linux-kernel - Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read

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Message-ID: <20200918110310.GO1362448@hirez.programming.kicks-ass.net>
Date:   Fri, 18 Sep 2020 13:03:10 +0200
From:   peterz@...radead.org
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jan Kara <jack@...e.cz>, Boaz Harrosh <boaz@...xistor.com>,
        Hou Tao <houtao1@...wei.com>, Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will@...nel.org>, Dennis Zhou <dennis@...nel.org>,
        Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for
 read_count

On Fri, Sep 18, 2020 at 12:48:24PM +0200, Oleg Nesterov wrote:

> Of course, this assumes that atomic_t->counter underflows "correctly", just
> like "unsigned int".

We're documented that we do. Lots of code relies on that.

See Documentation/atomic_t.txt TYPES

> But again, do we really want this?

I like the two counters better, avoids atomics entirely, some archs
hare horridly expensive atomics (*cough* power *cough*).

I just tried to be clever and use a single u64 load (where possible)
instead of two 32bit loads and got the sum vs split order wrong.