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.
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() ?
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: