On Wed, Jul 29, 2020 at 8:24 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > On Wed, Jul 15, 2020 at 10:45 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> 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> > > --- > > Once Eric's nits are resolved: > > Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Thanks Joel! I'm fixing the nits and will report the patch shortly. One note about adding the "Fixes: " tag - this is a fix for a false positive lockdep warning and it's unclear which patch should be quoted here (I could not find a clear cause that started this warning). In similar situations, for example here: https://lkml.org/lkml/2020/6/15/958 developers seem to skip that tag. So I'll do the same. > > Thanks.
