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SubjectRe: [PATCH 1/1] staging: android: ashmem: Fix lockdep warning for write operation
On Wed, Jul 15, 2020 at 10:45 PM Suren Baghdasaryan <surenb@google.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@google.com>
> ---

Once Eric's nits are resolved:

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

Thanks.

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