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Date:   Tue, 11 Aug 2020 08:27:33 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Peter Oskolkov <posk@google.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        linux-kernel@vger.kernel.org, Paul Turner <pjt@google.com>,
        Chris Kennelly <ckennelly@google.com>,
        Peter Oskolkov <posk@posk.io>
Subject: Re: [PATCH 1/2 v3] rseq/membarrier: add
 MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
Message-ID: <20200811062733.GP3982@worktop.programming.kicks-ass.net>
References: <20200811000959.2486636-1-posk@google.com>
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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;
 	}