On Thu, Mar 19, 2020 at 09:46:54PM +0000, Marciniszyn, Mike wrote: > > Subject: Re: [PATCH for-rc] IB/hfi1: Insure pq is not left on waitlist > > > > The only place that uses seqlock in infiniband is in hfi1 > > > > It only calls seqlock_init and write_seqlock > > > > Never read_seqlock > > The sdma code uses read_seqbegin() and read_seq_retry() to avoid the spin > that is in that is in read_seqlock(). Hm, I see.. I did not find these uses when I was grepping, but now I'm even less happy with this :( > The two calls together allow for detecting a race where the > interrupt handler detects if the base level submit routines > have enqueued to a waiter list due to a descriptor shortage > concurrently with the this interrupt handler. You can't use read seqlock to protect a linked list when the write side is doing list_del. It is just wrong. > The full write_seqlock() is gotten when the list is not empty and the > req_seq_retry() detects when a list entry might have been added. A write side inside a read_side? It is maddness. > SDMA interrupts frequently encounter no waiters, so the lock only slows > down the interrupt handler. So, if you don't care about the race with adding then just use list_empty with no lock and then a normal spin lock All this readlock stuff doesn't remove any races. > > Please clean this mess too. > > The APIs associated with SDMA and iowait are pretty loose and we > will clean the up in a subsequent patch series. The nature of the locking > should not bleed out to the client code of SDMA. We will adjust the > commit message to indicate this. So what is the explanation here? This uses a write seqlock for a linked list but it is OK because nothing uses the read side except to do list_empty, which is unnecessary, and will be fixed later? Jason
