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Re: Lots of bugs with current->state = TASK_*INTERRUPTIBLE

To: rostedt@goodmis.org
Subject: Re: Lots of bugs with current->state = TASK_*INTERRUPTIBLE
From: David Daney <ddaney@caviumnetworks.com>
Date: Thu, 21 Jan 2010 11:18:50 -0800
Cc: LKML <linux-kernel@vger.kernel.org>, kernel-janitors <kernel-janitors@vger.kernel.org>, Peter Zijlstra <peterz@infradead.org>, Andrew Morton <akpm@linux-foundation.org>, linux-arch@vger.kernel.org, Greg KH <greg@kroah.com>, Andy Whitcroft <apw@canonical.com>, Ralf Baechle <ralf@linux-mips.org>, linux-mips <linux-mips@linux-mips.org>
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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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