Messages in this thread | | | Date | Tue, 3 Oct 2017 11:32:57 -0700 | From | Tejun Heo <> | Subject | Re: [GIT PULL] percpu fixes for v4.14-rc3 |
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Hello, Linus.
On Tue, Oct 03, 2017 at 10:27:42AM -0700, Linus Torvalds wrote: > On Tue, Oct 3, 2017 at 6:26 AM, Tejun Heo <tj@kernel.org> wrote: > > > > * Mark noticed that the generic implementations of percpu local atomic > > reads aren't properly protected against irqs and there's a (slim) > > chance for split reads on some 32bit systems. > > Grr. > > Do we really want to support 64-bit percpu operations on 32-bit architectures?
I don't know. However, AFAICS, no 32bit arch provides 64bit optimizations and we've been doing the same irq-disable protection on all !read percpu ops, so bringing reads in line is at least the immediately right thing to do.
> It does kind of break the whole point of percpu operations, and I > would like to point out that I find things like > > static DEFINE_PER_CPU(u64, running_sample_length); > > which is preceded by a comment that talks about how this is accessed > from critical code and explicitly mentions NMI's. > > So protection them against interrupts isn't actually going to *fix* anything.
NMI users would have to do their own protection (running_sample_length seems to be accessed only from NMI context) but there are quite a few existing irq users which were risking a very slight chance of doing corrupt split reads.
> Doing a > > git grep DEFINE_PER_CPU.*64 > > isn't likely to find everything, but maybe it's a representative > sample. There aren't that many of those things, and some of them are > very much ok (ie only 64-bit architectures, or explicitly using > "atomic64_t" to avoid access issues) > > I dunno. I have pulled you change, but it does make me go "people are > doing something wrong". > > Maybe we could just aim to disallow everything but CPU-native accesses?
Here are a couple points to consider.
* On a lot of archs, most of percpu operations need to be protected explicitly anyway regardless of size. This patch shifts things a bit worse but not drastically. IOW, removing 64bit support on 32bit isn't gonna remove most of explicit context protections.
* Using 64bit percpu ops is a bit of cop-out, where we trade off some overhead on 32bit for performance / simplicity on 64bit, which doesn't seem too different from what we do when we use explicit 64bit variables in general. And we do the latter quite a bit.
The question is whether we want to force percpu users to explicitly worry about 32bit machines and shape the code accordingly, which can possibly incur overhead / complexity on 64bit while resulting in better code on 32bit.
Given that we need explicit protections on most operations anyway, I lean towards keeping it. I don't think removing 64-on-32 support will buy us anything noticeable enough to justify the inconvenience.
Thanks.
-- tejun
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