linux-kernel - Re: [PATCH]: Use cmpxchg() in WARN_*

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Date:	Thu, 31 Mar 2011 11:23:19 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, dzickus@...hat.com
Subject: Re: [PATCH]: Use cmpxchg() in WARN_*_ONCE() functions

On Thu, Mar 31, 2011 at 08:46:07AM -0400, Prarit Bhargava wrote:
> An issue popped up where WARN_ON_ONCE() was used in a callback function
> in smp_call_function().  This resulted in the WARN_ON executing multiple times
> when it should have only executed once.

But that is just once per cpu, correct?

> 
> I then did
> 
>         for (i = 0; i < 1000000; i++)
>                 on_each_cpu(prarit_callback, NULL, 0);
> 
> The current code, of course, explodes :).  That's the bug I'm trying to fix.

How exactly does it explode? How many CPUs do you have, and does this
still just print once per CPU?

> What is interesting in this test, however, is the impact that checking the
> !__warned flag has [Aside: Checking the !__warned flag is an enhancement
> and is not explicitly required for this code].
> 
> A run with just (!cmpxchg(&__warned, 0, 1)) results in an average of 21.323s,
> and a run with  (!__warned && !cmpxchg(&__warned, 0, 1)) results in an
> average of 20.233s.  Of course, the !__warned is not necessary for the code
> to work properly but it seems to be a significant impact to the time to run
> this code.

Yes adding the check for !__warned first should have obvious benefits.

I really do not see anything wrong with this patch, but personally, I
would rather fix what caused the WARN_ON_ONCE() than fix the warning
itself, as long as the warning itself does not really break anything
else.

-- Steve
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