Re: [Qemu-devel] [PATCH v2 3/6] tcg: cpu-exec: remove tb_lock from the hot-path

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 05/07/16 19:18, 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.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Reviewed-by: Richard Henderson <rth@twiddle.net>
>
> ---
> v3 (base-patches)
>   - fix merge conflicts with Sergey's patch
> v1 (hot path, split from base-patches series)
>   - revert name tweaking
>   - drop test jmp_list_next outside lock
>   - mention lock NOPs in comments
> v2 (hot path)
>   - Add r-b tags
> ---
>  cpu-exec.c | 48 +++++++++++++++++++++---------------------------
>  1 file changed, 21 insertions(+), 27 deletions(-)
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 10ce1cb..dd0bd50 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -291,35 +291,29 @@ static TranslationBlock *tb_find_slow(CPUState *cpu,
>       * Pairs with smp_wmb() in tb_phys_invalidate(). */
>      smp_rmb();
>      tb = tb_find_physical(cpu, pc, cs_base, flags);
> -    if (tb) {
> -        goto found;
> -    }
> +    if (!tb) {
>  
> -#ifdef CONFIG_USER_ONLY
> -    /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
> -     * taken outside tb_lock.  Since we're momentarily dropping
> -     * tb_lock, there's a chance that our desired tb has been
> -     * translated.
> -     */
> -    tb_unlock();
> -    mmap_lock();
> -    tb_lock();
> -    tb = tb_find_physical(cpu, pc, cs_base, flags);
> -    if (tb) {
> -        mmap_unlock();
> -        goto found;
> -    }
> -#endif
> +        /* mmap_lock is needed by tb_gen_code, and mmap_lock must be
> +         * taken outside tb_lock. As system emulation is currently
> +         * single threaded the locks are NOPs.
> +         */
> +        mmap_lock();
> +        tb_lock();
>  
> -    /* if no translated code available, then translate it now */
> -    tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
> +        /* There's a chance that our desired tb has been translated while
> +         * taking the locks so we check again inside the lock.
> +         */
> +        tb = tb_find_physical(cpu, pc, cs_base, flags);
> +        if (!tb) {
> +            /* if no translated code available, then translate it now */
> +            tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
> +        }
>  
> -#ifdef CONFIG_USER_ONLY
> -    mmap_unlock();
> -#endif
> +        tb_unlock();
> +        mmap_unlock();
> +    }
>  
> -found:
> -    /* we add the TB in the virtual pc hash table */
> +    /* We add the TB in the virtual pc hash table for the fast lookup */
>      atomic_set(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)], tb);

There can be a race with tb_phys_invalidate() if we do this out of
tb_lock. Suppose:
(1) Thread 1: The first tb_find_phys() out of the lock is successful
(2) Thread 2: Remove the TB from the global hash table with qht_remove()
(3) Thread 2: Reset the 'tb_jmp_cache' entry
(4) Thread 1: Set the 'tb_jmp_cache' entry back (this line)

So it is only safe to do 'tb_jmp_cache' lookup out of lock, not to set it.

Kind regards,
Sergey

>      return tb;
>  }
> @@ -337,7 +331,6 @@ static inline TranslationBlock *tb_find_fast(CPUState *cpu,
>         always be the same before a given translated block
>         is executed. */
>      cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
> -    tb_lock();
>      tb = atomic_read(&cpu->tb_jmp_cache[tb_jmp_cache_hash_func(pc)]);
>      if (unlikely(!tb || tb->pc != pc || tb->cs_base != cs_base ||
>                   tb->flags != flags)) {
> @@ -361,9 +354,10 @@ static inline TranslationBlock *tb_find_fast(CPUState *cpu,
>  #endif
>      /* See if we can patch the calling TB. */
>      if (*last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
> +        tb_lock();
>          tb_add_jump(*last_tb, tb_exit, tb);
> +        tb_unlock();
>      }
> -    tb_unlock();
>      return tb;
>  }
>