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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
In-reply-to: <1476107947-31430-3-git-send-email-pbonzini@redhat.com>
Date: Wed, 12 Oct 2016 09:44:17 +0100
Message-ID: <87zimanehq.fsf@linaro.org>
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Subject: Re: [Qemu-devel] [PATCH 2/5] cpus: use atomic_read to read
 seqlock-protected variables
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, cota@braap.org


Paolo Bonzini <pbonzini@redhat.com> writes:

> There is a data race if the variable is written concurrently to the
> read.  In C11 this has undefined behavior.  Use atomic_read.  The
> write side does not need atomic_set, because it is protected by a
> mutex.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

> ---
>  cpus.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/cpus.c b/cpus.c
> index b2fbe33..3fc2f6e 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -170,7 +170,8 @@ int64_t cpu_get_icount_raw(void)
>  static int64_t cpu_get_icount_locked(void)
>  {
>      int64_t icount = cpu_get_icount_raw();
> -    return timers_state.qemu_icount_bias + cpu_icount_to_ns(icount);
> +    int64_t ns = cpu_icount_to_ns(icount);
> +    return atomic_read(&timers_state.qemu_icount_bias) + ns;
>  }
>
>  int64_t cpu_get_icount(void)
> @@ -206,7 +207,7 @@ int64_t cpu_get_ticks(void)
>      }
>
>      ticks = timers_state.cpu_ticks_offset;
> -    if (timers_state.cpu_ticks_enabled) {
> +    if (atomic_read(&timers_state.cpu_ticks_enabled)) {
>          ticks += cpu_get_host_ticks();
>      }
>
> @@ -225,8 +226,8 @@ static int64_t cpu_get_clock_locked(void)
>  {
>      int64_t time;
>
> -    time = timers_state.cpu_clock_offset;
> -    if (timers_state.cpu_ticks_enabled) {
> +    time = atomic_read(&timers_state.cpu_clock_offset);
> +    if (atomic_read(&timers_state.cpu_ticks_enabled)) {
>          time += get_clock();
>      }


--
Alex Bennée