netdev - Re: [PATCH 1/6] sched: Unbreak wakeups

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Message-ID: <20210602144755.GA31179@willie-the-truck>
Date:   Wed, 2 Jun 2021 15:47:55 +0100
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Jens Axboe <axboe@...nel.dk>,
        Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>, dm-devel@...hat.com,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Felipe Balbi <balbi@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
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        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
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        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
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        Namhyung Kim <namhyung@...nel.org>,
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        Paolo Bonzini <pbonzini@...hat.com>,
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        netdev@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, cgroups@...r.kernel.org,
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Subject: Re: [PATCH 1/6] sched: Unbreak wakeups

On Wed, Jun 02, 2021 at 03:12:26PM +0200, Peter Zijlstra wrote:
> Remove broken task->state references and let wake_up_process() DTRT.
> 
> The anti-pattern in these patches breaks the ordering of ->state vs
> COND as described in the comment near set_current_state() and can lead
> to missed wakeups:
> 
> 	(OoO load, observes RUNNING)<-.
> 	for (;;) {                    |
> 	  t->state = UNINTERRUPTIBLE; |
> 	  smp_mb();          ,-----> ,' (OoO load, observed !COND)
>                              |       |
> 	                     |       |	COND = 1;
> 			     |	     `- if (t->state != RUNNING)
>                              |		  wake_up_process(t); // not done
> 	  if (COND) ---------'
> 	    break;
> 	  schedule(); // forever waiting
> 	}
> 	t->state = TASK_RUNNING;
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  drivers/net/ethernet/qualcomm/qca_spi.c |    6 ++----
>  drivers/usb/gadget/udc/max3420_udc.c    |   15 +++++----------
>  drivers/usb/host/max3421-hcd.c          |    3 +--
>  kernel/softirq.c                        |    2 +-
>  4 files changed, 9 insertions(+), 17 deletions(-)

Acked-by: Will Deacon <will@...nel.org>

I couldn't spot any others.

Will