Re: [Qemu-devel] [PATCH v2 10/11] tcg/mips: Make direct jump patching thread-safe

[Aurelien Jarno <aurelien@aurel32.net>]

On 2016-04-22 19:08, Sergey Fedorov wrote:
> From: Sergey Fedorov <serge.fdrv@gmail.com>
> 
> Ensure direct jump patching in MIPS is atomic by using
> atomic_read()/atomic_set() for code patching.
> 
> Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
> Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
> ---
> 
> Changes in v2:
>  * s/atomic_write/atomic_set/
> 
>  tcg/mips/tcg-target.inc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/tcg/mips/tcg-target.inc.c b/tcg/mips/tcg-target.inc.c
> index 682e19897db0..cefc0398018a 100644
> --- a/tcg/mips/tcg-target.inc.c
> +++ b/tcg/mips/tcg-target.inc.c
> @@ -1886,6 +1886,7 @@ static void tcg_target_init(TCGContext *s)
>  void tb_set_jmp_target1(uintptr_t jmp_addr, uintptr_t addr)
>  {
>      uint32_t *ptr = (uint32_t *)jmp_addr;
> -    *ptr = deposit32(*ptr, 0, 26, addr >> 2);
> +    uint32_t insn = atomic_read(ptr);
> +    atomic_set(ptr, deposit32(insn, 0, 26, addr >> 2));
>      flush_icache_range(jmp_addr, jmp_addr + 4);

Does it really make sense to read and write the value atomically? The
resulting operation is still not atomic, something can happen in
between.

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net