Chris Kirby wrote: > Mark Maybee wrote: >> Chris Kirby wrote: >> >>> Matthew Ahrens wrote: >>> >>>> So, we use RW_LOCK_HELD() to mean, "might this thread hold the >>>> lock?" and it is generally only used in assertions. Eg, some >>>> routine should only be called with the lock held, so we >>>> "ASSERT(RW_LOCK_HELD(lock))". The fact that sometimes the >>>> implementation of RW_LOCK_HELD() returns TRUE when this thread does >>>> not in fact hold the lock is fine. >>>> >>>> In this case, we are using it a bit differently. The current thread >>>> must *not* hold the lock for reader. We use RW_LOCK_HELD() to >>>> determine definitively if this thread holds the lock for writer or >>>> not. The behavior we want is: if this thread already holds the >>>> lock for writer, we don't need to re-grab it. Otherwise, we grab >>>> the lock for READER. >>>> >>>> However, now that I go look at the implementation of RW_LOCK_HELD(), >>>> it doesn't do what this code expects; it should be using >>>> RW_WRITE_HELD(). >>> >>> >>> The problem in this case turns out to be a bit harder. >>> dsl_dataset_open_obj can be called with dp_config_rwlock held for >>> read or write, or not held at all. An obvious way to fix this would >>> be to have the callers pass an arg saying whether or not the lock is >>> held, but that's esp. ugly in this case since there are ~30 callers, >>> and at least some of them probably don't grab the lock directly. >>> >> Its actually OK if we re-grab a read lock when we already hold it. This >> will just cause the readers count to increment in the lock, and we will >> drop it twice, so the count will be correct in the end. But we don't >> want to try to obtain a read lock if we already have a write lock. So >> it works to just use 'RW_WRITE_HELD()' here. > > > But doesn't the rw_enter code block if there's a waiting > writer? In other words, can't we cause a deadlock > if we already hold the read lock, there's a waiting writer, > and we try to grab the read lock again? > Ah right, this isn't the "unfair" version of the rw lock that we use in the SPA is it? Yup, you're right, a writer-waiting here could lead to a deadlock.
-Mark
