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

From: David Daney
Date: Thu Jan 21 2010 - 14:19:30 EST

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;

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:

if (do_i_need_to_sleep())

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

David Daney
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