Re: Deadlocks due to per-process plugging

[Mike Galbraith <efault@gmx.de>]

On Mon, 2012-07-16 at 12:19 +0200, Thomas Gleixner wrote:

> > @@ -647,8 +648,11 @@ static inline void rt_spin_lock_fastlock
> >  
> >  	if (likely(rt_mutex_cmpxchg(lock, NULL, current)))
> >  		rt_mutex_deadlock_account_lock(lock, current);
> > -	else
> > +	else {
> > +		if (blk_needs_flush_plug(current))
> > +			blk_schedule_flush_plug(current);
> >  		slowfn(lock);
> > +	}
> 
> That should do the trick.

Box has been grinding away long enough now to agree that it did.

rt: pull your plug before blocking

Queued IO can lead to IO deadlock should a task require wakeup from as task
which is blocked on that queued IO.

ext3: dbench1 queues a buffer, blocks on journal mutex, it's plug is not
pulled.  dbench2 mutex owner is waiting for kjournald, who is waiting for
the buffer queued by dbench1.  Game over.

Signed-off-by: Mike Galbraith <efault@gmx.de>

diff --git a/kernel/rtmutex.c b/kernel/rtmutex.c
index d58db99..39140a5 100644
--- a/kernel/rtmutex.c
+++ b/kernel/rtmutex.c
@@ -20,6 +20,7 @@
 #include <linux/module.h>
 #include <linux/sched.h>
 #include <linux/timer.h>
+#include <linux/blkdev.h>
 
 #include "rtmutex_common.h"
 
@@ -647,8 +648,11 @@ static inline void rt_spin_lock_fastlock(struct rt_mutex *lock,
 
 	if (likely(rt_mutex_cmpxchg(lock, NULL, current)))
 		rt_mutex_deadlock_account_lock(lock, current);
-	else
+	else {
+		if (blk_needs_flush_plug(current))
+			blk_schedule_flush_plug(current);
 		slowfn(lock);
+	}
 }
 
 static inline void rt_spin_lock_fastunlock(struct rt_mutex *lock,
@@ -1104,8 +1108,11 @@ rt_mutex_fastlock(struct rt_mutex *lock, int state,
 	if (!detect_deadlock && likely(rt_mutex_cmpxchg(lock, NULL, current))) {
 		rt_mutex_deadlock_account_lock(lock, current);
 		return 0;
-	} else
+	} else {
+		if (blk_needs_flush_plug(current))
+			blk_schedule_flush_plug(current);
 		return slowfn(lock, state, NULL, detect_deadlock);
+	}
 }
 
 static inline int