On Wed, Jul 19, 2023 at 11:43:30AM -0500, Bob Pearson wrote: > On 7/19/23 02:49, Leon Romanovsky wrote: > > On Tue, Jul 18, 2023 at 12:59:44PM -0500, Bob Pearson wrote: > >> Make rcu_read locking of critical sections with the indexed > >> verbs objects be protected from early freeing of those objects. > >> The AH, QP, MR and MW objects are looked up from their indices > >> contained in received packets or WQEs during I/O processing. > >> Make these objects be freed using kfree_rcu to avoid races. > > > > Sorry, how use of RCU avoid races? > > > > Thanks > > The races are between destroy/deallocate/dereg verbs API calls and packets arriving or completing send > or deferred processing of wqes. Packets and wqes contain indices/keys/numbers that refer to objects. > The rxe driver maintains xarrays for each type of object that allow to lookup the address of the object > from its index and then take a reference to protect the pointer. The destroy verbs defer completion > until the reference count falls to zero and then removes the entry in the xarray. These operations > need to be atomic. One alternative is to use spinlocks to protect them but that places a load on > performance under heavy load which is typically dominated by the lookup function since objects tend > to have a long lifetime. rcu readlocks are a better alternative but depend on the deferred destruction > of the objects used in the rcu critical section. You rarely can replace locks with RCU without careful design changes. Thanks
