Hi Guillaume, On Wed, Jun 29, 2005 at 11:32:19AM -0400, Guillaume Autran wrote: > > 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 ?
Execution is resumed exactly where it has been interrupted. > 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().
