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

[Paolo Bonzini]

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.

Paolo