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Date: Fri, 8 Jul 2016 04:40:05 -0400 (EDT)
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 3/3] tcg: Avoid bouncing tb_lock between
 tb_gen_code() and tb_add_jump()
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To: Sergey Fedorov <sergey.fedorov@linaro.org>
Cc: qemu-devel@nongnu.org, Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>, patches@linaro.org, Sergey Fedorov <serge.fdrv@gmail.com>, mttcg@greensocs.com, fred konrad <fred.konrad@greensocs.com>, a rigo <a.rigo@virtualopensystems.com>, cota@braap.org, bobby prani <bobby.prani@gmail.com>, rth@twiddle.net, mark burton <mark.burton@greensocs.com>, jan kiszka <jan.kiszka@siemens.com>, peter maydell <peter.maydell@linaro.org>, claudio fontana <claudio.fontana@huawei.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>


> diff --git a/cpu-exec.c b/cpu-exec.c
> index dd0bd5007701..54c935039592 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -295,7 +295,8 @@ static TranslationBlock *tb_find_slow(CPUState *cpu,
>  
>          /* 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.
> +         * single threaded the locks are NOPs. Both locks are to be
> +         * released at the end of tb_find_fast().
>           */

Even better: add a "bool *tb_locked" argument to tb_find_slow, and
don't move the mmap_lock release.  Then tb_find_fast knows directly
whether tb_lock is taken, and you don't need any of tb_lock_reset
or mmap_lock_reset.

Thanks,

Paolo

>          mmap_lock();
>          tb_lock();
> @@ -308,9 +309,6 @@ static TranslationBlock *tb_find_slow(CPUState *cpu,
>              /* if no translated code available, then translate it now */
>              tb = tb_gen_code(cpu, pc, cs_base, flags, 0);
>          }
> -
> -        tb_unlock();
> -        mmap_unlock();
>      }
>  
>      /* We add the TB in the virtual pc hash table for the fast lookup */
> @@ -354,10 +352,15 @@ 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();
> +        if (!tb_lock_locked()) {
> +            tb_lock();
> +        }
>          tb_add_jump(*last_tb, tb_exit, tb);
> -        tb_unlock();
>      }
> +
> +    tb_lock_reset();
> +    mmap_lock_reset();
> +
>      return tb;
>  }
>  
> --
> 1.9.1
> 
>