Re: [PATCH 0/6] locking/rtmutex: Avoid PI state recursion through sched_submit_work()

[Peter Zijlstra]

On Wed, Aug 16, 2023 at 03:46:30PM +0200, Sebastian Andrzej Siewior wrote:
> On 2023-08-16 12:19:04 [+0200], To Peter Zijlstra wrote:
> > On 2023-08-16 11:42:57 [+0200], Peter Zijlstra wrote:
> > > Not the same -- this is namespace_lock(), right? That's a regular rwsem
> > > afaict and that *should* be good. Clearly I messed something up.
> > 
> > Most likely. I do see it also fom inode_lock() which does down_write().
> > I see it only to originate from rwbase_write_lock().
> 
> I've been looking at what you did and what we had.
> I'm not sure if your additional debug/assert code figured it out or me
> looking at it, but in rwbase_write_lock() for down_write(), we had this
> beauty with a comment that you made go away:
> 
> |        * Take the rtmutex as a first step. For rwsem this will also
> |        * invoke sched_submit_work() to flush IO and workers.
> |	 */
> |         if (rwbase_rtmutex_lock_state(rtm, state))
> 

Yeah, I can't quite remember why I killed that comment, I think because
it was either 'obvious' or confusing at the time. Or perhaps I was too
lazy to type, ... :-)

> for rw_semaphore we don't have any explicit rwbase_sched_submit_work()
> but relied on this one. Now that I look at it again,
> rwbase_rtmutex_lock_state() can succeed in the fast path so we don't
> flush/ invoke rwbase_pre_schedule().
> So you rightfully removed the comment as it was misleading but we do
> need that rwbase_pre_schedule() thingy before
> raw_spin_lock_irqsave(&rtm->wait_lock).

Right, it's both the fast-path and the fact that rt_mutex_slowlock()
will also do post_schedule() and reset the flag.

I've ended up with the below, but it is quite horrible.. but let me go
stare at the futex wreckage before trying to clean things up.

--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -270,6 +270,7 @@ static int __sched rwbase_write_lock(str
 
 out_unlock:
 	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
+	rt_mutex_post_schedule();
 	return 0;
 }
 
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1412,8 +1412,30 @@ static inline void __downgrade_write(str
 #define rwbase_restore_current_state()			\
 	__set_current_state(TASK_RUNNING)
 
-#define rwbase_rtmutex_lock_state(rtm, state)		\
-	__rt_mutex_lock(rtm, state)
+/*
+ * Variant of __rt_mutex_lock() that unconditionally does
+ * rt_mutex_pre_schedule() and keeps it on success.
+ */
+static __always_inline int
+rwbase_rtmutex_lock_state(struct rt_mutex_base *lock, unsigned int state)
+{
+	unsigned long flags;
+	int ret;
+
+	rt_mutex_pre_schedule();
+
+	if (likely(rt_mutex_try_acquire(lock)))
+		return 0;
+
+	raw_spin_lock_irqsave(&lock->wait_lock, flags);
+	ret = __rt_mutex_slowlock_locked(lock, NULL, state);
+	raw_spin_unlock_irqrestore(&lock->wait_lock, flags);
+
+	if (ret)
+		rt_mutex_post_schedule();
+
+	return ret;
+}
 
 #define rwbase_rtmutex_slowlock_locked(rtm, state)	\
 	__rt_mutex_slowlock_locked(rtm, NULL, state)