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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Thu, 10 Sep 2015 14:22:49 +0100
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Subject: Re: [Qemu-devel] [RFC 11/38] qemu-thread: handle spurious
	futex_wait wakeups
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To: "Emilio G. Cota" <cota@braap.org>
Cc: mttcg@listserver.greensocs.com, mark.burton@greensocs.com, a.rigo@virtualopensystems.com, qemu-devel@nongnu.org, guillaume.delbergue@greensocs.com, pbonzini@redhat.com, Frederic Konrad <fred.konrad@greensocs.com>


Emilio G. Cota <cota@braap.org> writes:

> Signed-off-by: Emilio G. Cota <cota@braap.org>
> ---
>  util/qemu-thread-posix.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
> index 04dae0f..3760e27 100644
> --- a/util/qemu-thread-posix.c
> +++ b/util/qemu-thread-posix.c
> @@ -303,7 +303,16 @@ static inline void futex_wake(QemuEvent *ev, int n)
>  
>  static inline void futex_wait(QemuEvent *ev, unsigned val)
>  {
> -    futex(ev, FUTEX_WAIT, (int) val, NULL, NULL, 0);
> +    while (futex(ev, FUTEX_WAIT, (int) val, NULL, NULL, 0)) {
> +        switch (errno) {
> +        case EWOULDBLOCK:
> +            return;
> +        case EINTR:
> +            break; /* get out of switch and retry */
> +        default:
> +            abort();

I'd be tempted to error_exit with the errno in this case so additional
information is reported before we bail out. The man pages seems to
indicate other errnos are possible for FUTUX_WAIT although they may be
unlikely:

       EACCES No read access to futex memory.
       EFAULT Error retrieving timeout information from user space.

       I guess things would have gone very wrong for these

       EINVAL Invalid argument.

       Hard to get wrong

       ENFILE The system limit on the total number of open files has
       been reached.

       Might happen under system load?

       ENOSYS Invalid operation specified in op.

       Hardcoded op so no

       ETIMEDOUT
              Timeout during the FUTEX_WAIT operation.

       No timeout specified so we shouldn't hit it


> +        }
> +    }
>  }
>  #else
>  static inline void futex_wake(QemuEvent *ev, int n)

-- 
Alex Bennée