[Bug libstdc++/104442] atomic<T>::wait incorrectly loops in case of spurious notification when __waiter is shared
cvs-commit at gcc dot gnu.org
gcc-bugzilla@gcc.gnu.org
Wed Feb 9 20:31:10 GMT 2022
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104442
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Thomas Rodgers <rodgertq@gcc.gnu.org>:
https://gcc.gnu.org/g:4cf3c339815cdfa636b25a512f91b63d7c313fd6
commit r12-7151-g4cf3c339815cdfa636b25a512f91b63d7c313fd6
Author: Thomas Rodgers <rodgert@appliantology.com>
Date: Wed Feb 9 12:29:19 2022 -0800
libstdc++: Fix deadlock in atomic wait [PR104442]
This issue was observed as a deadlock in
29_atomics/atomic/wait_notify/100334.cc on vxworks. When a wait is
"laundered" (e.g. type T* does not suffice as a waitable address for the
platform's native waiting primitive), the address waited is that of the
_M_ver member of __waiter_pool_base, so several threads may wait on the
same address for unrelated atomic<T> objects. As noted in the PR, the
implementation correctly exits the wait for the thread whose data
changed, but not for any other threads waiting on the same address.
As noted in the PR the __waiter::_M_do_wait_v member was correctly exiting
but the other waiters were not reloading the value of _M_ver before
re-entering the wait.
Moving the spin call inside the loop accomplishes this, and is
consistent with the predicate accepting version of __waiter::_M_do_wait.
libstdc++-v3/ChangeLog:
PR libstdc++/104442
* include/bits/atomic_wait.h (__waiter::_M_do_wait_v): Move spin
loop inside do loop so that threads failing the wait, reload
_M_ver.
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