locking/rt-mutex: fix deadlock in device mapper / block-IO

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
spinlock.

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.

CC: [email protected]
[bigeasy: The deadlock is not device-mapper specific, it can also occur
          in plain EXT4]
Signed-off-by: Mikulas Patocka <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

Author: Mikulas Patocka

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"
 
@@ -1968,6 +1969,15 @@ rt_mutex_fastlock(struct rt_mutex *lock, int state,
 	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);
 }
 
@@ -1985,6 +1995,9 @@ rt_mutex_timed_fastlock(struct rt_mutex *lock, int state,
 	    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);
 }