[libcxx-commits] [PATCH] D68480: Implementation of C++20's P1135R6 for libcxx

[Olivier Giroux via Phabricator via libcxx-commits]

__simt__ added inline comments.


================
Comment at: libcxx/src/atomic.cpp:38
+    static constexpr timespec __timeout = { 2, 0 };
+    syscall(SYS_futex, __ptr, FUTEX_WAIT_PRIVATE, __val, &__timeout, 0, 0);
+}
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MBkkt wrote:
> Why do you use timed wait here?
> It's strange, and also different with macos(ulock) behavior 
Because it would be incorrect otherwise.

There's an infinitesimal but not zero probability that the serial number being waited on can roll over, incremented by precisely the right value, and then you might think it didn't change when it did.

There are a lot of weird discussions we could have from here. Does this negate Futex? I don't think it should. Is it even worth worrying about (like, the computer might be hit by a neutron flying in from space much more often than this)? I think it's cheap to mitigate.

There's a judgement-call here about how long to wait. It's arbitrary.

We wouldn't need to do this if Linux supported 64-bit Futex. Partly because we would rarely use serial numbers like this, and also partly because 64-bit numbers don't roll over inside of the useful life span of computer hardware.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D68480/new/

https://reviews.llvm.org/D68480