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


I'm submitting this patch for the CONFIG_PREEMPT_RT patch. It fixes 
deadlocks in device mapper when real time preemption is used.


From: Mikulas Patocka <mpatocka@xxxxxxxxxx>

When some block device driver creates a bio and submits it to another 
block device driver, the bio is added to current->bio_list (in order to 
avoid unbounded recursion).

However, this queuing of bios can cause deadlocks, in order to avoid them, 
device mapper registers a function flush_current_bio_list. This function 
is called when device mapper driver blocks. It redirects bios queued on 
current->bio_list to helper workqueues, so that these bios can proceed 
even if the driver is blocked.

The problem with CONFIG_PREEMPT_RT_FULL is that when the device mapper
driver blocks, it won't call flush_current_bio_list (because
tsk_is_pi_blocked returns true in sched_submit_work), so deadlocks in
block device stack can happen.

Note that we can't call blk_schedule_flush_plug if tsk_is_pi_blocked
returns true - that would cause
BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)) in
task_blocks_on_rt_mutex when flush_current_bio_list attempts to take a

So the proper fix is to call blk_schedule_flush_plug in rt_mutex_fastlock,
when fast acquire failed and when the task is about to block.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>

 kernel/locking/rtmutex.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

Index: linux-stable/kernel/locking/rtmutex.c
--- linux-stable.orig/kernel/locking/rtmutex.c
+++ linux-stable/kernel/locking/rtmutex.c
@@ -24,6 +24,7 @@
 #include <linux/sched/debug.h>
 #include <linux/timer.h>
 #include <linux/ww_mutex.h>
+#include <linux/blkdev.h>
 #include "rtmutex_common.h"
@@ -1939,6 +1940,15 @@ rt_mutex_fastlock(struct rt_mutex *lock,
 	if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
 		return 0;
+	/*
+	 * If rt_mutex blocks, the function sched_submit_work will not call
+	 * blk_schedule_flush_plug (because tsk_is_pi_blocked would be true).
+	 * We must call blk_schedule_flush_plug here, if we don't call it,
+	 * a deadlock in device mapper may happen.
+	 */
+	if (unlikely(blk_needs_flush_plug(current)))
+		blk_schedule_flush_plug(current);
 	return slowfn(lock, state, NULL, RT_MUTEX_MIN_CHAINWALK, ww_ctx);
@@ -1956,6 +1966,9 @@ rt_mutex_timed_fastlock(struct rt_mutex
 	    likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
 		return 0;
+	if (unlikely(blk_needs_flush_plug(current)))
+		blk_schedule_flush_plug(current);
 	return slowfn(lock, state, timeout, chwalk, ww_ctx);