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SubjectRe: [PATCH PREEMPT RT] rt-mutex: fix deadlock in device mapper
On 2017-11-18 19:37:10 [+0100], Mike Galbraith wrote:
> Below is my 2012 3.0-rt version of that for reference; at that time we
> were using slab, and slab_lock referenced below was a local_lock.  The
> comment came from crash analysis of a deadlock I met before adding the
> (yeah, hacky) __migrate_disabled() blocker.  At the time, that was not
> an optional hack, you WOULD deadlock if you ground disks to fine powder
> the way SUSE QA did.  Pulling the plug before blocking cured the
> xfs/ext[34] IO deadlocks they griped about, but I had to add that hack
> to not trade their nasty IO deadlocks for the more mundane variety.  So
> my question is: are we sure that in the here and now flush won't want
> any lock we may be holding?  In days of yore, it most definitely would
> turn your box into a doorstop if you tried hard enough.

I have a deadlock in ftest01/LTP which is cured by that.
The root-problem (as I understand it) is that !RT does
schedule() -> sched_submit_work() -> blk_schedule_flush_plug()

on a lock contention (that is that bit_spinlock or rwsem in jbd2/ext4
for instance). On RT this does not happen because of tsk_is_pi_blocked()
check in sched_submit_work(). That check is needed because an additional
(rtmutex) lock can not be acquired at this point.

With this change we get closer to what !RT does. In regard to that
migrate_disable() we could check if it is possible to do that after we
acquired the lock (which is something we tried before but failed due to
CPU-hotplug requirement, maybe something changed now) or flush the plug
before disabling migration if it is really a problem.

To your question whether or not delaying IO can cause any deadlocks is
something that I can't answer and this something that would affect !RT,
too. I tried to add lockdep to bit-spinlocks but this does not work
because one context acquires the lock and another does the unlock. It
has been explained to me that no deadlocks should happen as long as
the IO is flushed before we block/wait on a lock.

> Subject: rt: pull your plug before blocking
> Date: Wed Jul 18 14:43:15 CEST 2012
> From: Mike Galbraith <efault@gmx.de>
>
> Queued IO can lead to IO deadlock should a task require wakeup from a task
> which is blocked on that queued IO.
>
> ext3: dbench1 queues a buffer, blocks on journal mutex, it's plug is not
> pulled. dbench2 mutex owner is waiting for kjournald, who is waiting for
> the buffer queued by dbench1. Game over.
>
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> ---
> kernel/locking/rtmutex.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> --- 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"
>
> @@ -930,8 +931,18 @@ static inline void rt_spin_lock_fastlock
>
> if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
> rt_mutex_deadlock_account_lock(lock, current);
> - else
> + else {
> + /*
> + * We can't pull the plug if we're already holding a lock
> + * else we can deadlock. eg, if we're holding slab_lock,
> + * ksoftirqd can block while processing BLOCK_SOFTIRQ after
> + * having acquired q->queue_lock. If _we_ then block on
> + * that q->queue_lock while flushing our plug, deadlock.
> + */
> + if (__migrate_disabled(current) < 2 && blk_needs_flush_plug(current))
> + blk_schedule_flush_plug(current);
> slowfn(lock);
> + }
> }
>
> static inline void rt_spin_lock_fastunlock(struct rt_mutex *lock,
> @@ -1892,9 +1903,12 @@ rt_mutex_fastlock(struct rt_mutex *lock,
> if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current))) {
> rt_mutex_deadlock_account_lock(lock, current);
> return 0;
> - } else
> + } else {
> + if (blk_needs_flush_plug(current))
> + blk_schedule_flush_plug(current);
> return slowfn(lock, state, NULL, RT_MUTEX_MIN_CHAINWALK,
> ww_ctx);
> + }
> }
>
> static inline int

Sebastian

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