linux-kernel - Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging normal ones

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Date:	Wed, 1 Jul 2015 16:17:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	josh@...htriplett.org, linux-kernel@...r.kernel.org,
	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, dvhart@...ux.intel.com, fweisbec@...il.com,
	oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH RFC tip/core/rcu 0/5] Expedited grace periods encouraging
 normal ones

On Wed, Jul 01, 2015 at 07:00:31AM -0700, Paul E. McKenney wrote:

> That is a bit extreme, Peter.

Of course; but I'm really not seeing people taking due care with them

> Are a huge pile of them coming in this merge window or something?
> What raised your concerns on this issue?

This is complete horse manure (breaking the nvidiot binary blob is a
good thing):

74b51ee152b6 ("ACPI / osl: speedup grace period in acpi_os_map_cleanup")

Also, I'm not entirely convinced things like:

fd2ed4d25270 ("dm: add statistics support")
83d5e5b0af90 ("dm: optimize use SRCU and RCU")
ef3230880abd ("backing-dev: use synchronize_rcu_expedited instead of synchronize_rcu")

Are in the 'never' happens category. Esp. the backing-dev one, it
triggers every time you unplug a USB stick or similar.

Rejigging a DM might indeed be rare enough; but then again, people use
DM explicitly so they can rejig while in operation.

Also, they really do not explain how expedited really is the only option
available. Why things can't be batched etc..
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