On Wed, Sep 06, 2023 at 10:37:40PM +0100, Catalin Marinas wrote: > On Wed, Sep 06, 2023 at 12:11:12PM -0700, Paul E. McKenney wrote: > > On Wed, Sep 06, 2023 at 06:15:49PM +0100, Catalin Marinas wrote: > > > On Wed, Sep 06, 2023 at 02:35:29PM +0000, Joel Fernandes wrote: > > > > On Tue, Sep 05, 2023 at 03:41:32PM +0100, Catalin Marinas wrote: > > > > > On Tue, Sep 05, 2023 at 11:17:25AM +0000, Joel Fernandes wrote: > > > > > > The correct fix then should probably be to mark the object as > > > > > > kmemleak_not_leak() until a grace period elapses. This will cause the object > > > > > > to not be reported but still be scanned until eventually the lower layers > > > > > > will remove the object from kmemleak-tracking after the grace period. Per the > > > > > > docs also, that API is used to prevent false-positives. > > > > > > > > > > This should work as well but I'd use kmemleak_ignore() instead of > > > > > kmemleak_not_leak(). The former, apart from masking the false positive, > > > > > also tells kmemleak not to scan the object. After a kvfree_rcu(), the > > > > > object shouldn't have any valid references to other objects, so not > > > > > worth scanning. > > > > > > > > Yes I am also OK with that, however to me I consider the object as alive as > > > > long as the grace period does not end. But I agree with you and it may not be > > > > worth tracking them or scanning them. > > > > > > I guess from an RCU perspective, the object is still alive. From the > > > kvfree_rcu() caller perspective though, it can disappear at any point > > > after the grace period, so it shouldn't rely on its content being valid > > > and referencing other objects (other than transiently e.g. in RCU list > > > traversal). > > > > > > It probably only matters if we have some very long grace periods (I'm > > > not up to date with the recent RCU developments). In such cases, the > > > object still being scanned could introduce false negatives. That's my > > > reasoning for suggesting kmemleak_ignore() rather than > > > kmemleak_not_leak(). > > > > Very long RCU readers still result in very long RCU grace periods. And, > > after some tens of seconds, RCU CPU stall warnings. So don't let your > > RCU readers run for that long. But you knew that already. ;-) > > That's still ok. I was more thinking of deferred freeing well past the > RCU readers completing. Ah, that can happen. Some kernels are built with CONFIG_RCU_LAZY=y, which delays freeing in order to reduce power consumption. And kfree_rcu() will also delay for a bit. But in both cases, a flood of callbacks should get things going again. But an isolated kfree_rcu() might well see a few seconds delay. Saving your battery! ;-) > > > @@ -3388,6 +3389,14 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr) > > > success = true; > > > } > > > > > > + /* > > > + * The kvfree_rcu() caller considers the pointer freed at this point > > > + * and likely removes any references to it. Since the the actual slab > > > + * freeing (and kmemleak_free()) is deferred, tell kmemleak to ignore > > > + * this object (no scanning or false positives reporting). > > > + */ > > > + kmemleak_ignore(ptr); > > > > Do we want to un-ignore it at the end of the grace period? Or will that > > happen automatically when it is passed to kfree()? (My guess is that > > the answer to both questions is "yes", but I figured that I should ask.) > > No need to un-ignore. This function only tells kmemleak it's not a > leak and doesn't have any interesting data to scan. Kmemleak still keeps > track of it until properly freed. Got it, thank you! Thanx, Paul
