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

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Message-ID: <BANLkTi=_+kYvs3_dsXifd_rkxfmFzJLttA@mail.gmail.com>
Date:	Wed, 13 Apr 2011 18:57:13 -0400
From:	Mike Frysinger <vapier@...too.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Pavel Machek <pavel@....cz>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: freezer: should barriers be smp ?

On Wed, Apr 13, 2011 at 18:49, Rafael J. Wysocki wrote:
> On Thursday, April 14, 2011, Mike Frysinger wrote:
>> i guess the trouble for us is that you have one CPU posting writes to
>> task->flags (and doing so by grabbing the task's spinlock), but the
>> other CPU is simply reading those flags.  there are no SMP barriers in
>> between the read and write steps, nor is the reading CPU grabbing any
>> locks which would be an implicit SMP barrier.  since the Blackfin SMP
>> port lacks hardware cache coherency, there is no way for us to know
>> "we've got to sync the caches before we can do this read".  by using
>> the patch i posted above, we have that signal and so things work
>> correctly.,
>
> In theory I wouldn't expect the patch to work correctly, because it replaces
> _stronger_ memory barriers with _weaker_ SMP barriers.  However, looking at
> the blackfin's definitions of SMP barriers I see that it uses extra stuff that
> should _also_ be used in the definitions of the mandatory barriers.
>
> In my opinion is an architecture problem, not the freezer code problem.

OK, we have a patch pending locally which populates all barriers with
this logic, but based on my understanding of things, that didnt seem
correct.  i guess i'm reading too much into the names ... i'd expect
the opposite behavior where "rmb" is only for UP needs while "smp_rmb"
is a rmb which additionally covers SMP.
-mike
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