Messages in this thread | | | From | Marco Elver <> | Date | Thu, 30 Jan 2020 14:39:38 +0100 | Subject | Re: [PATCH] locking/osq_lock: fix a data race in osq_wait_next |
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On Wed, 29 Jan 2020 at 19:40, Peter Zijlstra <peterz@infradead.org> wrote: > > On Wed, Jan 29, 2020 at 04:29:43PM +0100, Marco Elver wrote: > > > On Tue, 28 Jan 2020 at 17:52, Peter Zijlstra <peterz@infradead.org> wrote: > > > I'm claiming that in the first case, the only thing that's ever done > > > with a racy load is comparing against 0, there is no possible bad > > > outcome ever. While obviously if you let the load escape, or do anything > > > other than compare against 0, there is. > > > > It might sound like a simple rule, but implementing this is anything > > but simple: This would require changing the compiler, > > Right. > > > which we said we'd like to avoid as it introduces new problems. > > Ah, I missed that brief. > > > This particular rule relies on semantic analysis that is beyond what > > the TSAN instrumentation currently supports. Right now we support GCC > > and Clang; changing the compiler probably means we'd end up with only > > one (probably Clang), and many more years before the change has > > propagated to the majority of used compiler versions. It'd be good if > > we can do this purely as a change in the kernel's codebase. > > *sigh*, I didn't know there was such a resistance to change the tooling. > That seems very unfortunate :-/
Unfortunately. Just wanted to highlight what to expect if we go down that path. We can put it on a nice-to-have list, but don't expect or rely on it to happen soon, given the implications above.
> > Keeping the bigger picture in mind, how frequent is this case, and > > what are we really trying to accomplish? > > It's trying to avoid the RmW pulling the line in exclusive/modified > state in a loop. The basic C-CAS pattern if you will. > > > Is it only to avoid a READ_ONCE? Why is the READ_ONCE bad here? If > > there is a racing access, why not be explicit about it? > > It's probably not terrible to put a READ_ONCE() there; we just need to > make sure the compiler doesn't do something stupid (it is known to do > stupid when 'volatile' is present).
Maybe we need to optimize READ_ONCE().
'if (data_race(..))' would also work here and has no cost.
> But the fact remains that it is entirely superfluous, there is no > possible way the compiler can wreck this.
Agree. Still thinking if there is a way to do it without changing the compiler, but I can't see it right now. :/
Thanks, -- Marco
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