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 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.

Kind regards,
Sergey