Re: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ

[Peter Zijlstra]

On Mon, Aug 10, 2020 at 05:09:58PM -0700, Peter Oskolkov wrote:
> @@ -27,6 +35,12 @@
>  
>  static void ipi_mb(void *info)
>  {

The #ifdef wants to behere, otherwise you'll get a compile warning for
!RSEQ builds.

> +     int *flags = info;
> +
> +#ifdef CONFIG_RSEQ
> +     if (flags && (*flags == MEMBARRIER_FLAG_RSEQ))
> +             rseq_preempt(current);
> +#endif
>       smp_mb();       /* IPIs should be serializing but paranoid. */
>  }

But yes, this looks much better.

Mathieu did mention a few other points that I didn't see addressed:

 - he didn't like abusing the @flags syscall argument for a CPUid;
 - he wondered if we should support SYNC_CORE + RSEQ.


Not sure we can easily change the syscall at this point, but the latter
point could be addressed with something like this.

---
Index: linux-2.6/kernel/sched/membarrier.c
===================================================================
--- linux-2.6.orig/kernel/sched/membarrier.c
+++ linux-2.6/kernel/sched/membarrier.c
@@ -374,8 +374,26 @@ static int membarrier_register_private_e
  */
 SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
 {
+       int cflags = 0, int cpuid = -1;
+
        if (unlikely(flags) && cmd != MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)
                return -EINVAL;
+
+       if (cmd & (MEMBARRIER_CMD_PRIVATE_EXPEDITED |
+                  MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE |
+                  MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)) {
+
+               if (cmd & MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ)
+                       cflags |= MEMBARRIER_FLAG_RSEQ;
+
+               if (cmd & MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE) {
+                       cflags |= MEMBARRIER_FLAG_SYNC_CORE;
+                       cpuid = flags;
+               }
+
+               cmd = MEMBARRIER_CMD_PRIVATE_EXPEDITED;
+       }
+
        switch (cmd) {
        case MEMBARRIER_CMD_QUERY:
        {
@@ -396,18 +414,16 @@ SYSCALL_DEFINE2(membarrier, int, cmd, in
                return membarrier_global_expedited();
        case MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED:
                return membarrier_register_global_expedited();
-       case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
-               return membarrier_private_expedited(0, -1);
        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, 
-1);
        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, 
flags);
        case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
                return 
membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ);
+
+       case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
+               return membarrier_private_expedited(cflags, cpuid);
+
        default:
                return -EINVAL;
        }