linux-kernel - Re: [PATCH] mm: use srcu for shrinkers

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Message-ID: <1434591216.1903.44.camel@stgolabs.net>
Date:	Wed, 17 Jun 2015 18:33:36 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: use srcu for shrinkers

On Wed, 2015-06-17 at 09:47 +0200, Michal Hocko wrote:
> On the other hand using srcu is a neat idea. Shrinkers only need the
> existence guarantee when racing with unregister. Register even shouldn't
> be that interesting because such a shrinker wouldn't have much to
> shrink anyway so we can safely miss it AFAIU. With the srcu read lock
> we can finally get rid of the try_lock. I do not think you need an
> ugly spin_is_locked as the replacement though. We have the existence
> guarantee and that should be sufficient.

So the reason for the spin_is_locked check was that I was concerned
about new reader(s) that come in while doing the registry. Currently
this is forbidden by the trylock and fake-ish retry. But yes, perhaps I
was being over safe and we shouldn't be blockling the reclaim simply
because a shrinker is registering. And it would be cleaner to get rid of
the whole retry idea and just use rcu guarantees.

This is probably a little late in the game to try to push for 4.2, so
I'll send a v2 with any other updates that might come up once the merge
window closes.

Thanks,
Davidlohr

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