On Thu, Jun 30, 2005 at 09:26:07AM +1000, Benjamin Herrenschmidt wrote: > > > 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; > > Ugh ? Can switch_mm() be preempted at all ? Did I miss yet another > "let's open 10 gazillion races for gun" Ingo patch ?
Doh nope it can't - my bad. > > 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 the race is only when destroy_context() is preempted, but maybe > I missed something. Nope, I think you are right. My "theory" is obviously flawed now. There seem to be several contexts where destroy_context() could be called with preempt enabled - I should have been shutup in the first place :) Lets wait for Guillaume to test...
