[RFC PATCH for 4.14 1/2] membarrier: Remove unused code for architectures without membarrier hooks

[RFC PATCH for 4.14 1/2] membarrier: Remove unused code for architectures without membarrier hooks

[Mathieu Desnoyers]

Architectures without membarrier hooks don't need to emit the
empty membarrier_arch_switch_mm() static inline when
CONFIG_MEMBARRIER=y.

Adapt the CONFIG_MEMBARRIER=n counterpart to only emit the empty
membarrier_arch_switch_mm() for architectures with membarrier hooks.

Reported-by: Nicholas Piggin <npiggin@gmail.com>
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: linuxppc-dev@lists.ozlabs.org
CC: linux-arch@vger.kernel.org
---
 include/linux/sched/mm.h | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index d5a9ab8f3836..b2767ecb21a8 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -215,10 +215,6 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
 #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)
 {
@@ -247,10 +243,12 @@ static inline void membarrier_execve(struct task_struct *t)
 	membarrier_arch_execve(t);
 }
 #else
+#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
 static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 		struct mm_struct *next, struct task_struct *tsk)
 {
 }
+#endif
 static inline void membarrier_fork(struct task_struct *t,
 		unsigned long clone_flags)
 {
-- 
2.11.0

[RFC PATCH v2 for 4.14 2/2] Fix: membarrier: Handle CLONE_VM + !CLONE_THREAD correctly on powerpc

[Mathieu Desnoyers]

Threads targeting the same VM but which belong to different thread
groups is a tricky case. It has a few consequences:

It turns out that we cannot rely on get_nr_threads(p) to count the
number of threads using a VM. We can use
(atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
instead to skip the synchronize_sched() for cases where the VM only has
a single user, and that user only has a single thread.

It also turns out that we cannot use for_each_thread() to set
thread flags in all threads using a VM, as it only iterates on the
thread group.

Therefore, test the membarrier state variable directly rather than
relying on thread flags. This means
membarrier_register_private_expedited() needs to set the
MEMBARRIER_STATE_SWITCH_MM flag, issue synchronize_sched(), and only
then set MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY which allows private
expedited membarrier commands to succeed. membarrier_arch_switch_mm()
now tests for the MEMBARRIER_STATE_SWITCH_MM flag.

Changes since v1:
- Remove membarrier thread flag on powerpc (now unused).

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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
---
 arch/powerpc/include/asm/membarrier.h  | 21 ++-------------------
 arch/powerpc/include/asm/thread_info.h |  3 ---
 arch/powerpc/kernel/membarrier.c       | 17 ++++-------------
 include/linux/mm_types.h               |  2 +-
 include/linux/sched/mm.h               | 28 ++++++----------------------
 kernel/fork.c                          |  2 --
 kernel/sched/membarrier.c              | 16 +++++++++++++---
 7 files changed, 26 insertions(+), 63 deletions(-)

diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h
index 61152a7a3cf9..0951646253d9 100644
--- a/arch/powerpc/include/asm/membarrier.h
+++ b/arch/powerpc/include/asm/membarrier.h
@@ -11,8 +11,8 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 	 * 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))
+	if (likely(!(atomic_read(&next->membarrier_state)
+			& MEMBARRIER_STATE_SWITCH_MM) || !prev))
 		return;
 
 	/*
@@ -21,23 +21,6 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 	 */
 	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 2a208487724b..a941cc6fc3e9 100644
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h
@@ -100,7 +100,6 @@ 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)
@@ -120,8 +119,6 @@ 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/membarrier.c b/arch/powerpc/kernel/membarrier.c
index b0d79a5f5981..4795ad59b833 100644
--- a/arch/powerpc/kernel/membarrier.c
+++ b/arch/powerpc/kernel/membarrier.c
@@ -19,24 +19,15 @@
 #include <linux/thread_info.h>
 #include <linux/spinlock.h>
 #include <linux/rcupdate.h>
+#include <linux/atomic.h>
 
 void membarrier_arch_register_private_expedited(struct task_struct *p)
 {
-	struct task_struct *t;
+	struct mm_struct *mm = p->mm;
 
-	if (get_nr_threads(p) == 1) {
-		set_thread_flag(TIF_MEMBARRIER_PRIVATE_EXPEDITED);
+	atomic_or(MEMBARRIER_STATE_SWITCH_MM, &mm->membarrier_state);
+	if (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1)
 		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.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5e0fe8ce053b..1861ea8dba77 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -446,7 +446,7 @@ struct mm_struct {
 
 	struct core_state *core_state; /* coredumping support */
 #ifdef CONFIG_MEMBARRIER
-	int membarrier_private_expedited;
+	atomic_t membarrier_state;
 #endif
 #ifdef CONFIG_AIO
 	spinlock_t			ioctx_lock;
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index b2767ecb21a8..2fb6089efcd7 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -212,35 +212,23 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
 
 #ifdef CONFIG_MEMBARRIER
 
+enum {
+	MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY	= (1U << 0),
+	MEMBARRIER_STATE_SWITCH_MM			= (1U << 1),
+};
+
 #ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
 #include <asm/membarrier.h>
 #else
-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);
+	atomic_set(&t->mm->membarrier_state, 0);
 }
 #else
 #ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
@@ -249,10 +237,6 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 {
 }
 #endif
-static inline void membarrier_fork(struct task_struct *t,
-		unsigned long clone_flags)
-{
-}
 static inline void membarrier_execve(struct task_struct *t)
 {
 }
diff --git a/kernel/fork.c b/kernel/fork.c
index bd4a93915e08..10646182440f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1840,8 +1840,6 @@ 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/membarrier.c b/kernel/sched/membarrier.c
index f06949a279ca..23bd256f8458 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -18,6 +18,7 @@
 #include <linux/membarrier.h>
 #include <linux/tick.h>
 #include <linux/cpumask.h>
+#include <linux/atomic.h>
 
 #include "sched.h"	/* for cpu_rq(). */
 
@@ -40,7 +41,8 @@ static int membarrier_private_expedited(void)
 	bool fallback = false;
 	cpumask_var_t tmpmask;
 
-	if (!READ_ONCE(current->mm->membarrier_private_expedited))
+	if (!(atomic_read(&current->mm->membarrier_state)
+			& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
 		return -EPERM;
 
 	if (num_online_cpus() == 1)
@@ -104,11 +106,19 @@ static int membarrier_private_expedited(void)
 static void membarrier_register_private_expedited(void)
 {
 	struct task_struct *p = current;
+	struct mm_struct *mm = p->mm;
 
-	if (READ_ONCE(p->mm->membarrier_private_expedited))
+	/*
+	 * We need to consider threads belonging to different thread
+	 * groups, which use the same mm. (CLONE_VM but not
+	 * CLONE_THREAD).
+	 */
+	if (atomic_read(&mm->membarrier_state)
+			& MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY)
 		return;
 	membarrier_arch_register_private_expedited(p);
-	WRITE_ONCE(p->mm->membarrier_private_expedited, 1);
+	atomic_or(MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
+			&mm->membarrier_state);
 }
 
 /**
-- 
2.11.0

Re: [RFC PATCH for 4.14 1/2] membarrier: Remove unused code for architectures without membarrier hooks

[Paul E. McKenney]

On Thu, Oct 05, 2017 at 06:33:26PM -0400, Mathieu Desnoyers wrote:
> Architectures without membarrier hooks don't need to emit the
> empty membarrier_arch_switch_mm() static inline when
> CONFIG_MEMBARRIER=y.
> 
> Adapt the CONFIG_MEMBARRIER=n counterpart to only emit the empty
> membarrier_arch_switch_mm() for architectures with membarrier hooks.
> 
> Reported-by: Nicholas Piggin <npiggin@gmail.com>
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Queued for further review and testing, targeting v4.15.  Please let me
know if you need it sooner.

							Thanx, Paul

> 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: linuxppc-dev@lists.ozlabs.org
> CC: linux-arch@vger.kernel.org
> ---
>  include/linux/sched/mm.h | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index d5a9ab8f3836..b2767ecb21a8 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -215,10 +215,6 @@ static inline void memalloc_noreclaim_restore(unsigned int flags)
>  #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)
>  {
> @@ -247,10 +243,12 @@ static inline void membarrier_execve(struct task_struct *t)
>  	membarrier_arch_execve(t);
>  }
>  #else
> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
>  static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
>  		struct mm_struct *next, struct task_struct *tsk)
>  {
>  }
> +#endif
>  static inline void membarrier_fork(struct task_struct *t,
>  		unsigned long clone_flags)
>  {
> -- 
> 2.11.0
> 

Re: [RFC PATCH for 4.14 1/2] membarrier: Remove unused code for architectures without membarrier hooks

[Mathieu Desnoyers]

----- On Oct 6, 2017, at 5:08 PM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:

> On Thu, Oct 05, 2017 at 06:33:26PM -0400, Mathieu Desnoyers wrote:
>> Architectures without membarrier hooks don't need to emit the
>> empty membarrier_arch_switch_mm() static inline when
>> CONFIG_MEMBARRIER=y.
>> 
>> Adapt the CONFIG_MEMBARRIER=n counterpart to only emit the empty
>> membarrier_arch_switch_mm() for architectures with membarrier hooks.
>> 
>> Reported-by: Nicholas Piggin <npiggin@gmail.com>
>> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> 
> Queued for further review and testing, targeting v4.15.  Please let me
> know if you need it sooner.

Hi Paul,

Given that the following patch in this patchset (2/2)
"Fix: membarrier: Handle CLONE_VM + !CLONE_THREAD correctly on powerpc"
is based on this patch, and that the fix needs to go into 4.14,
I would recommend queuing both patches for 4.14 is possible.

Otherwise I could try swapping the order of the two patches if
needed, but since both are touching nearby code areas, rebasing
is not straightforward.

Thanks,

Mathieu

> 
>							Thanx, Paul
> 
>> 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: linuxppc-dev@lists.ozlabs.org
>> CC: linux-arch@vger.kernel.org
>> ---
>>  include/linux/sched/mm.h | 6 ++----
>>  1 file changed, 2 insertions(+), 4 deletions(-)
>> 
>> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
>> index d5a9ab8f3836..b2767ecb21a8 100644
>> --- a/include/linux/sched/mm.h
>> +++ b/include/linux/sched/mm.h
>> @@ -215,10 +215,6 @@ static inline void memalloc_noreclaim_restore(unsigned int
>> flags)
>>  #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)
>>  {
>> @@ -247,10 +243,12 @@ static inline void membarrier_execve(struct task_struct
>> *t)
>>  	membarrier_arch_execve(t);
>>  }
>>  #else
>> +#ifdef CONFIG_ARCH_HAS_MEMBARRIER_HOOKS
>>  static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
>>  		struct mm_struct *next, struct task_struct *tsk)
>>  {
>>  }
>> +#endif
>>  static inline void membarrier_fork(struct task_struct *t,
>>  		unsigned long clone_flags)
>>  {
>> --
>> 2.11.0

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