linux-kernel - Re: [PATCH] aio: protect reqs_available updates from changes in interrupt handlers

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Message-ID: <53C413C3.7050308@kernel.dk>
Date:	Mon, 14 Jul 2014 19:30:43 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Benjamin LaHaise <bcrl@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Robert Elliot <Elliott@...com>
CC:	linux-aio@...ck.org, linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>, stable@...r.kernel.org
Subject: Re: [PATCH] aio: protect reqs_available updates from changes in interrupt
 handlers

On 2014-07-14 19:12, Benjamin LaHaise wrote:
> Hello everyone,
>
> Please pull the following commit (263782c1c95bbddbb022dc092fd89a36bb8d5577)
> from git://git.kvack.org/aio-fixes.git to fix an aio bug reported by Robert
> Elliot.
> ---
> As of commit f8567a3845ac05bb28f3c1b478ef752762bd39ef it is now possible to
> have put_reqs_available() called from irq context.  While put_reqs_available()
> is per cpu, it did not protect itself from interrupts on the same CPU.  This
> lead to aio_complete() corrupting the available io requests count when run
> under a heavy O_DIRECT workloads as reported by Robert Elliott.  Fix this by
> disabling irq updates around the per cpu batch updates of reqs_available.
>
> Many thanks to Robert and folks for testing and tracking this down.
>
> Reported-by: Robert Elliot <Elliott@...com>
> Tested-by: Robert Elliot <Elliott@...com>
> Signed-off-by: Benjamin LaHaise <bcrl@...ck.org>
> Cc: Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@...radead.org>
> Cc: stable@...r.kenel.org
> ---
>   fs/aio.c | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 955947e..1c9c5f0 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -830,16 +830,20 @@ void exit_aio(struct mm_struct *mm)
>   static void put_reqs_available(struct kioctx *ctx, unsigned nr)
>   {
>   	struct kioctx_cpu *kcpu;
> +	unsigned long flags;
>
>   	preempt_disable();
>   	kcpu = this_cpu_ptr(ctx->cpu);
>
> +	local_irq_save(flags);
>   	kcpu->reqs_available += nr;

Why not just makes this:

	local_irq_save(flags);
	kcpu = this_put_ptr(...);
	...

and get rid of the preemption bits, it's a bit redundant now you need to 
kill local interrupts anyway.

-- 
Jens Axboe

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