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

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Date:   Thu, 30 Jul 2020 12:15:44 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>,
        Christian Brauner <christian@...uner.io>,
        Hridya Valsaraju <hridya@...gle.com>,
        Michal Hocko <mhocko@...nel.org>,
        Hillf Danton <hdanton@...a.com>,
        Eric Biggers <ebiggers@...nel.org>,
        "open list:ANDROID DRIVERS" <devel@...verdev.osuosl.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/1] staging: android: ashmem: Fix lockdep warning for
 write operation

On Wed, Jul 29, 2020 at 8:24 PM Joel Fernandes <joel@...lfernandes.org> wrote:
>
> On Wed, Jul 15, 2020 at 10:45 PM Suren Baghdasaryan <surenb@...gle.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@...gle.com>
> > ---
>
> Once Eric's nits are resolved:
>
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

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.