Hi Marcelo,

Marcelo Tosatti wrote:

Hi Guillaume,

On Wed, Jun 29, 2005 at 11:32:19AM -0400, Guillaume Autran wrote:
 

quoted

Benjamin Herrenschmidt wrote:

   

quoted

On Tue, 2005-06-28 at 09:42 -0400, Guillaume Autran wrote:


     

quoted

Hi,

I happen to notice a race condition in the mmu_context code for the 8xx 
with very few context (16 MMU contexts) and kernel preemption enable. It 
is hard to reproduce has it shows only when many processes are 
created/destroy and the system is doing a lot of IRQ processing.

In short, one process is trying to steal a context that is in the 
process of being freed (mm->context == NO_CONTEXT) but not completely 
freed (nr_free_contexts == 0).
The steal_context() function does not do anything and the process stays 
in the loop forever.

Anyway, I got a patch that fixes this part. Does not seem to affect 
scheduling latency at all.

Comments are appreciated.
 

       

Your patch seems to do a hell lot more than fixing this race ... What
about just calling preempt_disable() in destroy_context() instead ?


     

I'm still a bit confused with "kernel preemption". One thing for sure is 
that disabling kernel preemption does indeed fix my problem.
So, my question is, what if a task in the middle of being schedule gets 
preempted by an IRQ handler, where will this task restart execution ? 
Back at the beginning of schedule or where it left of ?
   

Execution is resumed exactly where it has been interrupted.

In that case, what happen when a higher priority task steal the context 
of the lower priority task after get_mmu_context() but before 
set_mmu_context() ?
Then when the lower priority task resumes, its context may no longer be 
valid...
Do I get this right ?

quoted

The idea behind my patch was to get rid of that nr_free_contexts counter 
that is (I thing) redundant with the context_map.
   

Apparently its there to avoid the spinlock exactly on !FEW_CONTEXTS machines.

I suppose that what happens is that get_mmu_context() gets preempted after stealing
a context (so nr_free_contexts = 0), but before setting next_mmu_context to the 
next entry

next_mmu_context = (ctx + 1) & LAST_CONTEXT;

So if the now running higher prio tasks calls switch_mm() (which is likely to happen)
it loops forever on atomic_dec_if_positive(&nr_free_contexts), while steal_context()
sees "mm->context == CONTEXT".

I think that you should try "preempt_disable()/preempt_enable" pair at entry and 
exit of get_mmu_context() - I suppose around destroy_context() is not enough (you 
can try that also).

spinlock ends up calling preempt_disable().

 

I'm going to do like this instead of my previous attempt:

        /* Setup new userspace context */
        preempt_disable();
        get_mmu_context(next);
        set_context(next->context, next->pgd);
        preempt_enable();

To make sure we don't loose our context in between.



Thanks.
Guillaume.

-- 
=======================================
Guillaume Autran
Senior Software Engineer
MRV Communications, Inc.
Tel: (978) 952-4932 office
E-mail: gautran@mrv.com
=======================================