linux-kernel - Re: [PATCH] slow_work_thread() should do the exclusive wait

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Message-ID: <14561.1239873018@redhat.com>
Date:	Thu, 16 Apr 2009 10:10:18 +0100
From:	David Howells <dhowells@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	dhowells@...hat.com, Oleg Nesterov <oleg@...hat.com>,
	Trond.Myklebust@...app.com, serue@...ibm.com, steved@...hat.com,
	viro@...iv.linux.org.uk, Daire.Byrne@...mestore.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slow_work_thread() should do the exclusive wait

Andrew Morton <akpm@...ux-foundation.org> wrote:

> The patch itself is a little worrisome.  The wake-all semantics are
> very good at covering up little race bugs.  And switching to wake-once
> is a great way of exposing hitherto-unsuspected races.

It's something I'm intending to test, once I get MN10300 working again (which
for some reason it isn't).

> I wonder if slow_work_cull_timeout() should have some sort of barrier,
> so the write is suitably visible to the woken thread.

That's an interesting question.  Should wake_up() imply a barrier of any sort,
I wonder.  Well, __wake_up() does impose a barrier as it uses a spinlock, but
I wonder if that's sufficient.

> Bearing in mind that the thread might _already_ have been woken by someone
> else?

If the thread is woken by someone else, there must be work for it to do, in
which case it wouldn't be culled anyway.

> off-topic: afacit the code will cull a maximum of one thread per five
> seconds.  But the rate of thread _creation_ is, afacit, unbound.  Are
> there scenarios in which we can get a runaway thread count?

The maximum number of threads is limited (slow_work_max_threads).

David
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