[RFC PATCH v2 1/3] restartable sequences: user-space per-cpu critical sections

Paul Turner
2015-10-27T23:57:05Z

From: Paul Turner <pjt@google.com>

Introduce the notion of a restartable sequence. This is a piece of user code
that can be described in 3 components:

1) Establish where [e.g. which cpu] the thread is running
2) Preparatory work that is dependent on the state in [1].
3) A committing instruction that proceeds only if [2] and [1] have proceeded
in a coherent state (i.e. not thread from the same address state has run
on the same cpu).

Preemption, or other interruption beyond [1] guarantees that [3] will not
execute. With support for control being transferred to a user-defined restart
handler. This handler may arrange for the original operation to be retried,
including potentially resynchronizing with dependent state that may
have been updated in the interim.

This may be used in combination with an in-memory cpu-id to allow user programs
to implement cpu-local data-structures and primitives, without the use/overhead
of any atomics.

The kernel ABI generally consists of:
a) A shared TLS word which exports the current cpu and associated event-count
b) A shared TLS word which, when non-zero, stores the first post-commit
instruction if a sequence is active. (The kernel observing rips greater
than this takes no action and concludes user-space has completed its
critical section, but not yet cleared this state).
c) An architecture specific way to publish the state read from (a) which
user-space believes to be current.

[ (a) is only read by userspace, it is published by the kernel.
(b) is only ready by the kernel, it is published by userspace. ]

Signed-off-by: Paul Turner <pjt@google.com>
---
arch/Kconfig | 7 ++
arch/x86/Kconfig | 1
arch/x86/entry/syscalls/syscall_64.tbl | 1
fs/exec.c | 1
include/linux/sched.h | 27 ++++++++
include/uapi/asm-generic/unistd.h | 4 +
init/Kconfig | 11 +++
kernel/Makefile | 2 -
kernel/restartable_sequences.c | 112 ++++++++++++++++++++++++++++++++
kernel/sched/core.c | 4 +
kernel/sched/sched.h | 3 +
kernel/sys_ni.c | 2 +
12 files changed, 172 insertions(+), 3 deletions(-)
create mode 100644 kernel/restartable_sequences.c

diff --git a/arch/Kconfig b/arch/Kconfig
index 671810c..336880c 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -241,6 +241,13 @@ config HAVE_REGS_AND_STACK_ACCESS_API
declared in asm/ptrace.h
For example the kprobes-based event tracer needs this API.

+config HAVE_RESTARTABLE_SEQUENCE_SUPPORT
+ bool
+ depends on HAVE_REGS_AND_STACK_ACCESS_API
+ help
+ This symbol should be selected by an architecture if it supports an
+ implementation of restartable sequences.
+
config HAVE_CLK
bool
help
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 262b79c..25a0a60 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -135,6 +135,7 @@ config X86
select HAVE_PERF_REGS
select HAVE_PERF_USER_STACK_DUMP
select HAVE_REGS_AND_STACK_ACCESS_API
+ select HAVE_RESTARTABLE_SEQUENCE_SUPPORT
select HAVE_SYSCALL_TRACEPOINTS
select HAVE_UID16 if X86_32 || IA32_EMULATION
select HAVE_UNSTABLE_SCHED_CLOCK
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 278842f..0fd4243 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -331,6 +331,7 @@
322 64 execveat stub_execveat
323 common userfaultfd sys_userfaultfd
324 common membarrier sys_membarrier
+325 common restartable_sequences sys_restartable_sequences

#
# x32-specific system call numbers start at 512 to avoid cache impact
diff --git a/fs/exec.c b/fs/exec.c
index 0a77a69..120ef19 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1599,6 +1599,7 @@ static int do_execveat_common(int fd, struct filename *filename,
current->in_execve = 0;
acct_update_integrals(current);
task_numa_free(current);
+ rseq_clear_state_exec();
free_bprm(bprm);
kfree(pathbuf);
putname(filename);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 9e1e06c..03ab1f0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1189,6 +1189,21 @@ struct mempolicy;
struct pipe_inode_info;
struct uts_namespace;

+#ifdef CONFIG_RESTARTABLE_SEQUENCES
+struct restartable_sequence_state {
+ __user void *post_commit_instr_addr;
+ __user u64 *event_and_cpu;
+
+ u32 last_switch_count;
+ u32 event_counter;
+ struct preempt_notifier notifier;
+};
+
+void rseq_clear_state_exec(void);
+#else
+static inline void rseq_clear_state_exec(void) {}
+#endif
+
struct load_weight {
unsigned long weight;
u32 inv_weight;
@@ -1820,6 +1835,9 @@ struct task_struct {
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
unsigned long task_state_change;
#endif
+#ifdef CONFIG_RESTARTABLE_SEQUENCES
+ struct restartable_sequence_state rseq_state;
+#endif
int pagefault_disabled;
/* CPU-specific state of this task */
struct thread_struct thread;
@@ -3189,4 +3207,13 @@ static inline unsigned long rlimit_max(unsigned int limit)
return task_rlimit_max(current, limit);
}

+#ifdef CONFIG_RESTARTABLE_SEQUENCES
+static inline int rseq_active(struct task_struct *p)
+{
+ return p->rseq_state.event_and_cpu != NULL;
+}
+#else
+static inline int rseq_active(struct task_struct *p) { return 0; }
+#endif
+
#endif
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index ee12400..9659f31 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -713,9 +713,11 @@ __SC_COMP(__NR_execveat, sys_execveat, compat_sys_execveat)
__SYSCALL(__NR_userfaultfd, sys_userfaultfd)
#define __NR_membarrier 283
__SYSCALL(__NR_membarrier, sys_membarrier)
+#define __NR_restartable_sequences 284
+__SYSCALL(__NR_restartable_sequences, sys_restartable_sequences)

#undef __NR_syscalls
-#define __NR_syscalls 284
+#define __NR_syscalls 285

/*
* All syscalls below here should go away really,
diff --git a/init/Kconfig b/init/Kconfig
index c24b6f7..98c02f2 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2040,7 +2040,16 @@ config STOP_MACHINE
source "block/Kconfig"

config PREEMPT_NOTIFIERS
- bool
+ bool n
+
+config RESTARTABLE_SEQUENCES
+ bool "Userspace Restartable Sequences (RSEQ)"
+ default n
+ depends on HAVE_RESTARTABLE_SEQUENCE_SUPPORT && PREEMPT_NOTIFIERS
+ help
+ Allows binaries to define a region of user-text within which
+ execution will be restarted in the event of signal delivery or
+ preemption.

config PADATA
depends on SMP
diff --git a/kernel/Makefile b/kernel/Makefile
index 53abf00..dbe6963 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -101,8 +101,8 @@ obj-$(CONFIG_JUMP_LABEL) += jump_label.o
obj-$(CONFIG_CONTEXT_TRACKING) += context_tracking.o
obj-$(CONFIG_TORTURE_TEST) += torture.o
obj-$(CONFIG_MEMBARRIER) += membarrier.o
-
obj-$(CONFIG_HAS_IOMEM) += memremap.o
+obj-$(CONFIG_RESTARTABLE_SEQUENCES) += restartable_sequences.o

$(obj)/configs.o: $(obj)/config_data.h

diff --git a/kernel/restartable_sequences.c b/kernel/restartable_sequences.c
new file mode 100644
index 0000000..c99a574
--- /dev/null
+++ b/kernel/restartable_sequences.c
@@ -0,0 +1,112 @@
+/*
+ * Restartable Sequences are a lightweight interface that allows user-level
+ * code to be executed atomically relative to scheduler preemption. Typically
+ * used for implementing per-cpu operations.
+ *
+ * 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.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * Copyright (C) 2015, Google, Inc.,
+ * Paul Turner <pjt@google.com> and Andrew Hunter <ahh@google.com>
+ *
+ */
+
+#ifdef CONFIG_RESTARTABLE_SEQUENCES
+
+#include <linux/uaccess.h>
+#include <linux/preempt.h>
+#include <linux/syscalls.h>
+
+static void rseq_sched_in_nop(struct preempt_notifier *pn, int cpu) {}
+static void rseq_sched_out_nop(struct preempt_notifier *pn,
+ struct task_struct *next) {}
+
+static __read_mostly struct preempt_ops rseq_preempt_ops = {
+ .sched_in = rseq_sched_in_nop,
+ .sched_out = rseq_sched_out_nop,
+};
+
+int rseq_configure_current(__user u64 *event_and_cpu,
+ __user void *post_commit_instr_addr)
+{
+ struct restartable_sequence_state *rseq_state = &current->rseq_state;
+ int addr_size = test_tsk_thread_flag(current, TIF_IA32) ? 4 : 8;
+ int registered = 0;
+
+ /* Disable */
+ if (!event_and_cpu && !post_commit_instr_addr)
+ goto valid;
+
+ if (!event_and_cpu || !post_commit_instr_addr)
+ return -EINVAL;
+
+ if (!access_ok(VERIFY_WRITE, event_and_cpu, sizeof(*event_and_cpu)) ||
+ !access_ok(VERIFY_READ, post_commit_instr_addr, addr_size))
+ return -EINVAL;
+
+valid:
+ preempt_disable();
+
+ if (rseq_state->event_and_cpu)
+ registered = 1;
+
+ rseq_state->event_counter = 1;
+ rseq_state->event_and_cpu = event_and_cpu;
+ rseq_state->post_commit_instr_addr = post_commit_instr_addr;
+
+ if (event_and_cpu && !registered) {
+ preempt_notifier_init(&rseq_state->notifier,
+ &rseq_preempt_ops);
+ preempt_notifier_inc();
+ preempt_notifier_register(&rseq_state->notifier);
+ } else if (!event_and_cpu && registered) {
+ preempt_notifier_unregister(&rseq_state->notifier);
+ preempt_notifier_dec();
+ }
+
+ preempt_enable();
+
+ /* Will update *event_and_cpu on return. */
+ if (event_and_cpu)
+ set_thread_flag(TIF_NOTIFY_RESUME);
+
+ return 0;
+}
+
+void rseq_clear_state_exec()
+{
+ rseq_configure_current(NULL, NULL);
+}
+
+/*
+ * RSEQ syscall interface.
+ *
+ * Usage:
+ * sys_restartable_sequences(flags, &event_and_cpu, &post_commit_instr_addr)
+
+ * flags is currently unused.
+ */
+SYSCALL_DEFINE3(restartable_sequences,
+ int, flags, long, event_and_cpu, long, post_commit_instr_addr)
+{
+ return rseq_configure_current((__user u64 *)event_and_cpu,
+ (__user void *)post_commit_instr_addr);
+}
+#else
+SYSCALL_DEFINE0(restartable_sequences)
+{
+ return -ENOSYS;
+}
+#endif
+
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index aa59732..84e0641 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2120,6 +2120,10 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)

p->numa_group = NULL;
#endif /* CONFIG_NUMA_BALANCING */
+
+#ifdef CONFIG_RESTARTABLE_SEQUENCES
+ memset(&p->rseq_state, 0, sizeof(p->rseq_state));
+#endif
}

DEFINE_STATIC_KEY_FALSE(sched_numa_balancing);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index efd3bfc..e8de833 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -957,6 +957,9 @@ static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
{
set_task_rq(p, cpu);
#ifdef CONFIG_SMP
+ if (rseq_active(p))
+ set_tsk_thread_flag(p, TIF_NOTIFY_RESUME);
+
/*
* After ->cpu is set up to a new value, task_rq_lock(p, ...) can be
* successfuly executed on another CPU. We must ensure that updates of
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index a02decf..0bdf060 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -248,3 +248,5 @@ cond_syscall(sys_execveat);

/* membarrier */
cond_syscall(sys_membarrier);
+/* restartable sequences */
+cond_syscall(sys_restartable_sequences);