4.9.61-rt52-rc2 stable review patch. If anyone has any objections, please let me know. ------------------ From: Mikulas Patocka 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: stable-rt@vger.kernel.org [bigeasy: The deadlock is not device-mapper specific, it can also occur in plain EXT4] Signed-off-by: Mikulas Patocka Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Steven Rostedt (VMware) --- kernel/locking/rtmutex.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 78a6c4a223c1..b73cd7c87551 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -22,6 +22,7 @@ #include #include #include +#include #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); } -- 2.13.2