block: Shorten interrupt disabled regions
authorThomas Gleixner <tglx@linutronix.de>
Fri, 4 May 2018 14:32:47 +0000 (16:32 +0200)
committerJens Axboe <axboe@kernel.dk>
Mon, 7 May 2018 21:26:36 +0000 (15:26 -0600)
commit50864670b3ffce188e176822ac3dc0336f2489c7
treec03eba4af50d60d770e583a90f7786d48d23a562
parent656cb6d03e3df309d6e3954dc62f1d30f9462397
block: Shorten interrupt disabled regions

Commit 9c40cef2b799 ("sched: Move blk_schedule_flush_plug() out of
__schedule()") moved the blk_schedule_flush_plug() call out of the
interrupt/preempt disabled region in the scheduler. This allows to replace
local_irq_save/restore(flags) by local_irq_disable/enable() in
blk_flush_plug_list().

But it makes more sense to disable interrupts explicitly when the request
queue is locked end reenable them when the request to is unlocked. This
shortens the interrupt disabled section which is important when the plug
list contains requests for more than one queue. The comment which claims
that disabling interrupts around the loop is misleading as the called
functions can reenable interrupts unconditionally anyway and obfuscates the
scope badly:

 local_irq_save(flags);
   spin_lock(q->queue_lock);
   ...
   queue_unplugged(q...);
     scsi_request_fn();
       spin_unlock_irq(q->queue_lock);

-------------------^^^ ????

       spin_lock_irq(q->queue_lock);
     spin_unlock(q->queue_lock);
 local_irq_restore(flags);

Aside of that the detached interrupt disabling is a constant pain for
PREEMPT_RT as it requires patching and special casing when RT is enabled
while with the spin_*_irq() variants this happens automatically.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20110622174919.025446432@linutronix.de
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
block/blk-core.c