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

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

[Benjamin LaHaise]

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@hp.com>
Tested-by: Robert Elliot <Elliott@hp.com>
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Cc: Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@infradead.org>
Cc: stable@vger.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;
+
 	while (kcpu->reqs_available >= ctx->req_batch * 2) {
 		kcpu->reqs_available -= ctx->req_batch;
 		atomic_add(ctx->req_batch, &ctx->reqs_available);
 	}
 
+	local_irq_restore(flags);
 	preempt_enable();
 }
 
@@ -847,10 +851,12 @@ static bool get_reqs_available(struct kioctx *ctx)
 {
 	struct kioctx_cpu *kcpu;
 	bool ret = false;
+	unsigned long flags;
 
 	preempt_disable();
 	kcpu = this_cpu_ptr(ctx->cpu);
 
+	local_irq_save(flags);
 	if (!kcpu->reqs_available) {
 		int old, avail = atomic_read(&ctx->reqs_available);
 
@@ -869,6 +875,7 @@ static bool get_reqs_available(struct kioctx *ctx)
 	ret = true;
 	kcpu->reqs_available--;
 out:
+	local_irq_restore(flags);
 	preempt_enable();
 	return ret;
 }
-- 
1.8.2.1

-- 
"Thought is the essence of where you are now."

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

[Jens Axboe]

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@hp.com>
> Tested-by: Robert Elliot <Elliott@hp.com>
> Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
> Cc: Jens Axboe <axboe@kernel.dk>, Christoph Hellwig <hch@infradead.org>
> Cc: stable@vger.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

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

[Benjamin LaHaise]

On Mon, Jul 14, 2014 at 07:30:43PM +0200, Jens Axboe wrote:
> 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.

*nod*.  I'll add the following cleanup patch, as the first version already 
got merged into another tree.

		-ben

diff --git a/fs/aio.c b/fs/aio.c
index 1c9c5f0..104da62 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -832,10 +832,8 @@ 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 = this_cpu_ptr(ctx->cpu);
 	kcpu->reqs_available += nr;
 
 	while (kcpu->reqs_available >= ctx->req_batch * 2) {
@@ -844,7 +842,6 @@ static void put_reqs_available(struct kioctx *ctx, unsigned nr)
 	}
 
 	local_irq_restore(flags);
-	preempt_enable();
 }
 
 static bool get_reqs_available(struct kioctx *ctx)
@@ -853,10 +850,8 @@ static bool get_reqs_available(struct kioctx *ctx)
 	bool ret = false;
 	unsigned long flags;
 
-	preempt_disable();
-	kcpu = this_cpu_ptr(ctx->cpu);
-
 	local_irq_save(flags);
+	kcpu = this_cpu_ptr(ctx->cpu);
 	if (!kcpu->reqs_available) {
 		int old, avail = atomic_read(&ctx->reqs_available);
 
@@ -876,7 +871,6 @@ static bool get_reqs_available(struct kioctx *ctx)
 	kcpu->reqs_available--;
 out:
 	local_irq_restore(flags);
-	preempt_enable();
 	return ret;
 }
 

		-ben
-- 
"Thought is the essence of where you are now."