXPEDITED, otherwise that command returns EPERM.
>>>
>>> Changes since v1:
>>> - Use test_ti_thread_flag(next, ...) instead of test_thread_flag() in
>>>    powerpc membarrier_arch_sched_in(), given that we want to specifically
>>>    check the next thread state.
>>> - Add missing ARCH_HAS_MEMBARRIER_HOOKS in Kconfig.
>>> - Use task_thread_info() to pass thread_info from task to
>>>    *_ti_thread_flag().
>>>
>>> Changes since v2:
>>> - Move membarrier_arch_sched_in() call to finish_task_switch().
>>> - Check for NULL t->mm in membarrier_arch_fork().
>>> - Use membarrier_sched_in() in generic code, which invokes the
>>>    arch-specific membarrier_arch_sched_in(). This fixes allnoconfig
>>>    build on PowerPC.
>>> - Move asm/membarrier.h include under CONFIG_MEMBARRIER, fixing
>>>    allnoconfig build on PowerPC.
>>> - Build and runtime tested on PowerPC.
>>>
>>> Changes since v3:
>>> - Simply rely on copy_mm() to copy the membarrier_private_expedited mm
>>>    field on fork.
>>> - powerpc: test thread flag instead of reading
>>>    membarrier_private_expedited in membarrier_arch_fork().
>>> - powerpc: skip memory barrier in membarrier_arch_sched_in() if coming
>>>    from kernel thread, since mmdrop() implies a full barrier.
>>> - Set membarrier_private_expedited to 1 only after arch registration
>>>    code, thus eliminating a race where concurrent commands could succeed
>>>    when they should fail if issued concurrently with process
>>>    registration.
>>> - Use READ_ONCE() for membarrier_private_expedited field access in
>>>    membarrier_private_expedited. Matches WRITE_ONCE() performed in
>>>    process registration.
>>>
>>> Changes since v4:
>>> - Move powerpc hook from sched_in() to switch_mm(), based on feedback
>>>    from Nicholas Piggin.
>> For now, the powerpc approach is okay by me. I plan to test
>> others (e.g., taking runqueue locks) on larger systems, but that can
>> be sent as an incremental patch at a later time.
>>
>> The main thing I would like is for people to review the userspace API.
>>
> 
> As a future satisfied user of the expedited private membarrier syscall,
> I am happy with the change.

Thanks Avi for your input on the userspace API.

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Mathieu Desnoyers]

----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
>> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
>> new file mode 100644
>> index 000000000000..b0d79a5f5981
>> --- /dev/null
>> +++ b/arch/powerpc/kernel/membarrier.c
>> @@ -0,0 +1,45 @@
> 
>> +void membarrier_arch_register_private_expedited(struct task_struct *p)
>> +{
>> +	struct task_struct *t;
>> +
>> +	if (get_nr_threads(p) == 1) {
>> +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> +		return;
>> +	}
>> +	/*
>> +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
>> +	 * fork is protected by siglock.
>> +	 */
>> +	spin_lock(&p->sighand->siglock);
>> +	for_each_thread(p, t)
>> +		set_ti_thread_flag(task_thread_info(t),
>> +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> 
> I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.

The intent here is to hold the sighand siglock to provide mutual
exclusion against invocation of membarrier_fork(p, clone_flags)
by copy_process().

copy_process() grabs spin_lock(&current->sighand->siglock) for both
CLONE_THREAD and not CLONE_THREAD flags.

What am I missing here ?

> 
>> +	spin_unlock(&p->sighand->siglock);
>> +	/*
>> +	 * Ensure all future scheduler executions will observe the new
>> +	 * thread flag state for this process.
>> +	 */
>> +	synchronize_sched();
> 
> This relies on the flag being read inside rq->lock, right?

Yes. The flag is read by membarrier_arch_switch_mm(), invoked
within switch_mm_irqs_off(), called by context_switch() before
finish_task_switch() releases the rq lock.

Is the comment clear enough, or do you have suggestions for
improvements ?

Thanks,

Mathieu

> 
> > +}

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Mathieu Desnoyers]

----- On Oct 5, 2017, at 8:24 AM, Peter Zijlstra peterz@infradead.org wrote:

> On Thu, Oct 05, 2017 at 02:12:50PM +0200, Peter Zijlstra wrote:
>> On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
>> > diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
>> > new file mode 100644
>> > index 000000000000..b0d79a5f5981
>> > --- /dev/null
>> > +++ b/arch/powerpc/kernel/membarrier.c
>> > @@ -0,0 +1,45 @@
>> 
>> > +void membarrier_arch_register_private_expedited(struct task_struct *p)
>> > +{
>> > +	struct task_struct *t;
>> > +
>> > +	if (get_nr_threads(p) == 1) {
>> > +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> > +		return;
>> > +	}
>> > +	/*
>> > +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
>> > +	 * fork is protected by siglock.
>> > +	 */
>> > +	spin_lock(&p->sighand->siglock);
>> > +	for_each_thread(p, t)
>> > +		set_ti_thread_flag(task_thread_info(t),
>> > +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> 
>> I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.
> 
> Also, for easier reading I would suggest putting { } around the block.

Will do, thanks,

Mathieu

> 
>> > +	spin_unlock(&p->sighand->siglock);
>> > +	/*
>> > +	 * Ensure all future scheduler executions will observe the new
>> > +	 * thread flag state for this process.
>> > +	 */
>> > +	synchronize_sched();
>> 
>> This relies on the flag being read inside rq->lock, right?
>> 
> > > +}

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Peter Zijlstra]

On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote:
> ----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote:
> 
> > On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
> >> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
> >> new file mode 100644
> >> index 000000000000..b0d79a5f5981
> >> --- /dev/null
> >> +++ b/arch/powerpc/kernel/membarrier.c
> >> @@ -0,0 +1,45 @@
> > 
> >> +void membarrier_arch_register_private_expedited(struct task_struct *p)
> >> +{
> >> +	struct task_struct *t;
> >> +
> >> +	if (get_nr_threads(p) == 1) {
> >> +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> >> +		return;
> >> +	}
> >> +	/*
> >> +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
> >> +	 * fork is protected by siglock.
> >> +	 */
> >> +	spin_lock(&p->sighand->siglock);
> >> +	for_each_thread(p, t)
> >> +		set_ti_thread_flag(task_thread_info(t),
> >> +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> > 
> > I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.
> 
> The intent here is to hold the sighand siglock to provide mutual
> exclusion against invocation of membarrier_fork(p, clone_flags)
> by copy_process().
> 
> copy_process() grabs spin_lock(&current->sighand->siglock) for both
> CLONE_THREAD and not CLONE_THREAD flags.
> 
> What am I missing here ?

If you do CLONE_VM without CLONE_THREAD you'll end up sharing the mm but
you'll not be part of thread_head, so the for_each_thread() iteration
will not find the task.

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Mathieu Desnoyers]

----- On Oct 5, 2017, at 12:21 PM, Peter Zijlstra peterz@infradead.org wrote:

> On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote:
>> ----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote:
>> 
>> > On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
>> >> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
>> >> new file mode 100644
>> >> index 000000000000..b0d79a5f5981
>> >> --- /dev/null
>> >> +++ b/arch/powerpc/kernel/membarrier.c
>> >> @@ -0,0 +1,45 @@
>> > 
>> >> +void membarrier_arch_register_private_expedited(struct task_struct *p)
>> >> +{
>> >> +	struct task_struct *t;
>> >> +
>> >> +	if (get_nr_threads(p) == 1) {
>> >> +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> >> +		return;
>> >> +	}
>> >> +	/*
>> >> +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
>> >> +	 * fork is protected by siglock.
>> >> +	 */
>> >> +	spin_lock(&p->sighand->siglock);
>> >> +	for_each_thread(p, t)
>> >> +		set_ti_thread_flag(task_thread_info(t),
>> >> +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> > 
>> > I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.
>> 
>> The intent here is to hold the sighand siglock to provide mutual
>> exclusion against invocation of membarrier_fork(p, clone_flags)
>> by copy_process().
>> 
>> copy_process() grabs spin_lock(&current->sighand->siglock) for both
>> CLONE_THREAD and not CLONE_THREAD flags.
>> 
>> What am I missing here ?
> 
> If you do CLONE_VM without CLONE_THREAD you'll end up sharing the mm but
> you'll not be part of thread_head, so the for_each_thread() iteration
> will not find the task.

Excellent point. Please see the follow up RFC patch I posted taking care of
this matter.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Andrea Parri]

On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote:
> ----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote:
> 
> > On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
> >> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
> >> new file mode 100644
> >> index 000000000000..b0d79a5f5981
> >> --- /dev/null
> >> +++ b/arch/powerpc/kernel/membarrier.c
> >> @@ -0,0 +1,45 @@
> > 
> >> +void membarrier_arch_register_private_expedited(struct task_struct *p)
> >> +{
> >> +	struct task_struct *t;
> >> +
> >> +	if (get_nr_threads(p) == 1) {
> >> +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> >> +		return;
> >> +	}
> >> +	/*
> >> +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
> >> +	 * fork is protected by siglock.
> >> +	 */
> >> +	spin_lock(&p->sighand->siglock);
> >> +	for_each_thread(p, t)
> >> +		set_ti_thread_flag(task_thread_info(t),
> >> +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
> > 
> > I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.
> 
> The intent here is to hold the sighand siglock to provide mutual
> exclusion against invocation of membarrier_fork(p, clone_flags)
> by copy_process().
> 
> copy_process() grabs spin_lock(&current->sighand->siglock) for both
> CLONE_THREAD and not CLONE_THREAD flags.
> 
> What am I missing here ?
> 
> > 
> >> +	spin_unlock(&p->sighand->siglock);
> >> +	/*
> >> +	 * Ensure all future scheduler executions will observe the new
> >> +	 * thread flag state for this process.
> >> +	 */
> >> +	synchronize_sched();
> > 
> > This relies on the flag being read inside rq->lock, right?
> 
> Yes. The flag is read by membarrier_arch_switch_mm(), invoked
> within switch_mm_irqs_off(), called by context_switch() before
> finish_task_switch() releases the rq lock.

I fail to graps the relation between this synchronize_sched() and rq->lock.

(Besides, we made no reference to rq->lock while discussing:

  https://github.com/paulmckrcu/litmus/commit/47039df324b60ace0cf7b2403299580be730119b
  replace membarrier_arch_sched_in with switch_mm_irqs_off )

Could you elaborate?

  Andrea


> 
> Is the comment clear enough, or do you have suggestions for
> improvements ?



> 
> Thanks,
> 
> Mathieu
> 
> > 
> > > +}
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Andrea Parri]

On Fri, Oct 6, 2017 at 12:02 AM, Andrea Parri <parri.andrea@gmail.com> wrote:
> On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote:
>> ----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote:
>>
>> > On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
>> >> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
>> >> new file mode 100644
>> >> index 000000000000..b0d79a5f5981
>> >> --- /dev/null
>> >> +++ b/arch/powerpc/kernel/membarrier.c
>> >> @@ -0,0 +1,45 @@
>> >
>> >> +void membarrier_arch_register_private_expedited(struct task_struct *p)
>> >> +{
>> >> +  struct task_struct *t;
>> >> +
>> >> +  if (get_nr_threads(p) == 1) {
>> >> +          set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> >> +          return;
>> >> +  }
>> >> +  /*
>> >> +   * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
>> >> +   * fork is protected by siglock.
>> >> +   */
>> >> +  spin_lock(&p->sighand->siglock);
>> >> +  for_each_thread(p, t)
>> >> +          set_ti_thread_flag(task_thread_info(t),
>> >> +                          TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> >
>> > I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.
>>
>> The intent here is to hold the sighand siglock to provide mutual
>> exclusion against invocation of membarrier_fork(p, clone_flags)
>> by copy_process().
>>
>> copy_process() grabs spin_lock(&current->sighand->siglock) for both
>> CLONE_THREAD and not CLONE_THREAD flags.
>>
>> What am I missing here ?
>>
>> >
>> >> +  spin_unlock(&p->sighand->siglock);
>> >> +  /*
>> >> +   * Ensure all future scheduler executions will observe the new
>> >> +   * thread flag state for this process.
>> >> +   */
>> >> +  synchronize_sched();
>> >
>> > This relies on the flag being read inside rq->lock, right?
>>
>> Yes. The flag is read by membarrier_arch_switch_mm(), invoked
>> within switch_mm_irqs_off(), called by context_switch() before
>> finish_task_switch() releases the rq lock.
>
> I fail to graps the relation between this synchronize_sched() and rq->lock.

s/graps/grasp

  Andrea


>
> (Besides, we made no reference to rq->lock while discussing:
>
>   https://github.com/paulmckrcu/litmus/commit/47039df324b60ace0cf7b2403299580be730119b
>   replace membarrier_arch_sched_in with switch_mm_irqs_off )
>
> Could you elaborate?
>
>   Andrea
>
>
>>
>> Is the comment clear enough, or do you have suggestions for
>> improvements ?
>
>
>
>>
>> Thanks,
>>
>> Mathieu
>>
>> >
>> > > +}
>>
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> http://www.efficios.com

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Mathieu Desnoyers]

----- On Oct 5, 2017, at 6:02 PM, Andrea Parri parri.andrea@gmail.com wrote:

> On Thu, Oct 05, 2017 at 04:02:06PM +0000, Mathieu Desnoyers wrote:
>> ----- On Oct 5, 2017, at 8:12 AM, Peter Zijlstra peterz@infradead.org wrote:
>> 
>> > On Wed, Oct 04, 2017 at 02:37:53PM -0700, Paul E. McKenney wrote:
>> >> diff --git a/arch/powerpc/kernel/membarrier.c b/arch/powerpc/kernel/membarrier.c
>> >> new file mode 100644
>> >> index 000000000000..b0d79a5f5981
>> >> --- /dev/null
>> >> +++ b/arch/powerpc/kernel/membarrier.c
>> >> @@ -0,0 +1,45 @@
>> > 
>> >> +void membarrier_arch_register_private_expedited(struct task_struct *p)
>> >> +{
>> >> +	struct task_struct *t;
>> >> +
>> >> +	if (get_nr_threads(p) == 1) {
>> >> +		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> >> +		return;
>> >> +	}
>> >> +	/*
>> >> +	 * Coherence of TIF_MEMBARRIER_PRIVATE_EXPEDITED against thread
>> >> +	 * fork is protected by siglock.
>> >> +	 */
>> >> +	spin_lock(&p->sighand->siglock);
>> >> +	for_each_thread(p, t)
>> >> +		set_ti_thread_flag(task_thread_info(t),
>> >> +				TIF_MEMBARRIER_PRIVATE_EXPEDITED);
>> > 
>> > I'm not sure this works correctly vs CLONE_VM without CLONE_THREAD.
>> 
>> The intent here is to hold the sighand siglock to provide mutual
>> exclusion against invocation of membarrier_fork(p, clone_flags)
>> by copy_process().
>> 
>> copy_process() grabs spin_lock(&current->sighand->siglock) for both
>> CLONE_THREAD and not CLONE_THREAD flags.
>> 
>> What am I missing here ?
>> 
>> > 
>> >> +	spin_unlock(&p->sighand->siglock);
>> >> +	/*
>> >> +	 * Ensure all future scheduler executions will observe the new
>> >> +	 * thread flag state for this process.
>> >> +	 */
>> >> +	synchronize_sched();
>> > 
>> > This relies on the flag being read inside rq->lock, right?
>> 
>> Yes. The flag is read by membarrier_arch_switch_mm(), invoked
>> within switch_mm_irqs_off(), called by context_switch() before
>> finish_task_switch() releases the rq lock.
> 
> I fail to grap the relation between this synchronize_sched() and rq->lock.
> 
> (Besides, we made no reference to rq->lock while discussing:
> 
>  https://github.com/paulmckrcu/litmus/commit/47039df324b60ace0cf7b2403299580be730119b
>  replace membarrier_arch_sched_in with switch_mm_irqs_off )
> 
> Could you elaborate?

Hi Andrea,

AFAIU the scheduler rq->lock is held while preemption is disabled.
synchronize_sched() is used here to ensure that all pre-existing
preempt-off critical sections have completed.

So saying that we use synchronize_sched() to synchronize with rq->lock
would be stretching the truth a bit. It's actually only true because the
scheduler holding the rq->lock is surrounded by a preempt-off
critical section.

Thanks,

Mathieu



> 
>  Andrea
> 
> 
>> 
>> Is the comment clear enough, or do you have suggestions for
>> improvements ?
> 
> 
> 
>> 
>> Thanks,
>> 
>> Mathieu
>> 
>> > 
>> > > +}
>> 
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
> > http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Steven Rostedt]

On Thu, 5 Oct 2017 22:19:15 +0000 (UTC)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> AFAIU the scheduler rq->lock is held while preemption is disabled.
> synchronize_sched() is used here to ensure that all pre-existing
> preempt-off critical sections have completed.
> 
> So saying that we use synchronize_sched() to synchronize with rq->lock
> would be stretching the truth a bit. It's actually only true because the
> scheduler holding the rq->lock is surrounded by a preempt-off
> critical section.

Not only is preemption disabled (which is true for all spinlocks, at
least with non PREEMPT_RT), but the rq->lock must also only be taken
with interrupts disabled (for PREEMPT_RT as well). Because it can also
be taken in interrupt context.

-- Steve

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Peter Zijlstra]

> AFAIU the scheduler rq->lock is held while preemption is disabled.
> synchronize_sched() is used here to ensure that all pre-existing
> preempt-off critical sections have completed.
> 
> So saying that we use synchronize_sched() to synchronize with rq->lock
> would be stretching the truth a bit. It's actually only true because the
> scheduler holding the rq->lock is surrounded by a preempt-off
> critical section.

No, rq->lock is sufficient, note that rq->lock is a raw_spinlock_t which
implies !preempt. Yes, we also surround the rq->lock usage with a
slightly larger preempt_disable() section but that's not in fact
required for this.

Re: [PATCH tip/core/rcu 1/3] membarrier: Provide register expedited private command

[Andrea Parri]

On Fri, Oct 06, 2017 at 10:32:19AM +0200, Peter Zijlstra wrote:
> > AFAIU the scheduler rq->lock is held while preemption is disabled.
> > synchronize_sched() is used here to ensure that all pre-existing
> > preempt-off critical sections have completed.
> > 
> > So saying that we use synchronize_sched() to synchronize with rq->lock
> > would be stretching the truth a bit. It's actually only true because the
> > scheduler holding the rq->lock is surrounded by a preempt-off
> > critical section.
> 
> No, rq->lock is sufficient, note that rq->lock is a raw_spinlock_t which
> implies !preempt. Yes, we also surround the rq->lock usage with a
> slightly larger preempt_disable() section but that's not in fact
> required for this.

That's what it is, according to the current sources: we seemed to agree that
a preempt-off critical section is what we rely on here and that the start of
this critical section is not marked by that raw_spin_lock.

  Andrea