RE: [PATCH] sched/fair: remove the spin_lock operations

[David Laight]

From: Benjamin Segall
> Sent: 30 October 2020 18:48
> 
> Hui Su <sh_def@xxxxxxx> writes:
> 
> > Since 'ab93a4bc955b ("sched/fair: Remove
> > distribute_running fromCFS bandwidth")',there is
> > nothing to protect between raw_spin_lock_irqsave/store()
> > in do_sched_cfs_slack_timer().
> >
> > So remove it.
> 
> Reviewed-by: Ben Segall <bsegall@xxxxxxxxxx>
> 
> (I might nitpick the subject to be clear that it should be trivial
> because the lock area is empty, or call them dead or something, but it's
> not all that important)

I don't know about this case, but a lock+unlock can be used
to ensure that nothing else holds the lock when acquiring
the lock requires another lock be held.

So if the normal sequence is:
	lock(table)
	# lookup item
	lock(item)
	unlock(table)
	....
	unlock(item)

Then it can make sense to do:
	lock(table)
	lock(item)
	unlock(item)
	....
	unlock(table)

although that ought to deserve a comment.

	avid

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)