[PATCH 0/2] blk-mq: switch from preempt_disable/enable to get/put_cpu

[Paolo Bonzini]

blk-mq is using preempt_disable/enable in order to ensure that the
queue runners are placed on the right CPU.  This does not work with
the RT patches, because __blk_mq_run_hw_queue takes a non-raw
spinlock with the preemption-disabled region.  If there is contention
on the lock, this violates the rules for preemption-disabled regions.

While this could be fixed easily within the RT patches just by doing
migrate_disable/enable (note: this was not tested :)), we can do
better.  The first patch concentrates the preempt_disable/enable in
a single place in blk_mq_run_hw_queue, and also avoids useless calls
when the caller wants to start __blk_mq_run_hw_queue asynchronously.
(There is already a call to __blk_mq_run_hw_queue that does not disable
preemption in blk-flush.c; not coincidentially, it passes async=true).

Once this is done, it is trivial to use get/put_cpu instead of
preempt_disable/smp_processor_id/preempt_enable, which is what the
second patch does.  The RT patches then can change this to use
get_cpu_light.

With these changes (and the additional switch to get_cpu_light),
virtio-blk can be used again with RT kernels.

Paolo

Paolo Bonzini (2):
  blk_mq: call preempt_disable/enable in blk_mq_run_hw_queue, and only
    if needed
  blk-mq: use get_cpu/put_cpu instead of preempt_disable/preempt_enable

 block/blk-mq.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

-- 
2.1.0

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