[committed] libstdc++: Fix deadlock in debug iterator increment [PR108288]

[Jonathan Wakely <jwakely@redhat.com>]

Tested x86_64-linux. Pushed to trunk.

I think we should backport this too, after some soak time on trunk.

-- >8 --

With -fno-elide-constructors the debug iterator post-increment and
post-decrement operators are susceptible to deadlock. They take a mutex
lock and then return a temporary, which also attempts to take a lock to
attach itself to the sequence. If the return value and *this happen to
collide and use the same mutex from the pool, then you get a deadlock
trying to lock a mutex that is already held by the current thread.

The solution is to construct the return value before taking the lock.
The copy constructor and pre-inc/pre-dec operators already manage locks
correctly, without deadlock, so just implement post-inc/post-dec in the
conventional way, taking a copy then modifying *this, then returning the
copy.

libstdc++-v3/ChangeLog:

	PR libstdc++/108288
	* include/debug/safe_iterator.h (_Safe_iterator::operator++(int))
	(_Safe_iterator::operator--(int)): Do not hold lock around
	construction of return value.
---
 libstdc++-v3/include/debug/safe_iterator.h | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/libstdc++-v3/include/debug/safe_iterator.h b/libstdc++-v3/include/debug/safe_iterator.h
index 117dc93de60..f9068eaf8d6 100644
--- a/libstdc++-v3/include/debug/safe_iterator.h
+++ b/libstdc++-v3/include/debug/safe_iterator.h
@@ -761,12 +761,9 @@ namespace __gnu_debug
       _Safe_iterator
       operator++(int) _GLIBCXX_NOEXCEPT
       {
-	_GLIBCXX_DEBUG_VERIFY(this->_M_incrementable(),
-			      _M_message(__msg_bad_inc)
-			      ._M_iterator(*this, "this"));
-	__gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
-	return _Safe_iterator(this->base()++, this->_M_sequence,
-			      _Attach_single());
+	_Safe_iterator __ret = *this;
+	++*this;
+	return __ret;
       }
 
       // ------ Bidirectional iterator requirements ------
@@ -788,12 +785,9 @@ namespace __gnu_debug
       _Safe_iterator
       operator--(int) _GLIBCXX_NOEXCEPT
       {
-	_GLIBCXX_DEBUG_VERIFY(this->_M_decrementable(),
-			      _M_message(__msg_bad_dec)
-			      ._M_iterator(*this, "this"));
-	__gnu_cxx::__scoped_lock __l(this->_M_get_mutex());
-	return _Safe_iterator(this->base()--, this->_M_sequence,
-			      _Attach_single());
+	_Safe_iterator __ret = *this;
+	--*this;
+	return __ret;
       }
 
       // ------ Random access iterator requirements ------
-- 
2.39.0