Re: [PATCH for-rc] IB/hfi1: Insure pq is not left on waitlist

[Jason Gunthorpe <jgg@ziepe.ca>]

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