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aio: protect reqs_available updates from changes in interrupt handlers

PATCH In review Benjamin LaHaise 2 participants 3 messages Jul 14, 2014

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Stable branch VFS Aio
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Benjamin LaHaise

author maintainer
#0 lore raw

PATCH 1/1 aio: protect reqs_available updates from changes in interrupt handlers

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 <[email protected]>
Tested-by Robert Elliot <[email protected]>
Signed-off-by Benjamin LaHaise <[email protected]>
Cc Jens Axboe <[email protected]>, Christoph Hellwig <[email protected]>
Cc [email protected]
---
 fs/aio.c | 7 +++++++
 1 file changed, 7 insertions(+)
fs/aio.c +7 -0
index 955947e..1c9c5f0 100644
@@ -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;
 }
Signature · 4 lines
1.8.2.1

-- 
"Thought is the essence of where you are now."
#1 lore raw
Quoted 29 lines · 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 <[email protected]>
Tested-by Robert Elliot <[email protected]>
Signed-off-by Benjamin LaHaise <[email protected]>
Cc Jens Axboe <[email protected]>, Christoph Hellwig <[email protected]>
Cc [email protected]
---
  fs/aio.c | 7 +++++++
  1 file changed, 7 insertions(+)
fs/aio.c +0 -0
index 955947e..1c9c5f0 100644

fs/aio.c void exit_aio(struct mm_struct *mm) · line 839

Quoted 10 lines
fs/aio.c +2 -0
@@ -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.

Signature · 1 line
Jens Axboe
author maintainer
#2 lore raw
Quoted 8 lines · Jens Axboe

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

fs/aio.c +2 -8
index 1c9c5f0..104da62 100644
@@ -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

Signature · 1 line
"Thought is the essence of where you are now."