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SubjectRe: RT and Cascade interrupts
Oleg Nesterov wrote:
> CPU_0 CPU_1
> rpc_release_task(task)
> __run_timers:
> timer->base = NULL;
> rpc_delete_timer()
> if (timer->base)
> // not taken
> del_timer_sync();
> mempool_free(task);
> timer->function(task):
> // task already freed/reused
> __rpc_default_timer(task);

Ah ok, I was misreading the above. Now I see your point.
The race if hit loses the synchronization provided by
del_timer_sync() in my case. Thanks for being persistent.

The other possibility to fix the original problem within
the scope of the RPC code was to replace the bit state of
RPC_TASK_HAS_TIMER with a count so we don't obscure the
fact the timer was requeued during the preemption window.
This happens as rpc_run_timer() does an unconditional
clear_bit(RPC_TASK_HAS_TIMER,..) before returning.

Another would be to check timer->base/timer_pending()
in rpc_run_timer() and to omit doing a clear_bit()
if the timer is found to have been requeued.

The former has simpler locking requirements but requires
an rpc_task data structure modification. The latter does
not and is a bit more intuitive but needs additional
locking. Both appear to provide a solution though I
haven't yet attempted either.

Anyone with more ownership of this code care to comment
on a preferred fix?


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