Re: [Qemu-devel] [PATCH 2/2] cpu-exec: remove tb_lock from the hot-path

[Emilio G. Cota]

On Fri, Jul 01, 2016 at 17:16:10 +0100, Alex Bennée wrote:
> Lock contention in the hot path of moving between existing patched
> TranslationBlocks is the main drag in multithreaded performance. This
> patch pushes the tb_lock() usage down to the two places that really need
> it:
> 
>   - code generation (tb_gen_code)
>   - jump patching (tb_add_jump)
> 
> The rest of the code doesn't really need to hold a lock as it is either
> using per-CPU structures, atomically updated or designed to be used in
> concurrent read situations (qht_lookup).
> 
> To keep things simple I removed the #ifdef CONFIG_USER_ONLY stuff as the
> locks become NOPs anyway until the MTTCG work is completed.

>From a scalability point of view it would be better to have a single
critical section.

>From a correctness point of view, we're reading tb->page_addr[1]
without holding a lock. This field is set after qht_insert(tb),
so we might read a yet-uninitialized value.

I propose to just extend the critical section, like we used to
do with tcg_lock_reset.

                Emilio