On Fri, Apr 17, 2015 at 11:06 AM, Mathieu Desnoyers
<mathieu.desnoy...@efficios.com> wrote:
> diff --git a/init/Kconfig b/init/Kconfig
> index f5dbc6d..89bad6a 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1559,6 +1559,19 @@ config PCI_QUIRKS
> bugs/quirks. Disable this only if your target machine is
> unaffected by PCI quirks.
>
> +config MEMBARRIER
> + bool "Enable membarrier() system call" if EXPERT
> + default y
> + depends on SMP
> + help
> + Enable the membarrier() system call that allows issuing memory
> + barriers across all running threads, which can be used to distribute
> + the cost of user-space memory barriers asymmetrically by
> transforming
> + pairs of memory barriers into pairs consisting of membarrier() and a
> + compiler barrier.
> +
> + If unsure, say Y.
> +
I understand why this syscall makes sense on SMP only, but you are
anyways checking num_online_cpus() and returning if it is only one. Is
this limitation necessary then? How do !SMP systems handle this
syscall? (I am guessing glibc wrapper?)
...
> +SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> +{
> + switch (cmd) {
> + case MEMBARRIER_CMD_QUERY:
> + return MEMBARRIER_CMD_BITMASK;
> + case MEMBARRIER_CMD_SHARED:
> + if (num_online_cpus() > 1)
> + synchronize_sched();
> + return 0;
> + default:
> + return -EINVAL;
> + }
> +}
--
Pranith
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