Re: Deadlocks due to per-process plugging

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, 13 Jul 2012, Jan Kara wrote:
> On Thu 12-07-12 16:15:29, Thomas Gleixner wrote:
> > >   Ah, I didn't know this. Thanks for the hint. So in the kdump I have I can
> > > see requests queued in tsk->plug despite the process is sleeping in
> > > TASK_UNINTERRUPTIBLE state.  So the only way how unplug could have been
> > > omitted is if tsk_is_pi_blocked() was true. Rummaging through the dump...
> > > indeed task has pi_blocked_on = 0xffff8802717d79c8. The dump is from an -rt
> > > kernel (I just didn't originally thought that makes any difference) so
> > > actually any mutex is rtmutex and thus tsk_is_pi_blocked() is true whenever
> > > we are sleeping on a mutex. So this seems like a bug in rtmutex code.
> > 
> > Well, the reason why this check is there is that the task which is
> > blocked on a lock can hold another lock which might cause a deadlock
> > in the flush path.
>   OK. Let me understand the details. Block layer needs just queue_lock for
> unplug to succeed. That is a spinlock but in RT kernel, even a process
> holding a spinlock can be preempted if I remember correctly. So that
> condition is there effectively to not unplug when a task is being scheduled
> away while holding queue_lock? Did I get it right?

blk_flush_plug_list() is not only queue_lock. There can be other locks
taken in the callbacks, elevator ...

> > > Thomas, you seemed to have added that condition... Any idea how to avoid
> > > the deadlock?
> > 
> > Good question. We could do the flush when the blocked task does not
> > hold a lock itself. Might be worth a try.
>   Yeah, that should work for avoiding the deadlock as well.

Though we don't have a lock held count except when lockdep is enabled,
which you probably don't want to do when running a production system.

But we only care about stuff being scheduled out while blocked on a
"sleeping spinlock" - i.e. spinlock, rwlock.

So the patch below should allow the unplug to take place when blocked
on mutexes etc.

Thanks,

	tglx
----
Index: linux-stable-rt/include/linux/sched.h
===================================================================
--- linux-stable-rt.orig/include/linux/sched.h
+++ linux-stable-rt/include/linux/sched.h
@@ -2145,9 +2145,10 @@ extern unsigned int sysctl_sched_cfs_ban
 extern int rt_mutex_getprio(struct task_struct *p);
 extern void rt_mutex_setprio(struct task_struct *p, int prio);
 extern void rt_mutex_adjust_pi(struct task_struct *p);
+extern bool pi_blocked_on_rt_lock(struct task_struct *tsk);
 static inline bool tsk_is_pi_blocked(struct task_struct *tsk)
 {
-	return tsk->pi_blocked_on != NULL;
+	return tsk->pi_blocked_on != NULL && pi_blocked_on_rt_lock(tsk);
 }
 #else
 static inline int rt_mutex_getprio(struct task_struct *p)
Index: linux-stable-rt/kernel/rtmutex.c
===================================================================
--- linux-stable-rt.orig/kernel/rtmutex.c
+++ linux-stable-rt/kernel/rtmutex.c
@@ -699,6 +699,11 @@ static int adaptive_wait(struct rt_mutex
 # define pi_lock(lock)			raw_spin_lock_irq(lock)
 # define pi_unlock(lock)		raw_spin_unlock_irq(lock)
 
+bool pi_blocked_on_rt_lock(struct task_struct *tsk)
+{
+	return tsk->pi_blocked_on && tsk->pi_blocked_on->savestate;
+}
+
 /*
  * Slow path lock function spin_lock style: this variant is very
  * careful not to miss any non-lock wakeups.