linux-kernel - Re: [PATCH 4/5] locking/rwsem: Use acquire/release semantics

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Message-ID: <CA+55aFyVyKKNSSZJ6qZLA+RjMjtH1K9MZK+GRqUwy5CyC36xcQ@mail.gmail.com>
Date:	Mon, 21 Sep 2015 13:39:48 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 4/5] locking/rwsem: Use acquire/release semantics

On Mon, Sep 21, 2015 at 1:17 PM, Davidlohr Bueso <dave@...olabs.net> wrote:
> @@ -114,7 +114,7 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
>  {
>         long tmp;
>
> -       tmp = atomic_long_add_return(-RWSEM_WAITING_BIAS,
> +       tmp = atomic_long_add_return_acquire(-RWSEM_WAITING_BIAS,
>                                      (atomic_long_t *)&sem->count);
>         if (tmp < 0)
>                 rwsem_downgrade_wake(sem);

Careful. I'm pretty sure this is wrong.

When we downgrade exclusive ownership to non-exclusive, that should be
a *release* operation. Anything we did inside the write-locked region
had damn better _stay_ inside the write-locked region, we can not
allow it to escape down into the read-locked side. So it needs to be
at least a release.

In contrast, anything that we do in the read-locked part is fine to be
re-ordered into the write-locked exclusive part, so it does *not* need
acquire ordering (the original write locking obviously did use
acquire, and acts as a barrier for everything that comes in the locked
region).

I tried to look through everything, and I think this is the only thing
you got wrong, but I'd like somebody to double-checks. Getting the
acquire/release semantics wrong will cause some really really subtle
and hard-as-hell-to-find bugs.  So let's be careful out there, ok?

                Linus
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