linux-kernel - Re: [RFC][PATCH 12/13] stop

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Date:	Tue, 23 Jun 2015 12:55:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	paulmck@...ux.vnet.ibm.com, tj@...nel.org, mingo@...hat.com,
	linux-kernel@...r.kernel.org, der.herr@...r.at, dave@...olabs.net,
	riel@...hat.com, viro@...IV.linux.org.uk,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH 12/13] stop_machine: Remove lglock

On Tue, Jun 23, 2015 at 12:09:32PM +0200, Peter Zijlstra wrote:
> We can of course slap a percpu-rwsem in, but I wonder if there's
> anything smarter we can do here.

Urgh, we cannot use percpu-rwsem here, because that would require
percpu_down_write_trylock(), and I'm not sure we can get around the
sync_sched() for that.

Now try_stop_cpus(), which requires the down_write_trylock() is used to
implement synchronize_sched_expedited().

Using sync_sched() to implement sync_sched_expedited would make me
happy, but it does somewhat defeat the purpose.

Also, I think _expedited is used too eagerly, look at this:

+void dm_sync_table(struct mapped_device *md)
+{
+       synchronize_srcu(&md->io_barrier);
+       synchronize_rcu_expedited();
+}

sync_srcu() is slow already, why then bother with an
sync_rcu_expedited() :/
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