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

[Jason Gunthorpe <jgg@ziepe.ca>]

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