Re: [PATCH] sbitmap: only queue kyber's wait callback if not already active

[Omar Sandoval <osandov@osandov.com>]

On Tue, Dec 17, 2019 at 11:00:24AM -0500, David Jeffery wrote:
> Under heavy loads where the kyber I/O scheduler hits the token limits for
> its scheduling domains, kyber can become stuck.  When active requests
> complete, kyber may not be woken up leaving the I/O requests in kyber
> stuck.
> 
> This stuck state is due to a race condition with kyber and the sbitmap
> functions it uses to run a callback when enough requests have completed.
> The running of a sbt_wait callback can race with the attempt to insert the
> sbt_wait.  Since sbitmap_del_wait_queue removes the sbt_wait from the list
> first then sets the sbq field to NULL, kyber can see the item as not on a
> list but the call to sbitmap_add_wait_queue will see sbq as non-NULL. This
> results in the sbt_wait being inserted onto the wait list but ws_active
> doesn't get incremented.  So the sbitmap queue does not know there is a
> waiter on a wait list.
> 
> Since sbitmap doesn't think there is a waiter, kyber may never be
> informed that there are domain tokens available and the I/O never advances.
> With the sbt_wait on a wait list, kyber believes it has an active waiter
> so cannot insert a new waiter when reaching the domain's full state.
> 
> This race can be fixed by only adding the sbt_wait to the queue if the
> sbq field is NULL.  If sbq is not NULL, there is already an action active
> which will trigger the re-running of kyber.  Let it run and add the
> sbt_wait to the wait list if still needing to wait.

So the race here is:

Thread 1                        Thread 2
kyber_domain_wake
  sbitmap_del_wait_queue
    list_del_init
    atomic_dec sbq->ws_active
                                kyber_get_domain_token
                                  list_empty_careful
                                  sbitmap_add_wait_queue
                                    if (!sqb_wait->sb) // false
                                    add_wait_queue
    sbq_wait->sbq = NULL

Now sbq_wait->sbq == NULL, sbq->ws_active = 0, and
!list_empty(domain_wait), so sbq_wake_ptr returns NULL and
sbitmap_queue_wake_up does nothing.

I get the feeling that sbitmap_{add,del}_wait_queue need some memory
barriers... But ignoring that, this fix seems right.

Reviewed-by: Omar Sandoval <osandov@fb.com>

P.S. s/sbt_wait/sbq_wait/g in the commit message.