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

From: Jens Axboe
Date: Mon Jul 14 2014 - 13:30:57 EST

On 2014-07-14 19:12, Benjamin LaHaise wrote:
Hello everyone,

Please pull the following commit (263782c1c95bbddbb022dc092fd89a36bb8d5577)
from git:// to fix an aio bug reported by Robert
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@xxxxxx>
Tested-by: Robert Elliot <Elliott@xxxxxx>
Signed-off-by: Benjamin LaHaise <bcrl@xxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>, Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxx
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;

kcpu = this_cpu_ptr(ctx->cpu);

+ local_irq_save(flags);
kcpu->reqs_available += nr;

Why not just makes this:

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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