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SubjectRE: [patch] aio: fix buggy put_ioctx call in aio_complete
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jmoyer@redhat.com wrote on Thursday, December 21, 2006 9:35 AM
> kenneth.w.chen> Take ioctx_lock is one part, the other part is to move
> kenneth.w.chen> spin_unlock_irqrestore(&ctx->ctx_lock, flags);
> kenneth.w.chen> in aio_complete all the way down to the end of the
> kenneth.w.chen> function, after wakeup and put_ioctx. But then the ref
> kenneth.w.chen> counting on ioctx in aio_complete path is Meaningless,
> kenneth.w.chen> which is the thing I'm trying to remove.
>
> OK, right. But are we simply papering over the real problem? Earlier in
> this thread, you stated:
>
> > flush_workqueue() is not allowed to be called in the softirq context.
> > However, aio_complete() called from I/O interrupt can potentially call
> > put_ioctx with last ref count on ioctx and trigger a bug warning. It
> > is simply incorrect to perform ioctx freeing from aio_complete.
>
> But how do we end up with the last reference to the ioctx in the aio
> completion path? That's a question I haven't seen answered.


It is explained in this posting, A race between io_destroy and aio_complete:
http://groups.google.com/group/linux.kernel/msg/d2ba7d73aca1dd0c?&hl=en

Trond spotted a bug in that posting. The correct way of locking is to
move the spin_unlock_irqrestore in aio_complete all the way down instead
of calling aio_put_req at the end. Like this:


--- ./fs/aio.c.orig 2006-12-21 08:08:14.000000000 -0800
+++ ./fs/aio.c 2006-12-21 08:14:27.000000000 -0800
@@ -298,17 +298,23 @@ static void wait_for_all_aios(struct kio
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);

+ spin_lock_irq(&ctx->ctx_lock);
if (!ctx->reqs_active)
- return;
+ goto out;

add_wait_queue(&ctx->wait, &wait);
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
while (ctx->reqs_active) {
+ spin_unlock_irq(&ctx->ctx_lock);
schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ spin_lock_irq(&ctx->ctx_lock);
}
__set_task_state(tsk, TASK_RUNNING);
remove_wait_queue(&ctx->wait, &wait);
+
+out:
+ spin_unlock_irq(&ctx->ctx_lock);
}

/* wait_on_sync_kiocb:
@@ -424,7 +430,6 @@ static struct kiocb fastcall *__aio_get_
ring = kmap_atomic(ctx->ring_info.ring_pages[0], KM_USER0);
if (ctx->reqs_active < aio_ring_avail(&ctx->ring_info, ring)) {
list_add(&req->ki_list, &ctx->active_reqs);
- get_ioctx(ctx);
ctx->reqs_active++;
okay = 1;
}
@@ -536,8 +541,6 @@ int fastcall aio_put_req(struct kiocb *r
spin_lock_irq(&ctx->ctx_lock);
ret = __aio_put_req(ctx, req);
spin_unlock_irq(&ctx->ctx_lock);
- if (ret)
- put_ioctx(ctx);
return ret;
}

@@ -782,8 +785,7 @@ static int __aio_run_iocbs(struct kioctx
*/
iocb->ki_users++; /* grab extra reference */
aio_run_iocb(iocb);
- if (__aio_put_req(ctx, iocb)) /* drop extra ref */
- put_ioctx(ctx);
+ __aio_put_req(ctx, iocb);
}
if (!list_empty(&ctx->run_list))
return 1;
@@ -998,14 +1000,10 @@ put_rq:
/* everything turned out well, dispose of the aiocb. */
ret = __aio_put_req(ctx, iocb);

- spin_unlock_irqrestore(&ctx->ctx_lock, flags);
-
if (waitqueue_active(&ctx->wait))
wake_up(&ctx->wait);

- if (ret)
- put_ioctx(ctx);
-
+ spin_unlock_irqrestore(&ctx->ctx_lock, flags);
return ret;
}

-
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