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

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

[David Daney]

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

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

[Steven Rostedt]

On Thu, 2010-01-21 at 11:18 -0800, David Daney wrote:
> 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?

Nope, wmb() is not enough. Below is an explanation.

> 
> 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()?

Lets look at a hypothetical situation with:

	add_wait_queue();
	current->state = TASK_UNINTERRUPTIBLE;
	smp_wmb();
	if (!x)
		schedule();



Then somewhere we probably have:

	x = 1;
	smp_wmb();
	wake_up(queue);



	   CPU 0			   CPU 1
	------------			-----------
	add_wait_queue();
	(cpu pipeline sees a load
	 of x ahead, and preloads it)
					x = 1;
					smp_wmb();
					wake_up(queue);
					(task on CPU 0 is still at
					 TASK_RUNNING);

	current->state = TASK_INTERRUPTIBLE;
	smp_wmb(); <<-- does not prevent early loading of x
	if (!x)  <<-- returns true
		schedule();

Now the task on CPU 0 missed the wake up.

Note, places that call schedule() are not fast paths, and probably not
called often. Adding the overhead of smp_mb() to ensure correctness is a
small price to pay compared to search for why you have a stuck task that
was never woken up.

Read Documentation/memory-barriers.txt, it will be worth the time you
spend doing so.

-- Steve

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

[David Daney]

Steven Rostedt wrote:
> On Thu, 2010-01-21 at 11:18 -0800, David Daney wrote:
>> 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?
> 
> Nope, wmb() is not enough. Below is an explanation.
> 
>> 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()?
> 
> Lets look at a hypothetical situation with:
> 
> 	add_wait_queue();
> 	current->state = TASK_UNINTERRUPTIBLE;
> 	smp_wmb();
> 	if (!x)
> 		schedule();
> 
> 
> 
> Then somewhere we probably have:
> 
> 	x = 1;
> 	smp_wmb();
> 	wake_up(queue);
> 
> 
> 
> 	   CPU 0			   CPU 1
> 	------------			-----------
> 	add_wait_queue();
> 	(cpu pipeline sees a load
> 	 of x ahead, and preloads it)


This is what I thought.

My cpu (Cavium Octeon) does not have out of order reads, so my wmb() is 
in fact a full mb() from the point of view of the current CPU.  So I 
think I could weaken my bariers in set_current_state() and still get 
correct operation.  However as you say...


> 					x = 1;
> 					smp_wmb();
> 					wake_up(queue);
> 					(task on CPU 0 is still at
> 					 TASK_RUNNING);
> 
> 	current->state = TASK_INTERRUPTIBLE;
> 	smp_wmb(); <<-- does not prevent early loading of x
> 	if (!x)  <<-- returns true
> 		schedule();
> 
> Now the task on CPU 0 missed the wake up.
> 
> Note, places that call schedule() are not fast paths, and probably not
> called often. Adding the overhead of smp_mb() to ensure correctness is a
> small price to pay compared to search for why you have a stuck task that
> was never woken up.

... It may not be worth the trouble.


> 
> Read Documentation/memory-barriers.txt, it will be worth the time you
> spend doing so.

Indeed I have read it.  My questions arise because the semantics of my 
barrier primitives do not map exactly to the semantics prescribed for 
mb() and wmb().

A kernel programmer has only the types of barriers described in 
memory-barriers.txt available.  Since there is no 
mb_on_current_cpu_but_only_order_writes_as_seen_by_other_cpus(), we use 
  a full mb() instead.


Thanks for the explanation Steve,

David Daney

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

[Steven Rostedt]

On Thu, 2010-01-21 at 11:57 -0800, David Daney wrote:

> >> Since the current CPU sees the memory accesses in order, what can be 
> >> happening on other CPUs that would require a full mb()?
> > 
> > Lets look at a hypothetical situation with:
> > 
> > 	add_wait_queue();
> > 	current->state = TASK_UNINTERRUPTIBLE;
> > 	smp_wmb();
> > 	if (!x)
> > 		schedule();
> > 
> > 
> > 
> > Then somewhere we probably have:
> > 
> > 	x = 1;
> > 	smp_wmb();
> > 	wake_up(queue);
> > 
> > 
> > 
> > 	   CPU 0			   CPU 1
> > 	------------			-----------
> > 	add_wait_queue();
> > 	(cpu pipeline sees a load
> > 	 of x ahead, and preloads it)
> 
> 
> This is what I thought.
> 
> My cpu (Cavium Octeon) does not have out of order reads, so my wmb() is 

Can you have reads that are out of order wrt writes? Because the above
does not have out of order reads. It just had a read that came before a
write. The above code could look like:

(hypothetical assembly language)

	ld r2, TASK_UNINTERRUPTIBLE
	st r2, (current->state)
	wmb
	ld r1, (x)
	cmp r1, 0

Is it possible for the CPU to do the load of r1 before storing r2? If
so, then the bug still exists.

-- Steve


> in fact a full mb() from the point of view of the current CPU.  So I 
> think I could weaken my bariers in set_current_state() and still get 
> correct operation.  However as you say...
> 

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

[David Daney]

>>
>> This is what I thought.
>>
>> My cpu (Cavium Octeon) does not have out of order reads, so my wmb() is 
> 
> Can you have reads that are out of order wrt writes? Because the above
> does not have out of order reads. It just had a read that came before a
> write. The above code could look like:
> 
> (hypothetical assembly language)
> 
> 	ld r2, TASK_UNINTERRUPTIBLE
> 	st r2, (current->state)
> 	wmb
> 	ld r1, (x)
> 	cmp r1, 0
> 
> Is it possible for the CPU to do the load of r1 before storing r2? If
> so, then the bug still exists.
> 

Indeed it is.  Lockless operations make my head hurt.

Thanks for clarifying.

David Daney



> -- Steve
> 
> 
>> in fact a full mb() from the point of view of the current CPU.  So I 
>> think I could weaken my bariers in set_current_state() and still get 
>> correct operation.  However as you say...
>>
> 
> 
>