[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 <elli...@hp.com>
Tested-by: Robert Elliot <elli...@hp.com>
Signed-off-by: Benjamin LaHaise <b...@kvack.org>
Cc: Jens Axboe <ax...@kernel.dk>, Christoph Hellwig <h...@infradead.org>
Cc: sta...@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

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