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...