Re: [Qemu-devel] Using aio_poll for timer carrier threads

[Paolo Bonzini]

Il 13/08/2013 16:54, Jan Kiszka ha scritto:
>> > Using an AioContext lock for timers is somewhat complicated for lock
>> > ordering, because context A could try to modify a timer from context B,
>> > at the same time when context B is modifying a timer from context A.
>> > This would cause a deadlock.
> That's like MMIO access on device A triggers MMIO access on B and vice
> versa - why should we need this, so why should we support this? I think
> the typical case is that timers (and their lists) and data structures
> they access have a fairly close relation, thus can reuse the same lock.

Yes, that's true.  Still it would have to be documented, and using
too-coarse locks risks having many BQLs, which multiplies the complexity
(fine-grained locking at least keeps critical sections small and limits
the amount of nested locking).

I like Stefan's patches to make the timer list thread-safe, especially
if we can optimize it (with RCU?) to make the read side lockless.

Paolo