linux-kernel - Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

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Date:	Tue, 9 Jan 2007 15:03:02 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	David Howells <dhowells@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...l.org>,
	linux-kernel@...r.kernel.org, Gautham shenoy <ego@...ibm.com>
Subject: Re: [PATCH] flush_cpu_workqueue: don't flush an empty ->worklist

On Mon, Jan 08, 2007 at 09:26:56PM -0800, Andrew Morton wrote:
> That's not correct.  freeze_processes() will freeze *all* processes.

I am not arguing whether all processes will be frozen. However my question was 
on the freeze point. Let me ask the question with an example:

rtasd thread (arch/powerpc/platforms/pseries/rtasd.c) executes this simple
loop:

static int rtasd(void *unused)
{

	i = first_cpu(cpu_online_map);

	while (1) {

		set_cpus_allowed(current, cpumask_of_cpu(i));	/* can block */

		/* we should now be running on cpu i */

		do_something_on_a_cpu(i);

		/* sleep for some time */

		i = next_cpu(cpu, cpu_online_map);
	}

}

This thread makes absolutely -no- calls to try_to_freeze() in its lifetime.

1. Does this mean that the thread can't be frozen? (lets say that the
   thread's PF_NOFREEZE is not set)

   AFAICS it can still be frozen by sending it a signal and have the signal 
   delivery code call try_to_freeze() ..

2. If the thread can be frozen at any arbitrary point of its execution, then I 
   dont see what prevents cpu_online_map from changing under the feet of rtasd 
   thread, 

    In other words, we would have failed to provide the ability to *block* 
    hotplug operations from happening concurrently.

>  All of them are forced to enter refrigerator(). 
	           ^^^^^^

*forced*, yes ..that's the point of concern ..

Warm regards,
vatsa
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