lockdep false positive in double_lock_balance()?

From: Roland Dreier
Date: Wed May 16 2012 - 16:00:30 EST

Hi scheduler hackers,

I'm very occasionally seeing the lockdep warning below on our boxes
running 2.6.39 (PREEMPT=n, so "unfair" _double_lock_balance()). I
think I see the explanation, and it's probably not even worth fixing:

On the unlock side, we have:

static inline void double_unlock_balance(struct rq *this_rq, struct rq *busiest)
__releases(busiest->lock)
{
raw_spin_unlock(&busiest->lock);
lock_set_subclass(&this_rq->lock.dep_map, 0, _RET_IP_);
}

while on the lock side we have:

static int _double_lock_balance(struct rq *this_rq, struct rq *busiest)
__releases(this_rq->lock)
__acquires(busiest->lock)
__acquires(this_rq->lock)
{
int ret = 0;

if (unlikely(!raw_spin_trylock(&busiest->lock))) {
if (busiest < this_rq) {
raw_spin_unlock(&this_rq->lock);
raw_spin_lock(&busiest->lock);
raw_spin_lock_nested(&this_rq->lock,
SINGLE_DEPTH_NESTING);
ret = 1;
} else
raw_spin_lock_nested(&busiest->lock,
SINGLE_DEPTH_NESTING);
}
return ret;
}

So it seems we have the following (purely lockdep-related) race:

unlock: lock:

if (unlikely(!raw_spin_trylock(&busiest->lock))) { //fail to lock

raw_spin_unlock(&busiest->lock);

if (busiest < this_rq) { //not true
} else
raw_spin_lock_nested(&busiest->lock,
SINGLE_DEPTH_NESTING);

lock_set_subclass(&this_rq->lock.dep_map, 0, _RET_IP_); //too late

where we end up trying to take a second lock with SINGLE_DEPTH_NESTING
before we've promoted our first lock to subclass 0.

I'm not sure this is easily fixable (if we flipped the
lock_set_subclass() to before the raw_spin_unlock(), would we
introduce other lockdep problems?). Mostly I want to make sure
my diagnosis is correct and there's not some actual deadlock
that could happen.

So does this make sense?

Thanks,
Roland

Here's the actual lockdep warning:

[89945.638847] =============================================
[89945.638974] [ INFO: possible recursive locking detected ]
[89945.639033] 2.6.39.3-dbg+ #13245
[89945.639079] ---------------------------------------------
[89945.639131] foed/7820 is trying to acquire lock:
[89945.639180] (&rq->lock/1){..-.-.}, at: [<ffffffff8103fa1a>] double_lock_balance+0x5a/0x90
[89945.639291]
[89945.639292] but task is already holding lock:
[89945.639374] (&rq->lock/1){..-.-.}, at: [<ffffffff8103fa3b>] double_lock_balance+0x7b/0x90
[89945.639480]
[89945.639481] other info that might help us debug this:
[89945.639566] 2 locks held by foed/7820:
[89945.639611] #0: (&mm->mmap_sem){++++++}, at: [<ffffffff8156a445>] do_page_fault+0x1f5/0x560
[89945.639717] #1: (&rq->lock/1){..-.-.}, at: [<ffffffff8103fa3b>] double_lock_balance+0x7b/0x90
[89945.639829]
[89945.639830] stack backtrace:
[89945.639908] Pid: 7820, comm: foed Tainted: G W 2.6.39.3-dbg+ #13245
[89945.639965] Call Trace:
[89945.640011] [<ffffffff8109293f>] __lock_acquire+0x153f/0x1da0
[89945.640069] [<ffffffff81009235>] ? native_sched_clock+0x15/0x70
[89945.640126] [<ffffffff810812ef>] ? local_clock+0x6f/0x80
[89945.640179] [<ffffffff8103fa3b>] ? double_lock_balance+0x7b/0x90
[89945.640235] [<ffffffff8109384d>] lock_acquire+0x9d/0x130
[89945.640288] [<ffffffff8103fa1a>] ? double_lock_balance+0x5a/0x90
[89945.640345] [<ffffffff810fe47f>] ? cpupri_find+0xcf/0x140
[89945.640401] [<ffffffff81565ce4>] _raw_spin_lock_nested+0x34/0x70
[89945.640456] [<ffffffff8103fa1a>] ? double_lock_balance+0x5a/0x90
[89945.640512] [<ffffffff8103fa1a>] double_lock_balance+0x5a/0x90
[89945.640568] [<ffffffff8104c546>] push_rt_task+0xc6/0x290
[89945.640621] [<ffffffff8104c7c0>] push_rt_tasks+0x20/0x30
[89945.640674] [<ffffffff8104c83e>] post_schedule_rt+0xe/0x10
[89945.640729] [<ffffffff81562d8c>] schedule+0x53c/0xa70
[89945.640781] [<ffffffff810941c8>] ? mark_held_locks+0x78/0xb0
[89945.640781] [<ffffffff810941c8>] ? mark_held_locks+0x78/0xb0
[89945.640836] [<ffffffff81565b15>] rwsem_down_failed_common+0xb5/0x150
[89945.640893] [<ffffffff8108122d>] ? sched_clock_cpu+0xbd/0x110
[89945.640949] [<ffffffff8108e33d>] ? trace_hardirqs_off+0xd/0x10
[89945.641004] [<ffffffff81565be5>] rwsem_down_read_failed+0x15/0x17
[89945.641061] [<ffffffff812c0a54>] call_rwsem_down_read_failed+0x14/0x30
[89945.641120] [<ffffffff81139565>] ? sys_mprotect+0xe5/0x230
[89945.641174] [<ffffffff81564f1e>] ? down_read+0x7e/0xa0
[89945.641227] [<ffffffff8156a445>] ? do_page_fault+0x1f5/0x560
[89945.641281] [<ffffffff8156a445>] do_page_fault+0x1f5/0x560
[89945.641336] [<ffffffff812bb5fc>] ? rwsem_wake+0x4c/0x60
[89945.641388] [<ffffffff812c0aa7>] ? call_rwsem_wake+0x17/0x30
[89945.641443] [<ffffffff812c0b5d>] ? trace_hardirqs_off_thunk+0x3a/0x3c
[89945.641501] [<ffffffff81566d25>] page_fault+0x25/0x30
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/