linux-kernel - Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12

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Message-ID: <20170419131553.cii62nl2si3lwm7w@hirez.programming.kicks-ass.net>
Date:   Wed, 19 Apr 2017 15:15:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, 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,
        marc.zyngier@....com
Subject: Re: [PATCH v2 tip/core/rcu 0/13] Miscellaneous fixes for 4.12

On Wed, Apr 19, 2017 at 06:02:45AM -0700, Paul E. McKenney wrote:
> On Wed, Apr 19, 2017 at 01:28:45PM +0200, Peter Zijlstra wrote:
> > 
> > So the thing Maz complained about is because KVM assumes
> > synchronize_srcu() is 'free' when there is no srcu_read_lock() activity.
> > This series 'breaks' that.
> > 
> > I've not looked hard enough at the new SRCU to see if its possible to
> > re-instate that feature.
> 
> And with the fix I gave Maz, the parallelized version is near enough
> to being free as well.  It was just a stupid bug on my part: I forgot
> to check for expedited when scheduling callbacks.

Right, although for the old SRCU it was true for !expedited as well.
Just turns out the KVM memslots crud already uses
synchronize_srcu_expedited().

<rant>without a friggin' comment; hate @expedited</rant>