linux-kernel - Re: [PATCH v2] add function spin_event

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Date:	Mon, 9 Mar 2009 15:31:31 +0000
From:	Will Newton <will.newton@...il.com>
To:	Timur Tabi <timur@...escale.com>
Cc:	linux-kernel@...r.kernel.org, rdreier@...co.com,
	jirislaby@...il.com
Subject: Re: [PATCH v2] add function spin_event_timeout()

On Mon, Mar 9, 2009 at 3:18 PM, Timur Tabi <timur@...escale.com> wrote:
> The function spin_event_timeout() takes a condition and timeout value
> (in jiffies) as parameters.  It spins until either the condition is true
> or the timeout expires.  It returns non-zero if the condition is true,
> zero otherwise.
>
> Signed-off-by: Timur Tabi <timur@...escale.com>
> ---
>
> v2: changes based on feedback
>
>  include/linux/delay.h |   22 ++++++++++++++++++++++
>  1 files changed, 22 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/delay.h b/include/linux/delay.h
> index fd832c6..235ca25 100644
> --- a/include/linux/delay.h
> +++ b/include/linux/delay.h
> @@ -51,4 +51,26 @@ static inline void ssleep(unsigned int seconds)
>        msleep(seconds * 1000);
>  }
>
> +/**
> + * spin_event_timeout - spin until a condition gets true or a timeout elapses
> + * @condition: a C expression for the event to wait for
> + * @timeout: timeout, in jiffies
> + *
> + * The process spins until the @condition evaluates to true or the @timeout
> + * elapses.
> + *
> + * The function returns non-zero if the @condition evaluated to true, or
> + * zero if the @timeout elapsed.  If both occurs (e.g. the loop was
> + * pre-empted and the @condition became true in the meantime, but when the
> + * loop resumed the @timeout had already elapsed), then non-zero will be
> + * returned.
> + */
> +#define spin_event_timeout(condition, timeout)                 \
> +({                                                             \
> +       unsigned long __timeout = jiffies + (timeout);          \
> +       while (!(condition) && time_before(jiffies, __timeout)) \
> +               cpu_relax();                                    \
> +       (condition);                                            \
> +})
> +
>  #endif /* defined(_LINUX_DELAY_H) */

Are you sure you want to evaluate condition a second time when
returning? Some memory mapped registers don't have a stable value so
e.g. the first test could succeed but the return value could still be
zero.

> --
> 1.6.1.3
>
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