linux-kernel - Re: [PATCH 2/4] arm64: hibernate: remove WARN_ON in save_processor

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Message-ID: <20230608172656.GB1606@willie-the-truck>
Date:   Thu, 8 Jun 2023 18:26:58 +0100
From:   Will Deacon <will@...nel.org>
To:     Song Shuai <songshuaishuai@...ylab.org>
Cc:     catalin.marinas@....com, paul.walmsley@...ive.com,
        palmer@...belt.com, aou@...s.berkeley.edu, chris@...kel.net,
        jcmvbkbc@...il.com, steven.price@....com,
        vincenzo.frascino@....com, pcc@...gle.com, wangxiang@...rlc.com,
        ajones@...tanamicro.com, conor.dooley@...rochip.com,
        jeeheng.sia@...rfivetech.com, leyfoon.tan@...rfivetech.com,
        linux@...linux.org.uk, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 2/4] arm64: hibernate: remove WARN_ON in
 save_processor_state

On Wed, Jun 07, 2023 at 11:00:08AM +0800, Song Shuai wrote:
> 
> 
> 在 2023/6/5 22:28, Will Deacon 写道:
> > On Thu, May 25, 2023 at 10:55:53AM +0800, Song Shuai wrote:
> > > During hibernation or restoration, freeze_secondary_cpus
> > > checks num_online_cpus via BUG_ON, and the subsequent
> > > save_processor_state also does the checking with WARN_ON.
> > > 
> > > So remove the unnecessary checking in save_processor_state.
> > 
> > This is a very terse summary of why this is safe.
> > 
> > Looking at the code, freeze_secondary_cpus() does indeed check
> > num_online_cpus(), or it returns an error which then causes the hibernation
> > to fail. However, this is all in the CONFIG_PM_SLEEP_SMP=y case and it's
> > far less clear whether your assertion is true if that option is disabled.
> > 
> > Please can you describe your reasoning in more detail, and cover the case
> > where CONFIG_PM_SLEEP_SMP=n as well, please?
> 
> With HIBERNATION enabled, the sole possible condition to disable
> CONFIG_PM_SLEEP_SMP
> is !SMP where num_online_cpus is always 1. We also don't have to check it in
> save_processor_state.

Thanks. Please add that to the commit message.

Will