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SubjectRe: [PATCH 1/2 v3] rseq/membarrier: add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ
On Mon, Aug 10, 2020 at 11:27 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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.

Ack. Will do in the next version - for now waiting for the rest to be
worked out.

[...]

>
> 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;

@flags is not used now; maybe just rename it to something more
generic? @param? Or @options? Or maybe more specific, like @cpu_id?

> - he wondered if we should support SYNC_CORE + RSEQ.

It seems to me that CMD_PRIVATE_EXPEDITED_RSEQ is basically
CMD_PRIVATE_EXPEDITED_SYNC_CORE with the extra "restart RSEQ CSs"
behavior. Am I missing something? If not, what is the point of
complicating the code as suggested below? Maybe just renaming
CMD_PRIVATE_EXPEDITED_RSEQ to CMD_PRIVATE_EXPEDITED_SYNC_CORE_RSEQ
will do?

>
>
> 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;
> }
>

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