linux-kernel - Re: [PATCH] block: remove redundant explicit memory barrier from rq

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Message-ID: <ZxgEHW0UuuLcSY7_@telecaster.dhcp.thefacebook.com>
Date: Tue, 22 Oct 2024 12:59:25 -0700
From: Omar Sandoval <osandov@...ndov.com>
To: Muchun Song <muchun.song@...ux.dev>
Cc: Jens Axboe <axboe@...nel.dk>, Muchun Song <songmuchun@...edance.com>,
	josef@...icpanda.com, oleg@...hat.com, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: remove redundant explicit memory barrier from
 rq_qos waiter and waker

On Tue, Oct 22, 2024 at 02:31:53PM +0800, Muchun Song wrote:
> 
> 
> > On Oct 21, 2024, at 21:45, Jens Axboe <axboe@...nel.dk> wrote:
> > 
> > On 10/21/24 2:52 AM, Muchun Song wrote:
> >> The memory barriers in list_del_init_careful() and list_empty_careful()
> >> in pairs already handle the proper ordering between data.got_token
> >> and data.wq.entry. So remove the redundant explicit barriers. And also
> >> change a "break" statement to "return" to avoid redundant calling of
> >> finish_wait().
> > 
> > Not sure why you didn't CC Omar on this one, as he literally just last
> > week fixed an issue related to this.
> 
> Hi Jens,
> 
> Yes. I only CC the author of patch of adding the barriers, I thought
> they should be more confident about this. Thanks for your reminder.
> I saw Omar's great fix. And thanks for you help me CC Omar. I think
> he'll be also suitable for commenting on this patch.
> 
> Muchun,
> Thanks.

Well there goes my streak of not reading memory-barriers.txt for a few
months...

This looks fine to me. wake_up_process() also implies a full memory
barrier, so I that smp_wmb() was extra redundant.

Reviewed-by: Omar Sandoval <osandov@...com>