From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Thu, 19 Mar 2015 15:44:36 +0000 Subject: [PATCH] arm64: percpu: Make this_cpu accessors pre-empt safe In-Reply-To: <1426776751-20526-1-git-send-email-steve.capper@linaro.org> References: <1426776751-20526-1-git-send-email-steve.capper@linaro.org> Message-ID: <20150319154435.GC25967@leverpostej> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Steve, Thanks for putting this together! On Thu, Mar 19, 2015 at 02:52:31PM +0000, Steve Capper wrote: > this_cpu operations were implemented for arm64 in: > 5284e1b arm64: xchg: Implement cmpxchg_double > f97fc81 arm64: percpu: Implement this_cpu operations > > Unfortunately, it is possible for pre-emption to take place between > address generation and data access. This can lead to cases where data > is being manipulated by this_cpu for a different CPU than it was > called on. Which effectively breaks the spec. > > This patch disables pre-emption for the this_cpu operations > guaranteeing that address generation and data manipulation. Shouldn't that last sentence end with "occur on the same CPU", or something like that? [...] > +/* > + * Modules aren't allowed to use preempt_enable_no_resched, and it is > + * undef'ed. If we are unable to use preempt_enable_no_resched, then > + * fallback to the standard preempt_enable. > + */ > +#ifdef preempt_enable_no_resched > +#define __pcp_preempt_enable() preempt_enable_no_resched() > +#else > +#define __pcp_preempt_enable() preempt_enable() > +#endif /* preempt_enable_no_resched */ I think it would be worth mentioning in the comment why we want to use preempt_enable_no_resched where possible (e.g. read-modify-cmpxchg sequences where we want to have as few retries as possible). Other than those points, the patch looks good to me, feel free to add: Reviewed-by: Mark Rutland It's a shame there don't seem to be any this_cpu_* self-tests; I've booted a kernel with this applied, but I didn't have anything that exploded without this, so I'd feel uneasy giving a Tested-by. Mark.