Re: [PATCH PREEMPT RT] rt-mutex: fix deadlock in device mapper

[Mike Galbraith <efault@gmx.de>]

On Fri, 2017-11-17 at 15:57 +0100, Sebastian Siewior wrote:
> On 2017-11-13 12:56:53 [-0500], Mikulas Patocka wrote:
> > Hi
> Hi,
> 
> > I'm submitting this patch for the CONFIG_PREEMPT_RT patch. It fixes 
> > deadlocks in device mapper when real time preemption is used.
> 
> applied, thank you.

Below is my 2012 3.0-rt version of that for reference; at that time we
were using slab, and slab_lock referenced below was a local_lock.  The
comment came from crash analysis of a deadlock I met before adding the
(yeah, hacky) __migrate_disabled() blocker.  At the time, that was not
an optional hack, you WOULD deadlock if you ground disks to fine powder
the way SUSE QA did.  Pulling the plug before blocking cured the
xfs/ext[34] IO deadlocks they griped about, but I had to add that hack
to not trade their nasty IO deadlocks for the more mundane variety.  So
my question is: are we sure that in the here and now flush won't want
any lock we may be holding?  In days of yore, it most definitely would
turn your box into a doorstop if you tried hard enough.

Subject: rt: pull your plug before blocking
Date: Wed Jul 18 14:43:15 CEST 2012
From: Mike Galbraith <efault@gmx.de>

Queued IO can lead to IO deadlock should a task require wakeup from a 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>
---
 kernel/locking/rtmutex.c |   18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -22,6 +22,7 @@
 #include <linux/sched/deadline.h>
 #include <linux/timer.h>
 #include <linux/ww_mutex.h>
+#include <linux/blkdev.h>
 
 #include "rtmutex_common.h"
 
@@ -930,8 +931,18 @@ static inline void rt_spin_lock_fastlock
 
 	if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
 		rt_mutex_deadlock_account_lock(lock, current);
-	else
+	else {
+		/*
+		 * We can't pull the plug if we're already holding a lock
+		 * else we can deadlock.  eg, if we're holding slab_lock,
+		 * ksoftirqd can block while processing BLOCK_SOFTIRQ after
+		 * having acquired q->queue_lock.  If _we_ then block on
+		 * that q->queue_lock while flushing our plug, deadlock.
+		 */
+		if (__migrate_disabled(current) < 2 && blk_needs_flush_plug(current))
+			blk_schedule_flush_plug(current);
 		slowfn(lock);
+	}
 }
 
 static inline void rt_spin_lock_fastunlock(struct rt_mutex *lock,
@@ -1892,9 +1903,12 @@ rt_mutex_fastlock(struct rt_mutex *lock,
 	if (likely(rt_mutex_cmpxchg_acquire(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, RT_MUTEX_MIN_CHAINWALK,
 			      ww_ctx);
+	}
 }
 
 static inline int