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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. Reviewed-by: Omar Sandoval <osandov@fb.com> Signed-off-by: David Jeffery <djeffery@redhat.com> Reported-by: John Pittman <jpittman@redhat.com> Tested-by: John Pittman <jpittman@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.