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Wed, 29 Jul 2020 20:24:06 -0700 (PDT) MIME-Version: 1.0 References: <20200716024527.4009170-1-surenb@google.com> In-Reply-To: <20200716024527.4009170-1-surenb@google.com> From: Joel Fernandes Date: Wed, 29 Jul 2020 23:23:55 -0400 Message-ID: Subject: Re: [PATCH 1/1] staging: android: ashmem: Fix lockdep warning for write operation To: Suren Baghdasaryan Cc: Greg Kroah-Hartman , =?UTF-8?B?QXJ2ZSBIasO4bm5ldsOlZw==?= , Todd Kjos , Martijn Coenen , Christian Brauner , Hridya Valsaraju , Michal Hocko , hdanton@sina.com, ebiggers@kernel.org, "open list:ANDROID DRIVERS" , linux-mm , LKML , kernel-team Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: 6578F180F8B86 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Jul 15, 2020 at 10:45 PM 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 > --- Once Eric's nits are resolved: Reviewed-by: Joel Fernandes (Google) Thanks.