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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Benjamin Segall' <bsegall@google.com>, Hui Su <sh_def@163.com>
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Subject: RE: [PATCH] sched/fair: remove the spin_lock operations
Thread-Topic: [PATCH] sched/fair: remove the spin_lock operations
Thread-Index: AQHWru02j9v+7b8fw0CwtF3P4oRlh6mwtROg
Date:   Fri, 30 Oct 2020 22:16:29 +0000
Message-ID: <22f99ee1d9b245c2a356d4d555b54e6a@AcuMS.aculab.com>
References: <20201030144621.GA96974@rlk> <xm26mu0335zz.fsf@google.com>
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From: Benjamin Segall
> Sent: 30 October 2020 18:48
> 
> Hui Su <sh_def@163.com> 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@google.com>
> 
> (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

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