On 10/01, Peter Zijlstra wrote:
>
> On Tue, Sep 30, 2014 at 11:47:32PM +0200, Oleg Nesterov wrote:
>
> > > > This is minor, but this way CONFIG_DEBUG_ATOMIC_SLEEP will not imply
> > > > a subtle behavioural change.
> > >
> > > You mean the __set_current_state() that's extra?
> >
> > Yes, and note that it only does __set_current_state(RUNNING) if
> > CONFIG_DEBUG_ATOMIC_SLEEP. This means that disabling/enabling this
> > option can, silently hide/uncover a bug.
> >
> > > I would actually argue
> > > to keep that since it makes the 'problem' much worse.
> >
> > OK, I won't insist, but could you explain why the suggested change can
> > make the problem (and which problem ;) worse?
>
> Sure, so the trivial problem is not actually going to sleep in the outer
> wait primitive because the inner wait primitive reset ->state to
> TASK_RUNNING.
But this means that fixup_sleep() must not be used?
> So by always setting the ->state to TASK_RUNNING it never goes to sleep
> and it'll revert to spinning,
But I tried to suggest to not set TASK_RUNNING?
Peter, I am sorry for wasting your time, this is really minor, but still
I'd like to understand.
Let me try again. With this series we have
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
#define fixup_sleep() __set_current_state(TASK_RUNNING)
#else
#define fixup_sleep() do { } while (0)
#endif
and this means that we do not need __set_current_state(RUNNING) for
correctness, just we want to shut up the warning in __might_sleep().
This is fine (and the self-documenting helper is nice), but this means
that CONFIG_DEBUG_ATOMIC_SLEEP adds a subtle difference.
For example, let's suppose that we do not have 01/11 which fixes
mutex_lock(). Then this code
set_current_state(TASK_UNINTERRUPTIBLE);
...
fixup_sleep();
...
mutex_lock(some_mutex);
can hang, but only if !CONFIG_DEBUG_ATOMIC_SLEEP.
So perhaps it makes sense to redefine it
#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
#define fixup_sleep() (current->task_state_change = 0)
#else
#define fixup_sleep() do { } while (0)
#endif
and change __might_sleep()
- if (WARN(current->state != TASK_RUNNING,
+ if (WARN(current->state != TASK_RUNNING &&
current->task_state_change != 0,
?
Oleg.
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