linux-kernel - Re: [PATCH 1/1] staging: android: ashmem: Fix lockdep warning for write operation

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Date:   Wed, 29 Jul 2020 23:23:55 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Christian Brauner <christian@...uner.io>,
        Hridya Valsaraju <hridya@...gle.com>,
        Michal Hocko <mhocko@...nel.org>, hdanton@...a.com,
        ebiggers@...nel.org,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/1] staging: android: ashmem: Fix lockdep warning for
 write operation

On Wed, Jul 15, 2020 at 10:45 PM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> syzbot report [1] describes a deadlock when write operation against an
> ashmem fd executed at the time when ashmem is shrinking its cache results
> in the following lock sequence:
>
> Possible unsafe locking scenario:
>
>         CPU0                    CPU1
>         ----                    ----
>    lock(fs_reclaim);
>                                 lock(&sb->s_type->i_mutex_key#13);
>                                 lock(fs_reclaim);
>    lock(&sb->s_type->i_mutex_key#13);
>
> kswapd takes fs_reclaim and then inode_lock while generic_perform_write
> takes inode_lock and then fs_reclaim. However ashmem does not support
> writing into backing shmem with a write syscall. The only way to change
> its content is to mmap it and operate on mapped memory. Therefore the race
> that lockdep is warning about is not valid. Resolve this by introducing a
> separate lockdep class for the backing shmem inodes.
>
> [1]: https://lkml.kernel.org/lkml/0000000000000b5f9d059aa2037f@google.com/
>
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
> ---

Once Eric's nits are resolved:

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Thanks.