linux-kernel - Re: [Kgdb-bugreport] [PATCH 4/5] kgdb: Use atomic operators whichuse barriers

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Message-ID: <1271690516.17500.2.camel@e102144-lin.cambridge.arm.com>
Date:	Mon, 19 Apr 2010 16:21:56 +0100
From:	Will Deacon <will.deacon@....com>
To:	Jason Wessel <jason.wessel@...driver.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	kgdb-bugreport@...ts.sourceforge.net,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm@...r.kernel.org
Subject: Re: [Kgdb-bugreport] [PATCH 4/5] kgdb: Use atomic operators
 whichuse barriers

Hi Jason,

> Here is the revised patch, which is going into the kgdb-fixes branch.
> 
> Thanks,
> Jason.
> 
> --
> From: Jason Wessel <jason.wessel@...driver.com>
> Subject: [PATCH] kgdb: Use atomic operators which use barriers
> 
> The cpu_relax() does not mandate that there is an smp memory barrier.
> As a result on the arm smp architecture the kernel debugger can hang
> on entry from time to time, as shown by the kgdb regression tests.
> 
> The solution is simply to use the atomic operators which include a
> proper smp memory barrier, instead of using atomic_set().
> 
> Tested-by: Will Deacon <will.deacon@....com>
> Signed-off-by: Jason Wessel <jason.wessel@...driver.com>
> ---
>  kernel/kgdb.c |   15 +++++++--------
>  1 file changed, 7 insertions(+), 8 deletions(-)
> 
> --- a/kernel/kgdb.c
> +++ b/kernel/kgdb.c
> @@ -1379,8 +1379,7 @@ acquirelock:
>  	 * Make sure the above info reaches the primary CPU before
>  	 * our cpu_in_kgdb[] flag setting does:
>  	 */
> -	smp_wmb();
> -	atomic_set(&cpu_in_kgdb[cpu], 1);
> +	atomic_inc(&cpu_in_kgdb[cpu]);

As Dmitry pointed out here:

http://lkml.org/lkml/2010/4/8/214

This bit of code looks broken, especially since the comment has been left
alone by the patch. I think commit ae6bf53e should be reverted because
semantically all it does is remove the smp_wmb() above.

Please let me know what you think,

Will



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