linux-kernel - Re: Lots of bugs with current->state = TASK

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Date:	Thu, 21 Jan 2010 11:18:50 -0800
From:	David Daney <ddaney@...iumnetworks.com>
To:	rostedt@...dmis.org
CC:	LKML <linux-kernel@...r.kernel.org>,
	kernel-janitors <kernel-janitors@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch@...r.kernel.org, Greg KH <greg@...ah.com>,
	Andy Whitcroft <apw@...onical.com>,
	Ralf Baechle <ralf@...ux-mips.org>,
	linux-mips <linux-mips@...ux-mips.org>
Subject: Re: Lots of bugs with current->state = TASK_*INTERRUPTIBLE

Steven Rostedt wrote:
> Peter Zijlstra and I were doing a look over of places that assign
> current->state = TASK_*INTERRUPTIBLE, by simply looking at places with:
> 
>  $ git grep -A1 'state[[:space:]]*=[[:space:]]*TASK_[^R]'
> 
> and it seems there are quite a few places that looks like bugs. To be on
> the safe side, everything outside of a run queue lock that sets the
> current state to something other than TASK_RUNNING (or dead) should be
> using set_current_state().
> 
> 	current->state = TASK_INTERRUPTIBLE;
> 	schedule();
> 
> is probably OK, but it would not hurt to be consistent. Here's a few
> examples of likely bugs:
> 
[...]

This may be a bit off topic, but exactly which type of barrier should 
set_current_state() be implying?

On MIPS, set_mb() (which is used by set_current_state()) has a full mb().

Some MIPS based processors have a much lighter weight wmb().  Could 
wmb() be used in place of mb() here?

If not, an explanation of the required memory ordering semantics here 
would be appreciated.

I know the documentation says:

     set_current_state() includes a barrier so that the write of
     current->state is correctly serialised wrt the caller's subsequent
     test of whether to actually sleep:

  	set_current_state(TASK_UNINTERRUPTIBLE);
  	if (do_i_need_to_sleep())
  		schedule();


Since the current CPU sees the memory accesses in order, what can be 
happening on other CPUs that would require a full mb()?


Thanks,
David Daney
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