On Wed, Oct 28, 2020 at 08:42:09PM +0100, Peter Zijlstra wrote: > On Wed, Oct 28, 2020 at 05:40:48PM +0000, Chris Wilson wrote: > > Quoting Chris Wilson (2020-10-27 16:34:53) > > > Quoting Peter Zijlstra (2020-10-27 15:45:33) > > > > On Tue, Oct 27, 2020 at 01:29:10PM +0000, Chris Wilson wrote: > > > > > > > > > <4> [304.908891] hm#2, depth: 6 [6], 3425cfea6ff31f7f != 547d92e9ec2ab9af > > > > > <4> [304.908897] WARNING: CPU: 0 PID: 5658 at kernel/locking/lockdep.c:3679 check_chain_key+0x1a4/0x1f0 > > > > > > > > Urgh, I don't think I've _ever_ seen that warning trigger. > > > > > > > > The comments that go with it suggest memory corruption is the most > > > > likely trigger of it. Is it easy to trigger? > > > > > > For the automated CI, yes, the few machines that run that particular HW > > > test seem to hit it regularly. I have not yet reproduced it for myself. > > > I thought it looked like something kasan would provide some insight for > > > and we should get a kasan run through CI over the w/e. I suspect we've > > > feed in some garbage and called it a lock. > > > > I tracked it down to a second invocation of lock_acquire_shared_recursive() > > intermingled with some other regular mutexes (in this case ww_mutex). > > > > We hit this path in validate_chain(): > > /* > > * Mark recursive read, as we jump over it when > > * building dependencies (just like we jump over > > * trylock entries): > > */ > > if (ret == 2) > > hlock->read = 2; > > > > and that is modifying hlock_id() and so the chain-key, after it has > > already been computed. > > Ooh, interesting.. I'll have to go look at this in the morning, brain is > fried already. Thanks for digging into it. So that's commit f611e8cf98ec ("lockdep: Take read/write status in consideration when generate chainkey") that did that. So validate_chain() requires the new chain_key, but can change ->read which then invalidates the chain_key we just calculated. This happens when check_deadlock() returns 2, which only happens when: - next->read == 2 && ... ; however @hext is our @hlock, so that's pointless - when there's a nest_lock involved ; ww_mutex uses that !!! I suppose something like the below _might_ just do it, but I haven't compiled it, and like said, my brain is fried. Boqun, could you have a look, you're a few timezones ahead of us so your morning is earlier ;-) --- diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c index 3e99dfef8408..3caf63532bc2 100644 --- a/kernel/locking/lockdep.c +++ b/kernel/locking/lockdep.c @@ -3556,7 +3556,7 @@ static inline int lookup_chain_cache_add(struct task_struct *curr, static int validate_chain(struct task_struct *curr, struct held_lock *hlock, - int chain_head, u64 chain_key) + int chain_head, u64 *chain_key) { /* * Trylock needs to maintain the stack of held locks, but it @@ -3568,6 +3568,7 @@ static int validate_chain(struct task_struct *curr, * (If lookup_chain_cache_add() return with 1 it acquires * graph_lock for us) */ +again: if (!hlock->trylock && hlock->check && lookup_chain_cache_add(curr, hlock, chain_key)) { /* @@ -3597,8 +3598,12 @@ static int validate_chain(struct task_struct *curr, * building dependencies (just like we jump over * trylock entries): */ - if (ret == 2) + if (ret == 2) { hlock->read = 2; + *chain_key = iterate_chain_key(hlock->prev_chain_key, hlock_id(hlock)); + goto again; + } + /* * Add dependency only if this lock is not the head * of the chain, and if it's not a secondary read-lock: @@ -3620,7 +3625,7 @@ static int validate_chain(struct task_struct *curr, #else static inline int validate_chain(struct task_struct *curr, struct held_lock *hlock, - int chain_head, u64 chain_key) + int chain_head, u64 *chain_key) { return 1; } @@ -4834,7 +4839,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass, WARN_ON_ONCE(!hlock_class(hlock)->key); } - if (!validate_chain(curr, hlock, chain_head, chain_key)) + if (!validate_chain(curr, hlock, chain_head, &chain_key)) return 0; curr->curr_chain_key = chain_key;
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