> Subject: Re: [PATCH for-rc] IB/hfi1: Insure pq is not left on waitlist > > 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. > It is not actually doing that. The lock only protects the list_empty(). > > 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 > So are you suggesting the list_empty() can be uncontrolled? Perhaps list_empty_careful() is a better choice? > 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? > I suggest we fix the bug and submit a follow-up to clean the locking up and the open coding. The patch footprint would probably be too large for stable as a single patch. Mike
