linux-kernel - Re: [BUG] mm: backing-dev: a possible sleep-in-atomic-context bug in cgwb

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Date:   Wed, 20 Jun 2018 20:35:15 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Jia-Ju Bai <baijiaju1990@...il.com>
Cc:     axboe@...nel.dk, akpm@...ux-foundation.or, jack@...e.cz,
        zhangweiping@...ichuxing.com, sergey.senozhatsky@...il.com,
        andriy.shevchenko@...ux.intel.com, christophe.jaillet@...adoo.fr,
        aryabinin@...tuozzo.com, linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG] mm: backing-dev: a possible sleep-in-atomic-context bug in
 cgwb_create()

On Thu, Jun 21, 2018 at 11:02:58AM +0800, Jia-Ju Bai wrote:
> The kernel may sleep with holding a spinlock.
> The function call path (from bottom to top) in Linux-4.16.7 is:
> 
> [FUNC] schedule
> lib/percpu-refcount.c, 222:
>         schedule in __percpu_ref_switch_mode
> lib/percpu-refcount.c, 339:
>         __percpu_ref_switch_mode in percpu_ref_kill_and_confirm
> ./include/linux/percpu-refcount.h, 127:
>         percpu_ref_kill_and_confirm in percpu_ref_kill
> mm/backing-dev.c, 545:
>         percpu_ref_kill in cgwb_kill
> mm/backing-dev.c, 576:
>         cgwb_kill in cgwb_create
> mm/backing-dev.c, 573:
>         _raw_spin_lock_irqsave in cgwb_create
> 
> This bug is found by my static analysis tool (DSAC-2) and checked by my
> code review.

I disagree with your code review.

         * If the previous ATOMIC switching hasn't finished yet, wait for
         * its completion.  If the caller ensures that ATOMIC switching
         * isn't in progress, this function can be called from any context.

I believe cgwb_kill is always called under the spinlock, so we will never
sleep because the percpu ref will never be switching to atomic mode.

This is complex and subtle, so I could be wrong.