Messages in this thread | | | Date | Mon, 24 Sep 2018 11:51:33 +0100 | From | Will Deacon <> | Subject | Re: Traceback in ww_mutex test (test_cycle_work) on arm64/x86_64 |
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Hi Guenter,
On Sun, Sep 23, 2018 at 12:57:06PM -0700, Guenter Roeck wrote: > when enabling CONFIG_WW_MUTEX_SELFTEST on arm64 or x86_64, > I get the following traceback. > > [ 3.111852] ------------[ cut here ]------------ > [ 3.112100] DEBUG_LOCKS_WARN_ON(__owner_task(owner) != current) > [ 3.112753] WARNING: CPU: 1 PID: 771 at kernel/locking/mutex.c:1211 __mutex_unlock_slowpath+0x1a8/0x2e0 > [ 3.113238] Modules linked in: > [ 3.113774] CPU: 1 PID: 771 Comm: kworker/u16:8 Not tainted 4.19.0-rc5-dirty #1 > [ 3.114025] Hardware name: linux,dummy-virt (DT) > [ 3.114587] Workqueue: test-ww_mutex test_cycle_work > [ 3.114950] pstate: 40000005 (nZcv daif -PAN -UAO) > [ 3.115144] pc : __mutex_unlock_slowpath+0x1a8/0x2e0 > [ 3.115327] lr : __mutex_unlock_slowpath+0x1a8/0x2e0 > [ 3.115500] sp : ffff00000b7cbc40 > [ 3.115647] x29: ffff00000b7cbc40 x28: 0000000000000000 > [ 3.115921] x27: ffff00000942f000 x26: ffff00000a204da0 > [ 3.116155] x25: ffff00000a1c93d0 x24: ffff000009103cd8 > [ 3.116376] x23: ffff00000a1c9000 x22: ffff00000942f000 > [ 3.116596] x21: ffff00000b7cbca8 x20: ffff80001c05f8c8 > [ 3.116817] x19: 0000000000000000 x18: ffffffffffffffff > [ 3.117036] x17: 0000000000000000 x16: 0000000000000000 > [ 3.117256] x15: ffff00000942f808 x14: ffff00008a1c8bb7 > [ 3.117476] x13: ffff00000a1c8bc5 x12: ffff00000944f000 > [ 3.117695] x11: 0000000005f5e0ff x10: ffff0000094b3000 > [ 3.117947] x9 : 0000000000000000 x8 : ffff00000942f808 > [ 3.118172] x7 : ffff00000816153c x6 : 0000000000000000 > [ 3.118392] x5 : 0000000000000000 x4 : ffff00000b7cc000 > [ 3.118612] x3 : 6172e063a21fe200 x2 : ffff00000944fd80 > [ 3.118830] x1 : 6172e063a21fe200 x0 : 0000000000000000 > [ 3.119169] Call trace: > [ 3.119348] __mutex_unlock_slowpath+0x1a8/0x2e0 > [ 3.119540] ww_mutex_unlock+0x48/0xa0 > [ 3.119709] test_cycle_work+0x10c/0x220 > [ 3.119864] process_one_work+0x29c/0x708 > [ 3.120016] worker_thread+0x40/0x458 > [ 3.120179] kthread+0x12c/0x130 > [ 3.120317] ret_from_fork+0x10/0x18
Fun: I can reproduce this all the way back to 4.11, when the selftests were merged!
> Debugging shows that the traceback occurs in the following code > in test_cycle_work(). > > + err = ww_mutex_lock(cycle->b_mutex, &ctx); > + if (err == -EDEADLK) { > # true > + ww_mutex_unlock(&cycle->a_mutex); > + ww_mutex_lock_slow(cycle->b_mutex, &ctx); > + err = ww_mutex_lock(&cycle->a_mutex, &ctx); > # returns with err == -EDEADLK > + } > + > + if (!err) > + ww_mutex_unlock(cycle->b_mutex); > + ww_mutex_unlock(&cycle->a_mutex); > # traceback seen here: > # unlocks a_mutex even though it was not > # acquired by this thread > > Details don't really matter as long as the number of CPUs is at least 8 > (I have not seen the problem with 1, 2, 4, or 6 CPUs). My test system > has 8 CPU cores (times 2 for hyperthreading), so that may be related. > > The test case above is clearly wrong if both calls to ww_mutex_lock() > fail with -EDEADLK. Unfortunately I don't know the expected behavior > in this case, so I'll have to pass this on without a proposed fix.
Yeah, I think the test code isn't robust in the face of CONFIG_DEBUG_WW_MUTEX_SLOWPATH, which can spuriously return -EDEADLK from mutex_lock(). It looks like it's assuming that err will always be reset to 0 when it takes a_mutex the second time. Chris?
Will
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