[gcc r11-5330] libstdc++: Fix handling of futex wake [PR 97936]

[gcc r11-5330] libstdc++: Fix handling of futex wake [PR 97936]

[Jonathan Wakely]

https://gcc.gnu.org/g:ad9cbcee543ecccd79fa49dafcd925532d2ce210

commit r11-5330-gad9cbcee543ecccd79fa49dafcd925532d2ce210
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Nov 25 10:26:09 2020 +0000

    libstdc++: Fix handling of futex wake [PR 97936]
    
    The __platform_wait function is supposed to wait until *addr != old.
    The futex syscall checks the initial value and returns EAGAIN if *addr
    != old is already true, which should cause __platform_wait to return.
    Instead it loops and keeps doing a futex wait, which keeps returning
    EAGAIN.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/97936
            * include/bits/atomic_wait.h (__platform_wait): Return if futex
            sets EAGAIN.
            * testsuite/30_threads/latch/3.cc: Re-enable test.
            * testsuite/30_threads/semaphore/try_acquire_until.cc: Likewise.

Diff:
---
 libstdc++-v3/include/bits/atomic_wait.h                          | 4 ++--
 libstdc++-v3/testsuite/30_threads/latch/3.cc                     | 1 -
 libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc | 1 -
 3 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/bits/atomic_wait.h b/libstdc++-v3/include/bits/atomic_wait.h
index cd756f68de6..fdf7c4586f2 100644
--- a/libstdc++-v3/include/bits/atomic_wait.h
+++ b/libstdc++-v3/include/bits/atomic_wait.h
@@ -100,9 +100,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    auto __e = syscall (SYS_futex, static_cast<const void*>(__addr),
 				  static_cast<int>(__futex_wait_flags::__wait_private),
 				    __val, nullptr);
-	    if (!__e)
+	    if (!__e || EAGAIN)
 	      break;
-	    else if (!(errno == EINTR || errno == EAGAIN))
+	    else if (errno != EINTR)
 	      __throw_system_error(__e);
 	  }
       }
diff --git a/libstdc++-v3/testsuite/30_threads/latch/3.cc b/libstdc++-v3/testsuite/30_threads/latch/3.cc
index 6304135a877..70f2d183e87 100644
--- a/libstdc++-v3/testsuite/30_threads/latch/3.cc
+++ b/libstdc++-v3/testsuite/30_threads/latch/3.cc
@@ -19,7 +19,6 @@
 // { dg-do run { target c++2a } }
 // { dg-require-gthreads "" }
 // { dg-additional-options "-pthread" { target pthread } }
-// { dg-skip-if "broken" { *-*-* } }
 
 #include <latch>
 #include <atomic>
diff --git a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc
index 5e1141425f7..58f68ce9e1e 100644
--- a/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc
+++ b/libstdc++-v3/testsuite/30_threads/semaphore/try_acquire_until.cc
@@ -19,7 +19,6 @@
 // { dg-do run { target c++2a } }
 // { dg-require-gthreads "" }
 // { dg-additional-options "-pthread" { target pthread } }
-// { dg-skip-if "broken" { *-*-* } }
 
 #include <semaphore>
 #include <chrono>