From: Jonathan Wakely <jwakely@redhat.com>
Date: Wed, 25 Nov 2020 10:26:09 +0000 (+0000)
Subject: libstdc++: Fix handling of futex wake [PR 97936]
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=ad9cbcee543ecccd79fa49dafcd925532d2ce210;p=gcc.git

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 --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>