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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>, boqun.feng@gmail.com
Cc:     linux-kernel@vger.kernel.org, peterz@infradead.org,
        Ingo Molnar <mingo@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Will Deacon <will@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Mike Galbraith <efault@gmx.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
Subject: Re: [PATCH 3/4] locking/rwbase: Fix rwbase_write_lock() vs
 __rwbase_read_lock()
In-Reply-To: <20210909110203.893845303@infradead.org>
References: <20210909105915.757320973@infradead.org>
 <20210909110203.893845303@infradead.org>
Date:   Tue, 14 Sep 2021 09:45:12 +0200
Message-ID: <87k0jjeh2v.ffs@tglx>
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On Thu, Sep 09 2021 at 12:59, Peter Zijlstra wrote:

> Boqun noticed that the write-trylock sequence of load+set is broken in
> rwbase_write_lock()'s wait-loop since they're not both under the same
> wait_lock instance.

Confused.

lock(); A

for (; atomic_read(readers);) {
   ...
   unlock();
   ..
   lock(); B
}

atomic_set();
unlock(); A or B

The read/set is always in the same lock instance.

> Restructure the code to make this more obvious and correct.

I agree that it's easier to read, but I disagree that it makes the code
more correct.

Thanks,

        tglx