3.18.91-rt98-rc1 stable review patch. If anyone has any objections, please let me know.
------------------ From: Mikulas Patocka <[email protected]> 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]> --- kernel/locking/rtmutex.c | 25 ++++++++++++++++++++----- 1 file changed, 20 insertions(+), 5 deletions(-) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index da35f7ef5324..d58780b59583 100644 --- 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" @@ -1843,9 +1844,19 @@ rt_mutex_fastlock(struct rt_mutex *lock, int state, if (likely(rt_mutex_cmpxchg(lock, NULL, current))) { rt_mutex_deadlock_account_lock(lock, current); return 0; - } else - return slowfn(lock, state, NULL, RT_MUTEX_MIN_CHAINWALK, - ww_ctx); + } + + /* + * 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); } static inline int @@ -1862,8 +1873,12 @@ rt_mutex_timed_fastlock(struct rt_mutex *lock, int state, likely(rt_mutex_cmpxchg(lock, NULL, current))) { rt_mutex_deadlock_account_lock(lock, current); return 0; - } else - return slowfn(lock, state, timeout, chwalk, ww_ctx); + } + + if (unlikely(blk_needs_flush_plug(current))) + blk_schedule_flush_plug(current); + + return slowfn(lock, state, timeout, chwalk, ww_ctx); } static inline int -- 2.13.2
