Chen, Kenneth wrote on Thursday, December 14, 2006 5:59 PM
> > It seems utterly insane to have aio_complete() flush a workqueue. That
> > function has to be called from a number of different environments,
> > including non-sleep tolerant environments.
> >
> > For instance it means that directIO on NFS will now cause the rpciod
> > workqueues to call flush_workqueue(aio_wq), thus slowing down all RPC
> > activity.
>
> The bug appears to be somewhere else, somehow the ref count on ioctx is
> all messed up.
>
> In aio_complete, __put_ioctx() should not be invoked because ref count
> on ioctx is supposedly more than 2, aio_complete decrement it once and
> should return without invoking the free function.
>
> The real freeing ioctx should be coming from exit_aio() or io_destroy(),
> in which case both wait until no further pending AIO request via
> wait_for_all_aios().
Ah, I think I see the bug: it must be a race between io_destroy() and
aio_complete(). A possible scenario:
cpu0 cpu1
io_destroy aio_complete
wait_for_all_aios { __aio_put_req
... ctx->reqs_active--;
if (!ctx->reqs_active)
return;
}
...
put_ioctx(ioctx)
put_ioctx(ctx);
bam! Bug trigger!
AIO finished on cpu1 and while in the middle of aio_complete, cpu0 starts
io_destroy sequence, sees no pending AIO, went ahead decrement the ref
count on ioctx. At a later point in aio_complete, the put_ioctx decrement
last ref count and calls the ioctx freeing function and there it triggered
the bug warning.
A simple fix would be to access ctx->reqs_active inside ctx spin lock in wait_for_all_aios(). At the mean time, I would like to
remove ref counting
for each iocb because we already performing ref count using reqs_active. This
would also prevent similar buggy code in the future.
Signed-off-by: Ken Chen <[email protected]>
--- ./fs/aio.c.orig 2006-11-29 13:57:37.000000000 -0800
+++ ./fs/aio.c 2006-12-14 20:45:14.000000000 -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:
@@ -425,7 +431,6 @@ 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;
}
@@ -538,8 +543,6 @@ 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;
}
@@ -795,8 +798,7 @@ 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;
@@ -942,7 +944,6 @@ int fastcall aio_complete(struct kiocb *
struct io_event *event;
unsigned long flags;
unsigned long tail;
- int ret;
/*
* Special case handling for sync iocbs:
@@ -1011,18 +1012,12 @@ int fastcall aio_complete(struct kiocb *
pr_debug("%ld retries: %zd of %zd\n", iocb->ki_retried,
iocb->ki_nbytes - iocb->ki_left, iocb->ki_nbytes);
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);
-
- return ret;
+ return aio_put_req(iocb);
}
/* aio_read_evt
-
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