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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Thu, 16 Jul 2020 12:30:27 -0700
Message-ID: <CAJuCfpGwgmAD0v+F3CxieZmXHnjnbzrZ4Cpugi+8=-fDPgCU7w@mail.gmail.com>
Subject: Re: [PATCH 1/1] staging: android: ashmem: Fix lockdep warning for
 write operation
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= <arve@android.com>,
        Todd Kjos <tkjos@android.com>,
        Martijn Coenen <maco@android.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Christian Brauner <christian@brauner.io>,
        Hridya Valsaraju <hridya@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Hillf Danton <hdanton@sina.com>,
        "open list:ANDROID DRIVERS" <devel@driverdev.osuosl.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
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On Wed, Jul 15, 2020 at 8:30 PM Eric Biggers <ebiggers@kernel.org> wrote:
>
> 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.

Will add in v2. Thanks!

>
> - Eric