RE: 2.6.18.4: flush_workqueue calls mutex_lock in interrupt environment

[Chen, Kenneth W]

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 <kenneth.w.chen@xxxxxxxxx>

--- ./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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