Benjamin Herrenschmidt wrote:

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

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 ?

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

Regards,
Guillaume.

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