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Date:   Wed, 15 Jul 2020 20:30:48 -0700
From:   Eric Biggers <ebiggers@kernel.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     gregkh@linuxfoundation.org, arve@android.com, tkjos@android.com,
        maco@android.com, joel@joelfernandes.org, christian@brauner.io,
        hridya@google.com, mhocko@kernel.org, hdanton@sina.com,
        devel@driverdev.osuosl.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH 1/1] staging: android: ashmem: Fix lockdep warning for
 write operation
Message-ID: <20200716033048.GG1167@sol.localdomain>
References: <20200716024527.4009170-1-surenb@google.com>
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On Wed, Jul 15, 2020 at 07:45:27PM -0700, Suren Baghdasaryan 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@google.com>

Please add proper tags:

Reported-by: syzbot+7a0d9d0b26efefe61780@syzkaller.appspotmail.com
Fixes: ...
Cc: stable@vger.kernel.org


The Reported-by tag to use was given in the original syzbot report.

- Eric