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Re: [PATCH] Don't divide by zero when trying to destroy an uninitialised barrier.
- From: Szabolcs Nagy <szabolcs dot nagy at arm dot com>
- To: Mark Thompson <mark dot thompson at starleaf dot com>, GNU C Library <libc-alpha at sourceware dot org>
- Cc: nd <nd at arm dot com>
- Date: Wed, 20 Apr 2016 18:03:19 +0100
- Subject: Re: [PATCH] Don't divide by zero when trying to destroy an uninitialised barrier.
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- References: <5717B2F4 dot 9050105 at starleaf dot com>
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On 20/04/16 17:48, Mark Thompson wrote:
> nptl/pthread_barrier_destroy.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
> diff --git a/nptl/pthread_barrier_destroy.c b/nptl/pthread_barrier_destroy.c
> index 92d2027..d114084 100644
> --- a/nptl/pthread_barrier_destroy.c
> +++ b/nptl/pthread_barrier_destroy.c
> @@ -36,6 +36,15 @@ pthread_barrier_destroy (pthread_barrier_t *barrier)
> they have exited as well. To get the notification, pretend that we have
> reached the reset threshold. */
> unsigned int count = bar->count;
> + /* For an initialised barrier, count must be greater than zero here. An
> + uninitialised barrier may still have zero, however, and in this case it is
> + preferable to fail immediately rather than to invoke undefined behaviour
> + by dividing by zero on the next line. (POSIX allows the implementation to
> + diagnose invalid state and return EINVAL in this case.) */
> + if (__glibc_unlikely (count == 0))
> + return EINVAL;
this case is undefined behaviour in posix, and
i think the best way to handle that is crashing.
(because no behaviour can be portably relied upon)
nowadays posix says
"The [EINVAL] error for an uninitialized barrier
attributes object is removed; this condition
results in undefined behavior."
> unsigned int max_in_before_reset = BARRIER_IN_THRESHOLD
> - BARRIER_IN_THRESHOLD % count;
> /* Relaxed MO sufficient because the program must have ensured that all