linux-kernel - Re: [PATCH] locking/percpu_rwsem: Rewrite to not use rwsem

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Message-ID: <20190806171515.GR2349@hirez.programming.kicks-ass.net>
Date:   Tue, 6 Aug 2019 19:15:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Will Deacon <will.deacon@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, bigeasy@...utronix.de,
        juri.lelli@...hat.com, williams@...hat.com, bristot@...hat.com,
        longman@...hat.com, dave@...olabs.net, jack@...e.com
Subject: Re: [PATCH] locking/percpu_rwsem: Rewrite to not use rwsem

On Tue, Aug 06, 2019 at 06:17:42PM +0200, Oleg Nesterov wrote:

> but this will also wake all the pending readers up. Every reader will burn
> CPU for no reason and likely delay the writer.
> 
> In fact I'm afraid this can lead to live-lock, because every reader in turn
> will call __percpu_up_read().

I didn't really consider that case important; because of how heavy the
write side is, it should be relatively rare.

> How about 2 wait queues?

That said, I can certainly try that.