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

[Suren Baghdasaryan <surenb@google.com>]

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