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

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Message-ID: <20221216103141.6qxxmlzuqznp54qu@techsingularity.net>
Date:   Fri, 16 Dec 2022 10:31:41 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>, Jan Kara <jack@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Pierre Gondois <pierre.gondois@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Davidlohr Bueso <dave@...olabs.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-RT <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH] rtmutex: Add acquire semantics for rtmutex lock
 acquisition

On Tue, Dec 06, 2022 at 12:43:51PM +0100, Sebastian Andrzej Siewior wrote:
> > > 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.
> 
> but in general, it should succeed on the first iteration. It can only
> fail (and retry) if the owner was able to unlock it first. A second
> locker will spin on the wait_lock so.
> 

Sure, generally it would be fine but it also costs us nothing
to avoid additional overhead in the contended case. The pattern of
atomic_relaxed+smp_mb__after_atomic is unusual but I think the comment is
sufficient to explain why it's structured like that.

-- 
Mel Gorman
SUSE Labs