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

[Sergey Fedorov]

On 11/07/16 17:03, Paolo Bonzini wrote:
>
> On 11/07/2016 15:06, Sergey Fedorov wrote:
>> On 08/07/16 23:52, Paolo Bonzini wrote:
>>> On 08/07/2016 22:24, Sergey Fedorov wrote:
>>>> I remember, I've just found that we discussed it in this thread:
>>>>
>>>> http://thread.gmane.org/gmane.comp.emulators.qemu/401723/focus=406852
>>>>
>>>> I was thinking of just doing 'tb_jmp_cache' lookup out of the lock, not
>>>> tb_find_physical(). Now thanks to QHT, we could do tb_find_physical()
>>>> out of the lock, too. This changes things.
>>> But in my patch ("tcg: move tb_find_fast outside the tb_lock critical
>>> section", which originally was written by Fred---most of my contribution
>>> was getting the invalidation right, not the lock-free lookup)
>>> tb_find_physical was also done out of the lock.  It was then retried
>>> inside the lock, if it failed.
>>>
>>> This is why I needed to fail all concurrent lookups as the first step in
>>> the invalidation.
>>>
>>> Emilio's QHT resulted in a rewrite of tb_find_physical, but the basic
>>> concepts are the same.
>> That could work, I think, if we re-check under tb_lock whether the TB is
>> still valid before adding a direct jump to it.
> Right, this can still happen:
>
>       tb_find_fast                    tb_phys_invalidate
>                                        tb_lock
>        jmp_cache miss
>        -> tb_find_slow
>         -> tb_find_physical
>          QHT hit
>        tb_lock
>                                        invalidate tb->pc
>                                        remove from lists
>                                        tb_unlock
>        tb_add_jump
>        tb_unlock
>
> I seem to recall that Emilio added a seqlock for this purpose, but
> adding a tb_check_invalidated(TranslationBlock *tb) inline function will
> also do.

He used seqlock for 'tb_jmp_cache' only:
http://thread.gmane.org/gmane.comp.emulators.qemu/356765/focus=356774

He also added a dedicated field into TranslationBlock struction to mark
it invalid:
http://thread.gmane.org/gmane.comp.emulators.qemu/356765/focus=356785

Kind regards,
Sergey