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

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Date:   Fri, 2 Dec 2022 12:21:06 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Mel Gorman <mgorman@...hsingularity.net>
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>,
        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 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")

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?

Sebastian