Hi I'm submitting this patch for the CONFIG_PREEMPT_RT patch. It fixes deadlocks in device mapper when real time preemption is used. Mikulas 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 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. 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); } -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
