Messages in this thread | | | Date | Tue, 20 Aug 2019 13:29:32 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates |
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On Tue, Aug 20, 2019 at 03:56:12PM +0200, Peter Zijlstra wrote: > On Sat, Aug 17, 2019 at 01:08:02AM -0700, Linus Torvalds wrote: > > > The data tearing issue is almost a non-issue. We're not going to add > > WRITE_ONCE() to these kinds of places for no good reason. > > Paulmck actually has an example of that somewhere; ISTR that particular > case actually got fixed by GCC, but I'd really _love_ for some compiler > people (both GCC and LLVM) to state that their respective compilers will > not do load/store tearing for machine word sized load/stores.
I do very much recall such an example, but I am now unable to either find it or reproduce it. :-/
If I cannot turn it up in a few days, I will ask the LWN editors to make appropriate changes to the "Who is afraid" article.
> Without this written guarantee (which supposedly was in older GCC > manuals but has since gone missing), I'm loathe to rely on it. > > Yes, it is very rare, but it is a massive royal pain to debug if/when it > does do happen.
But from what I can see, Linus is OK with use of WRITE_ONCE() for data races on any variable for which there is at least one READ_ONCE(). So we can still use WRITE_ONCE() as we would like in our own code. Yes, you or I might be hit by someone else's omission of WRITE_ONCE(), it is better than the proverbial kick in the teeth.
Of course, if anyone knows of a compiler/architecture combination that really does tear stores of 32-bit constants, please do not keep it a secret! After all, it would be good to get that addressed easily starting now rather than after a difficult and painful series of debugging sessions.
Thanx, Paul
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