Re: [Qemu-devel] [PATCH 3/3] tcg: Avoid bouncing tb_lock between tb_gen_code() and tb_add_jump()

[Sergey Fedorov]

On 08/07/16 14:02, Paolo Bonzini wrote:
>> On 08/07/16 11:40, Paolo Bonzini wrote:
>>> Even better: add a "bool *tb_locked" argument to tb_find_slow, and
>>> don't move the mmap_lock release.  Then tb_find_fast knows directly
>>> whether tb_lock is taken, and you don't need any of tb_lock_reset
>>> or mmap_lock_reset.
>> I think we can do even better. One option is using a separate tiny lock
>> to protect direct jump set/reset instead of tb_lock.
> If you have to use a separate tiny lock, you don't gain anything compared
> to the two critical sections, do you?

If we have a separate lock for direct jump set/reset then we can do fast
TB lookup + direct jump patching without taking tb_lock at all. How much
this would reduce lock contention largely depends on the workload we use.

>
> In any case, this seems to be more complex than necessary.  The "bool *"
> convention is pretty common in Linux for example---it works well.

It could work, no doubts.

>
> The one below is even more complicated.  I'm all for simple lock-free
> stacks (e.g. QSLIST_INSERT_HEAD_ATOMIC and QSLIST_MOVE_ATOMIC), but lock-free
> lists are too much, especially if with the complicated first/next mechanism
> of TCG's chained block lists.

Direct jump handling code is pretty isolated and self-contained. It
would require to back out of tb_remove_from_jmp_list() and sprinkle a
couple of atomic_rcu_read()/atomic_rcu_set() with some comments, I
think. Maybe it could be easier to justify looking at actual patches.

Thanks,
Sergey

>
> Paolo
>
>> Another option which I've had in my mind for some time is to make direct
>> jump set/reset thread-safe. We already have thread-safe TB patching. The
>> only question is the right order of operations and handling
>> jmp_list_next/jmp_list_first safely. I think that could be done by
>> removing tb_remove_from_jmp_list() and making RCU-like manipulation with
>> jmp_list_next/jmp_list_first. What do you think?
>>
>> Kind regards,
>> Sergey
>>