On Thu, 2012-12-20 at 13:02 -0500, Steven Rostedt wrote:
> On Wed, 2012-12-19 at 20:31 -0500, Steven Rostedt wrote:
>
> Talking with Thomas, the proper solution is:
>
> > Then we have this lovely code:
> >
> > block/blk-ioc.c: put_io_context_active()
> >
> > retry:
> > spin_lock_irqsave_nested(&ioc->lock, flags, 1);
> > hlist_for_each_entry(icq, n, &ioc->icq_list, ioc_node) {
> > if (icq->flags & ICQ_EXITED)
> > continue;
> > if (spin_trylock(icq->q->queue_lock)) {
> > ioc_exit_icq(icq);
> > spin_unlock(icq->q->queue_lock);
> > } else {
> > spin_unlock_irqrestore(&ioc->lock, flags);
> > cpu_relax();
>
> s/cpu_relax/cpu_chill/
>
> -- Steve
>
> > goto retry;
> > }
> > }
> >
Thomas,
As you hate the cpu_chill() as all it does is to do a msleep, what about
adding a cpu_chill_on_lock(), that, instead of doing a sleep, grabs and
releases the lock it wants. This is only safe for those locations that
are just trying to do a reverse lock grab:
-- Steve
diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index fab4cdd..263e021 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -110,7 +110,7 @@ static void ioc_release_fn(struct work_struct *work)
spin_unlock(q->queue_lock);
} else {
spin_unlock_irqrestore(&ioc->lock, flags);
- cpu_relax();
+ cpu_chill_on_lock(ioc->icq_list);
spin_lock_irqsave_nested(&ioc->lock, flags, 1);
}
}
@@ -188,7 +188,7 @@ retry:
spin_unlock(icq->q->queue_lock);
} else {
spin_unlock_irqrestore(&ioc->lock, flags);
- cpu_relax();
+ cpu_chill_on_lock(ioc->q->queue_lock);
goto retry;
}
}
diff --git a/include/linux/delay.h b/include/linux/delay.h
index e23a7c0..40d8db8 100644
--- a/include/linux/delay.h
+++ b/include/linux/delay.h
@@ -54,8 +54,20 @@ static inline void ssleep(unsigned int seconds)
#ifdef CONFIG_PREEMPT_RT_FULL
# define cpu_chill() msleep(1)
+/*
+ * Use cpu_chill_on_lock() when trying to grab a lock in
+ * reverse order, and you have already released all locks
+ * that may cause a deadlock with the given lock.
+ */
+# define cpu_chill_on_lock(lock) \
+ do { \
+ spin_lock(&(lock)); \
+ spin_unlock(&(lock)); \
+ cpu_relax(); \
+ } while (0)
#else
-# define cpu_chill() cpu_relax()
+# define cpu_chill() cpu_relax()
+# define cpu_chill_on_lock(lock) cpu_relax()
#endif
#endif /* defined(_LINUX_DELAY_H) */
--
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