IPoIB: fix MCAST_FLAG_BUSY usage
Commit a9c8ba58 ("IPoIB: Fix usage of uninitialized multicast objects") added a new flag MCAST_JOIN_STARTED, but was not very strict in how it was used. We didn't always initialize the completion struct before we set the flag, and we didn't always call complete on the completion struct from all paths that complete it. This made it less than totally effective, and certainly made its use confusing. And in the flush function we would use the presence of this flag to signal that we should wait on the completion struct, but we never cleared this flag, ever. This is further muddied by the fact that we overload the MCAST_FLAG_BUSY flag to mean two different things: we have a join in flight, and we have succeeded in getting an ib_sa_join_multicast. In order to make things clearer and aid in resolving the rtnl deadlock bug I've been chasing, I cleaned this up a bit. 1) Remove the MCAST_JOIN_STARTED flag entirely 2) Un-overload MCAST_FLAG_BUSY so it now only means a join is in-flight 3) Test on mcast->mc directly to see if we have completed ib_sa_join_multicast (using IS_ERR_OR_NULL) 4) Make sure that before setting MCAST_FLAG_BUSY we always initialize the mcast->done completion struct 5) Make sure that before calling complete(&mcast->done), we always clear the MCAST_FLAG_BUSY bit 6) Take the mcast_mutex before we call ib_sa_multicast_join and also take the mutex in our join callback. This forces ib_sa_multicast_join to return and set mcast->mc before we process the callback. This way, our callback can safely clear mcast->mc if there is an error on the join and we will do the right thing as a result in mcast_dev_flush. 7) Because we need the mutex to synchronize mcast->mc, we can no longer call mcast_sendonly_join directly from mcast_send and instead must add sendonly join processing to the mcast_join_task A number of different races are resolved with these changes. These races existed with the old MCAST_FLAG_BUSY usage, the MCAST_JOIN_STARTED flag was an attempt to address them, and while it helped, a determined effort could still trip things up. One race looks something like this: Thread 1 Thread 2 ib_sa_join_multicast (as part of running restart mcast task) alloc member call callback ifconfig ib0 down wait_for_completion callback call completes wait_for_completion in mcast_dev_flush completes mcast->mc is PTR_ERR_OR_NULL so we skip ib_sa_leave_multicast return from callback return from ib_sa_join_multicast set mcast->mc = return from ib_sa_multicast We now have a permanently unbalanced join/leave issue that trips up the refcounting in core/multicast.c Another like this: Thread 1 Thread 2 Thread 3 ib_sa_multicast_join ifconfig ib0 down priv->broadcast = NULL join_complete wait_for_completion mcast->mc is not yet set, so don't clear return from ib_sa_join_multicast and set mcast->mc complete return -EAGAIN (making mcast->mc invalid) call ib_sa_multicast_leave on invalid mcast->mc, hang forever By holding the mutex around ib_sa_multicast_join and taking the mutex early in the callback, we force mcast->mc to be valid at the time we run the callback. This allows us to clear mcast->mc if there is an error and the join is going to fail. We do this before we complete the mcast. In this way, mcast_dev_flush always sees consistent state in regards to mcast->mc membership at the time that the wait_for_completion() returns. Signed-off-by:Doug Ledford <dledford@redhat.com> Signed-off-by:
Roland Dreier <roland@purestorage.com>
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