Re: [tip: locking/core] lockdep: Fix usage_traceoverflow

Peter Zijlstra
2020-10-28T21:45:17Z

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;