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

[Eric Biggers]

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@xxxxxxxxxx/
> 
> Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Please add proper tags:

Reported-by: syzbot+7a0d9d0b26efefe61780@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: ...
Cc: stable@xxxxxxxxxxxxxxx


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

- Eric