linux-kernel - Re: [PATCH] md/raid5: fix atomicity violation in raid5_cache

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Date: Fri, 22 Dec 2023 12:56:06 +0800
From: 20 39 <2045gemini@...il.com>
To: Yu Kuai <yukuai1@...weicloud.com>
Cc: song@...nel.org, linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org, 
	baijiaju1990@...look.com, BassCheck <bass@...a.edu.cn>, 
	"yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] md/raid5: fix atomicity violation in raid5_cache_count

Hi,
We've updated to use READ_ONCE() in PATCH v2, Thank you for helpful advice.
Best regards,
Han

Yu Kuai <yukuai1@...weicloud.com> 于2023年12月22日周五 10:53写道：
>
> Hi,
>
> 在 2023/12/22 10:34, 20 39 写道:
> > Hi Kuai,
> >
> > Thank you for your patience. This email is essentially the same as my
> > previous one, only now adjusted to plain text format. I apologize for
> > any inconvenience caused earlier.
> >
> > Thanks for your email and the insightful points you've raised. Let me
> > clarify a few aspects regarding the raid5_cache_count() and
> > raid5_set_cache_size() functions.
> >
> > 1. Callback Function in setup_conf(): You mentioned that
> > raid5_cache_count() is called from setup_conf() where reconfig_mutex
> > is held. While this is true, it's important to note that
> > raid5_cache_count() is actually initialized as a callback function in
> > setup_conf(), as described in /include/linux/shrinker.h. This means it
> > could be invoked later in a context where the reconfig_mutex isn't
> > necessarily held. The documentation in shrinker.h indicates potential
> > invocation scenarios beyond the initial setup context.
>
> Yes, you're right. I misread the code. Then this patch looks good to me,
> just one nit below.
>
> >>>> @@ -7390,11 +7390,12 @@ static unsigned long raid5_cache_count(struct shrinker *shrink,
> >>>>                                       struct shrink_control *sc)
> >>>>    {
> >>>>        struct r5conf *conf = shrink->private_data;
> >>>> -
> >>>> -     if (conf->max_nr_stripes < conf->min_nr_stripes)
> >>>> +     int max_stripes = conf->max_nr_stripes;
> >>>> +     int min_stripes = conf->min_nr_stripes;
>
> Since read and write can concurrent, I'll suggest to use READ_ONCE() and
> WRITE_ONCE() for max/min_nr_stripes.
>
> Thanks,
> Kuai
> >>>> +     if (max_stripes < min_stripes)
> >>>>                /* unlikely, but not impossible */
> >>>>                return 0;
> >>>> -     return conf->max_nr_stripes - conf->min_nr_stripes;
> >>>> +     return max_stripes - min_stripes;
> >>>>    }
> >>>>
> >>>>    static struct r5conf *setup_conf(struct mddev *mddev)
> >>>>
> >>>
> > .
> >
>