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.
v7->v8:
  - replaced BUG_ON with WARN_ON_ONCE in membarrier.c.

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

Signed-off-by: Peter Oskolkov <p...@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..e23e74d52db5 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 {
+               WARN_ON_ONCE(flags);
                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 {
+               WARN_ON_ONCE(flags);
        }
 
        /*
@@ -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;
        }
-- 
2.28.0.709.gb0816b6eb0-goog

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