I have also now tested this v2 patch and can confirm that it also
fixes the race in fscache that we were reliably able to reproduce with
our (re-export) workloads..
Tested-by: Daire Byrne <daire@xxxxxxxx>
Daire
On Thu, 17 Nov 2022 at 14:30, Dave Wysochanski <dwysocha@xxxxxxxxxx> wrote:
>
> If a cookie expires from the LRU and the LRU_DISCARD flag is set,
> but the state machine has not run yet, it's possible another thread
> can call fscache_use_cookie and begin to use it. When the
> cookie_worker finally runs, it will see the LRU_DISCARD flag set,
> transition the cookie->state to LRU_DISCARDING, which will then
> withdraw the cookie. Once the cookie is withdrawn the object is
> removed the below oops will occur because the object associated
> with the cookie is now NULL.
>
> Fix the oops by clearing the LRU_DISCARD bit if another thread
> uses the cookie before the cookie_worker runs.
>
> BUG: kernel NULL pointer dereference, address: 0000000000000008
> ...
> CPU: 31 PID: 44773 Comm: kworker/u130:1 Tainted: G E 6.0.0-5.dneg.x86_64 #1
> Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 08/26/2022
> Workqueue: events_unbound netfs_rreq_write_to_cache_work [netfs]
> RIP: 0010:cachefiles_prepare_write+0x28/0x90 [cachefiles]
> ...
> Call Trace:
> netfs_rreq_write_to_cache_work+0x11c/0x320 [netfs]
> process_one_work+0x217/0x3e0
> worker_thread+0x4a/0x3b0
> ? process_one_work+0x3e0/0x3e0
> kthread+0xd6/0x100
> ? kthread_complete_and_exit+0x20/0x20
> ret_from_fork+0x1f/0x30
>
> Fixes: 12bb21a29c19 ("fscache: Implement cookie user counting and resource pinning")
> Reported-by: Daire Byrne <daire.byrne@xxxxxxxxx>
> Signed-off-by: Dave Wysochanski <dwysocha@xxxxxxxxxx>
> ---
> fs/fscache/cookie.c | 8 ++++++++
> include/trace/events/fscache.h | 2 ++
> 2 files changed, 10 insertions(+)
>
> diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
> index 451d8a077e12..bce2492186d0 100644
> --- a/fs/fscache/cookie.c
> +++ b/fs/fscache/cookie.c
> @@ -605,6 +605,14 @@ void __fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify)
> set_bit(FSCACHE_COOKIE_DO_PREP_TO_WRITE, &cookie->flags);
> queue = true;
> }
> + /*
> + * We could race with cookie_lru which may set LRU_DISCARD bit
> + * but has yet to run the cookie state machine. If this happens
> + * and another thread tries to use the cookie, clear LRU_DISCARD
> + * so we don't end up withdrawing the cookie while in use.
> + */
> + if (test_and_clear_bit(FSCACHE_COOKIE_DO_LRU_DISCARD, &cookie->flags))
> + fscache_see_cookie(cookie, fscache_cookie_see_lru_discard_clear);
> break;
>
> case FSCACHE_COOKIE_STATE_FAILED:
> diff --git a/include/trace/events/fscache.h b/include/trace/events/fscache.h
> index c078c48a8e6d..a6190aa1b406 100644
> --- a/include/trace/events/fscache.h
> +++ b/include/trace/events/fscache.h
> @@ -66,6 +66,7 @@ enum fscache_cookie_trace {
> fscache_cookie_put_work,
> fscache_cookie_see_active,
> fscache_cookie_see_lru_discard,
> + fscache_cookie_see_lru_discard_clear,
> fscache_cookie_see_lru_do_one,
> fscache_cookie_see_relinquish,
> fscache_cookie_see_withdraw,
> @@ -149,6 +150,7 @@ enum fscache_access_trace {
> EM(fscache_cookie_put_work, "PQ work ") \
> EM(fscache_cookie_see_active, "- activ") \
> EM(fscache_cookie_see_lru_discard, "- x-lru") \
> + EM(fscache_cookie_see_lru_discard_clear,"- lrudc") \
> EM(fscache_cookie_see_lru_do_one, "- lrudo") \
> EM(fscache_cookie_see_relinquish, "- x-rlq") \
> EM(fscache_cookie_see_withdraw, "- x-wth") \
> --
> 2.31.1
>
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