On Fri, Mar 20, 2020 at 12:26:32AM +0000, Marciniszyn, Mike wrote: > > 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(). Which is now running concurrently with list_del - fortunately list_empty() is safe to run unlocked. > > > 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? Yes. list_empty() is defined to work for RCU readers, so it is safe to call unlocked. > Perhaps list_empty_careful() is a better choice? The comments for this say it is not safe if list_add is happening concurrently. list_empty has a single concurrent safe READ_ONCE. > > > > 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. Yes, but I still can't send this to Linus without a commit message explaining why it is like this. Like I say, protecting list calls with seqlock does not make any sense. > The patch footprint would probably be too large for stable as a single patch. Yes Jason
