Re: [PATCH 3/4] locking/ww_mutex: Treat ww_mutex_lock() like a trylock

Davidlohr Bueso
2021-03-17T03:02:47Z

On Tue, 16 Mar 2021, Waiman Long wrote:

>It was found that running the ww_mutex_lock-torture test produced the
>following lockdep splat almost immediately:
>
>[ 103.892638] ======================================================
>[ 103.892639] WARNING: possible circular locking dependency detected
>[ 103.892641] 5.12.0-rc3-debug+ #2 Tainted: G S W
>[ 103.892643] ------------------------------------------------------
>[ 103.892643] lock_torture_wr/3234 is trying to acquire lock:
>[ 103.892646] ffffffffc0b35b10 (torture_ww_mutex_2.base){+.+.}-{3:3}, at: torture_ww_mutex_lock+0x316/0x720 [locktorture]
>[ 103.892660]
>[ 103.892660] but task is already holding lock:
>[ 103.892661] ffffffffc0b35cd0 (torture_ww_mutex_0.base){+.+.}-{3:3}, at: torture_ww_mutex_lock+0x3e2/0x720 [locktorture]
>[ 103.892669]
>[ 103.892669] which lock already depends on the new lock.
>[ 103.892669]
>[ 103.892670]
>[ 103.892670] the existing dependency chain (in reverse order) is:
>[ 103.892671]
>[ 103.892671] -> #2 (torture_ww_mutex_0.base){+.+.}-{3:3}:
>[ 103.892675] lock_acquire+0x1c5/0x830
>[ 103.892682] __ww_mutex_lock.constprop.15+0x1d1/0x2e50
>[ 103.892687] ww_mutex_lock+0x4b/0x180
>[ 103.892690] torture_ww_mutex_lock+0x316/0x720 [locktorture]
>[ 103.892694] lock_torture_writer+0x142/0x3a0 [locktorture]
>[ 103.892698] kthread+0x35f/0x430
>[ 103.892701] ret_from_fork+0x1f/0x30
>[ 103.892706]
>[ 103.892706] -> #1 (torture_ww_mutex_1.base){+.+.}-{3:3}:
>[ 103.892709] lock_acquire+0x1c5/0x830
>[ 103.892712] __ww_mutex_lock.constprop.15+0x1d1/0x2e50
>[ 103.892715] ww_mutex_lock+0x4b/0x180
>[ 103.892717] torture_ww_mutex_lock+0x316/0x720 [locktorture]
>[ 103.892721] lock_torture_writer+0x142/0x3a0 [locktorture]
>[ 103.892725] kthread+0x35f/0x430
>[ 103.892727] ret_from_fork+0x1f/0x30
>[ 103.892730]
>[ 103.892730] -> #0 (torture_ww_mutex_2.base){+.+.}-{3:3}:
>[ 103.892733] check_prevs_add+0x3fd/0x2470
>[ 103.892736] __lock_acquire+0x2602/0x3100
>[ 103.892738] lock_acquire+0x1c5/0x830
>[ 103.892740] __ww_mutex_lock.constprop.15+0x1d1/0x2e50
>[ 103.892743] ww_mutex_lock+0x4b/0x180
>[ 103.892746] torture_ww_mutex_lock+0x316/0x720 [locktorture]
>[ 103.892749] lock_torture_writer+0x142/0x3a0 [locktorture]
>[ 103.892753] kthread+0x35f/0x430
>[ 103.892755] ret_from_fork+0x1f/0x30
>[ 103.892757]
>[ 103.892757] other info that might help us debug this:
>[ 103.892757]
>[ 103.892758] Chain exists of:
>[ 103.892758] torture_ww_mutex_2.base --> torture_ww_mutex_1.base --> torture_ww_mutex_0.base
>[ 103.892758]
>[ 103.892763] Possible unsafe locking scenario:
>[ 103.892763]
>[ 103.892764] CPU0 CPU1
>[ 103.892765] ---- ----
>[ 103.892765] lock(torture_ww_mutex_0.base);
>[ 103.892767] lock(torture_ww_mutex_1.base);
>[ 103.892770] lock(torture_ww_mutex_0.base);
>[ 103.892772] lock(torture_ww_mutex_2.base);
>[ 103.892774]
>[ 103.892774] *** DEADLOCK ***
>
>Since ww_mutex is supposed to be deadlock-proof if used properly, such
>deadlock scenario should not happen. To avoid this false positive splat,
>treat ww_mutex_lock() like a trylock().
>
>After applying this patch, the locktorture test can run for a long time
>without triggering the circular locking dependency splat.
>
>Signed-off-by: Waiman Long <longman@redhat.com>

Acked-by Davidlohr Bueso <dbueso@suse.de>