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Subject[PATCH 1/2 v7] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
From
This patchset is based on Google-internal RSEQ
work done by Paul Turner and Andrew Hunter.

When working with per-CPU RSEQ-based memory allocations,
it is sometimes important to make sure that a global
memory location is no longer accessed from RSEQ critical
sections. For example, there can be two per-CPU lists,
one is "active" and accessed per-CPU, while another one
is inactive and worked on asynchronously "off CPU" (e.g.
garbage collection is performed). Then at some point
the two lists are swapped, and a fast RCU-like mechanism
is required to make sure that the previously active
list is no longer accessed.

This patch introduces such a mechanism: in short,
membarrier() syscall issues an IPI to a CPU, restarting
a potentially active RSEQ critical section on the CPU.

v1->v2:
- removed the ability to IPI all CPUs in a single sycall;
- use task->mm rather than task->group_leader to identify
tasks belonging to the same process.
v2->v3:
- re-added the ability to IPI all CPUs in a single syscall;
- integrated with membarrier_private_expedited() to
make sure only CPUs running tasks with the same mm as
the current task are interrupted;
- also added MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ;
- flags in membarrier_private_expedited are never actually
bit flags but always distinct values (i.e. never two flags
are combined), so I modified bit testing to full equation
comparison for simplicity (otherwise the code needs to
work when several bits are set, for example).
v3->v4:
- added the third parameter to membarrier syscall: @cpu_id:
if @flags == MEMBARRIER_CMD_FLAG_CPU, then @cpu_id indicates
the cpu on which RSEQ CS should be restarted.
v4->v5:
- added @cpu_id parameter to sys_membarrier in syscalls.h.
v5->v6:
- made membarrier_private_expedited more efficient in a
single-cpu case;
- a couple of minor refactorings.
v6->v7:
- made @flags an unsigned int in sys_membarrier;
- a couple of minor refactorings.

The second patch in the patchset adds a selftest
of this feature.

Signed-off-by: Peter Oskolkov <posk@google.com>
---
include/linux/sched/mm.h | 3 +
include/linux/syscalls.h | 2 +-
include/uapi/linux/membarrier.h | 26 ++++++
kernel/sched/membarrier.c | 136 +++++++++++++++++++++++++-------
4 files changed, 136 insertions(+), 31 deletions(-)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index f889e332912f..15bfb06f2884 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -348,10 +348,13 @@ enum {
MEMBARRIER_STATE_GLOBAL_EXPEDITED = (1U << 3),
MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY = (1U << 4),
MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE = (1U << 5),
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY = (1U << 6),
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ = (1U << 7),
};

enum {
MEMBARRIER_FLAG_SYNC_CORE = (1U << 0),
+ MEMBARRIER_FLAG_RSEQ = (1U << 1),
};

#ifdef CONFIG_ARCH_HAS_MEMBARRIER_CALLBACKS
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 75ac7f8ae93c..466c993e52bf 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -974,7 +974,7 @@ asmlinkage long sys_execveat(int dfd, const char __user *filename,
const char __user *const __user *argv,
const char __user *const __user *envp, int flags);
asmlinkage long sys_userfaultfd(int flags);
-asmlinkage long sys_membarrier(int cmd, int flags);
+asmlinkage long sys_membarrier(int cmd, int flags, int cpu_id);
asmlinkage long sys_mlock2(unsigned long start, size_t len, int flags);
asmlinkage long sys_copy_file_range(int fd_in, loff_t __user *off_in,
int fd_out, loff_t __user *off_out,
diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
index 5891d7614c8c..737605897f36 100644
--- a/include/uapi/linux/membarrier.h
+++ b/include/uapi/linux/membarrier.h
@@ -114,6 +114,26 @@
* If this command is not implemented by an
* architecture, -EINVAL is returned.
* Returns 0 on success.
+ * @MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
+ * Ensure the caller thread, upon return from
+ * system call, that all its running thread
+ * siblings have any currently running rseq
+ * critical sections restarted if @flags
+ * parameter is 0; if @flags parameter is
+ * MEMBARRIER_CMD_FLAG_CPU,
+ * then this operation is performed only
+ * on CPU indicated by @cpu_id. If this command is
+ * not implemented by an architecture, -EINVAL
+ * is returned. A process needs to register its
+ * intent to use the private expedited rseq
+ * command prior to using it, otherwise
+ * this command returns -EPERM.
+ * @MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
+ * Register the process intent to use
+ * MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ.
+ * If this command is not implemented by an
+ * architecture, -EINVAL is returned.
+ * Returns 0 on success.
* @MEMBARRIER_CMD_SHARED:
* Alias to MEMBARRIER_CMD_GLOBAL. Provided for
* header backward compatibility.
@@ -131,9 +151,15 @@ enum membarrier_cmd {
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4),
MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE = (1 << 5),
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE = (1 << 6),
+ MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ = (1 << 7),
+ MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ = (1 << 8),

/* Alias for header backward compatibility. */
MEMBARRIER_CMD_SHARED = MEMBARRIER_CMD_GLOBAL,
};

+enum membarrier_cmd_flag {
+ MEMBARRIER_CMD_FLAG_CPU = (1 << 0),
+};
+
#endif /* _UAPI_LINUX_MEMBARRIER_H */
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 168479a7d61b..04729fb96fba 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -18,6 +18,14 @@
#define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK 0
#endif

+#ifdef CONFIG_RSEQ
+#define MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ_BITMASK \
+ (MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ \
+ | MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ_BITMASK)
+#else
+#define MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ_BITMASK 0
+#endif
+
#define MEMBARRIER_CMD_BITMASK \
(MEMBARRIER_CMD_GLOBAL | MEMBARRIER_CMD_GLOBAL_EXPEDITED \
| MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED \
@@ -30,6 +38,11 @@ static void ipi_mb(void *info)
smp_mb(); /* IPIs should be serializing but paranoid. */
}

+static void ipi_rseq(void *info)
+{
+ rseq_preempt(current);
+}
+
static void ipi_sync_rq_state(void *info)
{
struct mm_struct *mm = (struct mm_struct *) info;
@@ -129,19 +142,27 @@ static int membarrier_global_expedited(void)
return 0;
}

-static int membarrier_private_expedited(int flags)
+static int membarrier_private_expedited(int flags, int cpu_id)
{
- int cpu;
cpumask_var_t tmpmask;
struct mm_struct *mm = current->mm;
+ smp_call_func_t ipi_func = ipi_mb;

- if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
+ if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
return -EINVAL;
if (!(atomic_read(&mm->membarrier_state) &
MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY))
return -EPERM;
+ } else if (flags == MEMBARRIER_FLAG_RSEQ) {
+ if (!IS_ENABLED(CONFIG_RSEQ))
+ return -EINVAL;
+ if (!(atomic_read(&mm->membarrier_state) &
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY))
+ return -EPERM;
+ ipi_func = ipi_rseq;
} else {
+ BUG_ON(flags != 0);
if (!(atomic_read(&mm->membarrier_state) &
MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY))
return -EPERM;
@@ -156,35 +177,59 @@ static int membarrier_private_expedited(int flags)
*/
smp_mb(); /* system call entry is not a mb. */

- if (!zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
+ if (cpu_id < 0 && !zalloc_cpumask_var(&tmpmask, GFP_KERNEL))
return -ENOMEM;

cpus_read_lock();
- rcu_read_lock();
- for_each_online_cpu(cpu) {
+
+ if (cpu_id >= 0) {
struct task_struct *p;

- /*
- * Skipping the current CPU is OK even through we can be
- * migrated at any point. The current CPU, at the point
- * where we read raw_smp_processor_id(), is ensured to
- * be in program order with respect to the caller
- * thread. Therefore, we can skip this CPU from the
- * iteration.
- */
- if (cpu == raw_smp_processor_id())
- continue;
- p = rcu_dereference(cpu_rq(cpu)->curr);
- if (p && p->mm == mm)
- __cpumask_set_cpu(cpu, tmpmask);
+ if (cpu_id >= nr_cpu_ids || !cpu_online(cpu_id))
+ goto out;
+ if (cpu_id == raw_smp_processor_id())
+ goto out;
+ rcu_read_lock();
+ p = rcu_dereference(cpu_rq(cpu_id)->curr);
+ if (!p || p->mm != mm) {
+ rcu_read_unlock();
+ goto out;
+ }
+ rcu_read_unlock();
+ } else {
+ int cpu;
+
+ rcu_read_lock();
+ for_each_online_cpu(cpu) {
+ struct task_struct *p;
+
+ /*
+ * Skipping the current CPU is OK even through we can be
+ * migrated at any point. The current CPU, at the point
+ * where we read raw_smp_processor_id(), is ensured to
+ * be in program order with respect to the caller
+ * thread. Therefore, we can skip this CPU from the
+ * iteration.
+ */
+ if (cpu == raw_smp_processor_id())
+ continue;
+ p = rcu_dereference(cpu_rq(cpu)->curr);
+ if (p && p->mm == mm)
+ __cpumask_set_cpu(cpu, tmpmask);
+ }
+ rcu_read_unlock();
}
- rcu_read_unlock();

preempt_disable();
- smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
+ if (cpu_id >= 0)
+ smp_call_function_single(cpu_id, ipi_func, NULL, 1);
+ else
+ smp_call_function_many(tmpmask, ipi_func, NULL, 1);
preempt_enable();

- free_cpumask_var(tmpmask);
+out:
+ if (cpu_id < 0)
+ free_cpumask_var(tmpmask);
cpus_read_unlock();

/*
@@ -283,11 +328,18 @@ static int membarrier_register_private_expedited(int flags)
set_state = MEMBARRIER_STATE_PRIVATE_EXPEDITED,
ret;

- if (flags & MEMBARRIER_FLAG_SYNC_CORE) {
+ if (flags == MEMBARRIER_FLAG_SYNC_CORE) {
if (!IS_ENABLED(CONFIG_ARCH_HAS_MEMBARRIER_SYNC_CORE))
return -EINVAL;
ready_state =
MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY;
+ } else if (flags == MEMBARRIER_FLAG_RSEQ) {
+ if (!IS_ENABLED(CONFIG_RSEQ))
+ return -EINVAL;
+ ready_state =
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY;
+ } else {
+ BUG_ON(flags != 0);
}

/*
@@ -299,6 +351,8 @@ static int membarrier_register_private_expedited(int flags)
return 0;
if (flags & MEMBARRIER_FLAG_SYNC_CORE)
set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE;
+ if (flags & MEMBARRIER_FLAG_RSEQ)
+ set_state |= MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ;
atomic_or(set_state, &mm->membarrier_state);
ret = sync_runqueues_membarrier_state(mm);
if (ret)
@@ -310,8 +364,15 @@ static int membarrier_register_private_expedited(int flags)

/**
* sys_membarrier - issue memory barriers on a set of threads
- * @cmd: Takes command values defined in enum membarrier_cmd.
- * @flags: Currently needs to be 0. For future extensions.
+ * @cmd: Takes command values defined in enum membarrier_cmd.
+ * @flags: Currently needs to be 0 for all commands other than
+ * MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ: in the latter
+ * case it can be MEMBARRIER_CMD_FLAG_CPU, indicating that @cpu_id
+ * contains the CPU on which to interrupt (= restart)
+ * the RSEQ critical section.
+ * @cpu_id: if @flags == MEMBARRIER_CMD_FLAG_CPU, indicates the cpu on which
+ * RSEQ CS should be interrupted (@cmd must be
+ * MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ).
*
* If this system call is not implemented, -ENOSYS is returned. If the
* command specified does not exist, not available on the running
@@ -337,10 +398,21 @@ static int membarrier_register_private_expedited(int flags)
* smp_mb() X O O
* sys_membarrier() O O O
*/
-SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
+SYSCALL_DEFINE3(membarrier, int, cmd, unsigned int, flags, int, cpu_id)
{
- if (unlikely(flags))
- return -EINVAL;
+ switch (cmd) {
+ case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
+ if (unlikely(flags && flags != MEMBARRIER_CMD_FLAG_CPU))
+ return -EINVAL;
+ break;
+ default:
+ if (unlikely(flags))
+ return -EINVAL;
+ }
+
+ if (!(flags & MEMBARRIER_CMD_FLAG_CPU))
+ cpu_id = -1;
+
switch (cmd) {
case MEMBARRIER_CMD_QUERY:
{
@@ -362,13 +434,17 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
return membarrier_register_global_expedited();
case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
- return membarrier_private_expedited(0);
+ return membarrier_private_expedited(0, cpu_id);
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
return membarrier_register_private_expedited(0);
case MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE:
- return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
+ return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE, cpu_id);
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE:
return membarrier_register_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
+ case MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ:
+ return membarrier_private_expedited(MEMBARRIER_FLAG_RSEQ, cpu_id);
+ case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
+ return membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ);
default:
return -EINVAL;
}
--
2.28.0.618.gf4bc123cb7-goog
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