Re: [RFC PATCH] sys_membarrier(): system/process-wide memory barrier (x86) (v12)

[Josh Triplett <josh@joshtriplett.org>]

On Sun, Mar 15, 2015 at 03:24:19PM -0400, Mathieu Desnoyers wrote:
> Here is an implementation of a new system call, sys_membarrier(), which
> executes a memory barrier on either all running threads of the current
> process (MEMBARRIER_PRIVATE_FLAG) or calls synchronize_sched() to issue
> a memory barrier on all threads running on the system. It can be used to
> distribute the cost of user-space memory barriers asymmetrically by
> transforming pairs of memory barriers into pairs consisting of
> sys_membarrier() and a compiler barrier. For synchronization primitives
> that distinguish between read-side and write-side (e.g. userspace RCU,
> rwlocks), the read-side can be accelerated significantly by moving the
> bulk of the memory barrier overhead to the write-side.

>From a quick review, this seems quite reasonable (as it did 5 years
ago).

One request: Could you please add a config option (default y) in the
EXPERT menu to disable this?  You actually seem to already have it
marked as a cond_syscall.

Also, a very minor nit: flags in kernel APIs aren't typically named with
a _FLAG suffix.

With the syscall made optional, and with or without that naming nit
fixed:
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

- Josh Triplett