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Date:   Fri, 18 Sep 2020 13:03:10 +0200
From:   peterz@infradead.org
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Jan Kara <jack@suse.cz>, Boaz Harrosh <boaz@plexistor.com>,
        Hou Tao <houtao1@huawei.com>, Ingo Molnar <mingo@redhat.com>,
        Will Deacon <will@kernel.org>, Dennis Zhou <dennis@kernel.org>,
        Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for
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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.