Re: [PATCH] rtmutex: Add acquire semantics for rtmutex lock acquisition

[Mel Gorman]

On Fri, Dec 02, 2022 at 12:21:06PM +0100, Sebastian Andrzej Siewior wrote:
> On 2022-12-02 10:02:23 [+0000], Mel Gorman wrote:
> > The lock owner is updated with an IRQ-safe raw spinlock held but the
> > spin_unlock does not provide acquire semantics which are needed when
> > acquiring a mutex. This patch adds the necessary acquire semantics for a
> > lock operation when the lock owner is updated. It successfully completed
> > 10 iterations of the dbench workload while the vanilla kernel fails on
> > the first iteration.
> 
> I *think* it is
> 
> Fixes: 700318d1d7b38 ("locking/rtmutex: Use acquire/release semantics")
> 

Adding Davidlohr to cc.

It might have made the problem worse but even then rt_mutex_set_owner was
just a plain assignment and while I didn't check carefully, at a glance
try_to_take_rt_mutex didn't look like it guaranteed ACQUIRE semantics.

> Before that, it did cmpxchg() which should be fine.
> 
> Regarding mark_rt_mutex_waiters(). Isn't acquire semantic required in
> order for the lock-owner not perform the fastpath but go to the slowpath
> instead?
> 

Good spot, it does. While the most straight-forward solution is to use
cmpxchg_acquire, I think it is overkill because it could incur back-to-back
ACQUIRE operations in the event of contention. There could be a smp_wmb
after the cmpxchg_relaxed but that impacts all arches and a non-paired
smp_wmb is generally frowned upon.

I'm thinking this on top of the patch should be sufficient even though
it's a heavier operation than is necesary for ACQUIRE as well as being
"not typical" according to Documentation/atomic_t.txt. Will, as this
affects ARM primarily do you have any preference?

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 35212f260148..af0dbe4d5e97 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -238,6 +238,13 @@ static __always_inline void mark_rt_mutex_waiters(struct rt_mutex_base *lock)
 		owner = *p;
 	} while (cmpxchg_relaxed(p, owner,
 				 owner | RT_MUTEX_HAS_WAITERS) != owner);
+
+	/*
+	 * The cmpxchg loop above is relaxed to avoid back-to-back ACQUIRE
+	 * operations in the event of contention. Ensure the successful
+	 * cmpxchg is visible.
+	 */
+	smp_mb__after_atomic();
 }
 
 /*