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

["Marciniszyn, Mike" <mike.marciniszyn@intel.com>]

> 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