linux-kernel - Re: [PATCH] rwsem: fix missed wakeup due to reordering of load

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Message-ID: <20170927212012.GA26599@linux-80c1.suse>
Date:   Wed, 27 Sep 2017 14:20:12 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Andrea Parri <parri.andrea@...il.com>
Cc:     Prateek Sood <prsood@...eaurora.org>, mingo@...nel.org,
        longman@...hat.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org, sramana@...eaurora.org
Subject: Re: [PATCH] rwsem: fix missed wakeup due to reordering of load

On Wed, 20 Sep 2017, Andrea Parri wrote:

>> Instead, how about just removing the release from atomic_long_sub_return_release()
>> such that the osq load is not hoisted over the atomic compound (along with Peter's
>> comment):
>
>This solution will actually enforce a stronger (full) ordering w.r.t. the
>solution described by Prateek and Peter. Also, it will "trade" two lwsync
>for two sync (powerpc), one dmb.ld for one dmb (arm64).
>
>What are the reasons you would prefer this?

It was mainly to maintain consistency about dealing with sem->count, but sure
I won't argue with the above.