Messages in this thread | | | Date | Mon, 14 May 2018 06:31:05 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] nvme: fix lockdep warning in nvme_mpath_clear_current_path |
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On Mon, May 14, 2018 at 05:42:30AM -0700, Christoph Hellwig wrote: > > extern unsigned int nvme_io_timeout; > > #define NVME_IO_TIMEOUT (nvme_io_timeout * HZ) > > @@ -454,7 +455,9 @@ static inline void nvme_mpath_clear_current_path(struct nvme_ns *ns) > > { > > struct nvme_ns_head *head = ns->head; > > > > - if (head && ns == srcu_dereference(head->current_path, &head->srcu)) > > + if (head && > > + ns == rcu_dereference_protected(head->current_path, > > + lockdep_is_held(&ns->ctrl->subsys->lock))) > > rcu_assign_pointer(head->current_path, NULL); > > } > > struct nvme_ns *nvme_find_path(struct nvme_ns_head *head); > > We don't really dereference it at all in fact, but just check the > pointers for equality. I wonder if there is a better way to do this, > as my ANA patches add a caller without the lock (and withou SRU > protection either now that I think of it) - for a pure pointer compare > we really should not need any sort of protection.
If you are just looking at the value of an RCU-protected pointer, then using rcu_access_pointer() will cause RCU to just read out the value and otherwise keeps its mouth shut.
If you use rcu_access_pointer() and later dereference the value without protection, you will of course get what you deserve, good and hard. ;-)
Thanx, Paul
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