linux-kernel - Re: freezer: should barriers be smp ?

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Message-ID: <BANLkTiknfeTj6GD0t-ekMoaS1QYQBVyBjQ@mail.gmail.com>
Date:	Wed, 13 Apr 2011 17:02:45 -0400
From:	Mike Frysinger <vapier@...too.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-pm@...ts.linux-foundation.org
Subject: Re: freezer: should barriers be smp ?

On Wed, Apr 13, 2011 at 16:58, Rafael J. Wysocki wrote:
> On Wednesday, April 13, 2011, Mike Frysinger wrote:
>> when we suspend/resume Blackfin SMP systems, we notice that the
>> freezer code runs on multiple cores.  this is of course what you want
>> -- freeze processes in parallel.  however, the code only uses non-smp
>> based barriers which causes us problems ... our cores need software
>> support to keep caches in sync, so our smp barriers do just that.  but
>> the non-smp barriers do not, and so the frozen/thawed processes
>> randomly get stuck in the wrong task state.
>>
>> thinking about it, shouldnt the freezer code be using smp barriers ?
>
> Yes, it should, but rmb() and wmb() are supposed to be SMP barriers.
>
> Or do you mean something different?

then what's the diff between smp_rmb() and rmb() ?

this is what i'm proposing:
--- a/kernel/freezer.c
+++ b/kernel/freezer.c
@@ -17,7 +17,7 @@ static inline void frozen_process(void)
 {
    if (!unlikely(current->flags & PF_NOFREEZE)) {
        current->flags |= PF_FROZEN;
-       wmb();
+       smp_wmb();
    }
    clear_freeze_flag(current);
 }
@@ -93,7 +93,7 @@ bool freeze_task(struct task_struct *p, bool sig_only)
     * the task as frozen and next clears its TIF_FREEZE.
     */
    if (!freezing(p)) {
-       rmb();
+       smp_rmb();
        if (frozen(p))
            return false;

-mike
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