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Merge libcxxrt commit f2e55091e2e878386c9f7974d4922bbdc4faed84

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Fix unlock in two-word version and add missing comment.

  Fixes #15
  Fixes #16

This should fix the hangs in __cxa_guard_acquire() reported on i386 (and
possibly other 32-bit platforms).

Obtained from:	https://github.com/libcxxrt/libcxxrt/commit/f2e5509
Fixes:		56aaed388b
MFC after:	2 weeks
This commit is contained in:
Dimitry Andric 2022-03-20 22:21:28 +01:00
parent bfffb66e7f
commit 7819a911ff
1 changed files with 7 additions and 2 deletions
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9
contrib/libcxxrt/guard.cc
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@ -69,7 +69,12 @@
* progress). The bit to use depends on the byte order of the target.
*
* On many 32-bit platforms, 64-bit atomics are unavailable (or slow) and so we
* treat the two halves of the 64-bit word as independent values and
* treat the two halves of the 64-bit word as independent values and establish
* an ordering on them such that the guard word is never modified unless the
* lock word is in the locked state. This means that we can do double-checked
* locking by loading the guard word and, if it is not initialised, trying to
* transition the lock word from the unlocked to locked state, and then
* manipulate the guard word.
*/
namespace
{
@ -227,7 +232,7 @@ namespace
// If another thread did manage to initialise this, release
// the lock and notify the caller that initialisation is
// done.
lock_word.store(initialised, memory_order::release);
lock_word.store(0, memory_order::release);
return GuardState::InitDone;
}
return GuardState::InitLockSucceeded;