block: don't use non-syncing event blocking in disk_check_events()

This patch is part of fix for triggering of WARN_ON_ONCE() in
disk_clear_events() reported in bug#34662.

  https://bugzilla.kernel.org/show_bug.cgi?id=34662



disk_clear_events() blocks events, schedules and flushes the event
work.  It expects the work to have started execution on schedule and
finished on return from flush.  WARN_ON_ONCE() triggers if the event
work hasn't executed as expected.  This problem happens because
__disk_block_events() fails to guarantee that the event work item is
not in flight on return from the function in race-free manner.  The
problem is two-fold and this patch addresses one of them.

When __disk_block_events() is called with @sync == %false, it bumps
event block count, calls cancel_delayed_work() and return.  This makes
it impossible to guarantee that event polling is not in flight on
return from syncing __disk_block_events() - if the first blocker was
non-syncing, polling could still be in progress and later syncing ones
would assume that the first blocker already canceled it.

Making __disk_block_events() cancel_sync regardless of block count
isn't feasible either as it may race with forced event checking in
disk_clear_events().

As disk_check_events() is the only user of non-syncing
__disk_block_events(), updating it to directly cancel and schedule
event work is the easiest way to solve the issue.

Note that there's another bug in __disk_block_events() and this patch
doesn't fix the issue completely.  Later patch will fix the other bug.

Signed-off-by: Tejun Heo <tj@kernel.org>
Tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Reported-by: Borislav Petkov <bp@alien8.de>
Reported-by: Meelis Roos <mroos@linux.ee>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>