Discourage use of pthread_atfork() for call_rcu handlers

[userspace-rcu.git] / README

diff --git a/README b/README
index a6e29c875de973ec9020cad76de9e28aa564fdf2..97fba69462cc6e862578b942715316d40dfedecb 100644 (file)
--- a/README
+++ b/README
@@ -8,6 +8,7 @@ BUILDING
        ./configure
        make
        make install
+       ldconfig
 
        Hints:  Forcing 32-bit build:
                * CFLAGS="-m32 -g -O2" ./configure
@@ -38,9 +39,26 @@ supported, with the following exceptions:
   therefore not compatible with liburcu on x86 32-bit (i386, i486, i586, i686).
   The problem has been reported to the gcc community:
     http://www.mail-archive.com/gcc-bugs@gcc.gnu.org/msg281255.html
+- gcc 3.3 cannot match the "xchg" instruction on 32-bit x86 build.
+  See: http://kerneltrap.org/node/7507
 - Alpha, ia64 and ARM architectures depend on 4.x gcc with atomic builtins
   support.
 
+For developers using the git tree:
+
+This source tree is based on the autotools suite from GNU to simplify
+portability. Here are some things you should have on your system in order to
+compile the git repository tree :
+
+- GNU autotools (automake >=1.10, autoconf >=2.50, autoheader >=2.50)
+  (make sure your system wide "automake" points to a recent version!)
+- GNU Libtool >=2.2
+  (for more information, go to http://www.gnu.org/software/autoconf/)
+
+If you get the tree from the repository, you will need to use the "bootstrap"
+script in the root of the tree. It calls all the GNU tools needed to prepare the
+tree configuration.
+
 
 QUICK START GUIDE
 -----------------
@@ -184,6 +202,15 @@ Interaction with mutexes
        mutex in its dependency chain) should not be acquired from within a RCU
        read-side critical section.
 
+       This is especially important to understand in the context of the
+       QSBR flavor: a registered reader thread being "online" by
+       default should be considered as within a RCU read-side critical
+       section unless explicitly put "offline". Therefore, if
+       synchronize_rcu() is called with a mutex held, this mutex, as
+       well as any mutex which has this mutex in its dependency chain
+       should only be taken when the RCU reader thread is "offline"
+       (this can be performed by calling rcu_thread_offline()).
+
 Usage of DEBUG_RCU
 
        DEBUG_RCU is used to add internal debugging self-checks to the
@@ -226,3 +253,17 @@ Interaction with fork()
        call_rcu_after_fork_parent() after the fork().  The child
        process must invoke call_rcu_after_fork_child().
        These three APIs are suitable for passing to pthread_atfork().
+       Even though these three APIs are suitable for passing to
+       pthread_atfork(), use of pthread_atfork() is *STRONGLY
+       DISCOURAGED* for programs calling the glibc memory allocator
+       (malloc(), calloc(), free(), ...) within call_rcu callbacks.
+       This is due to limitations in the way glibc memory allocator
+       handles calls to the memory allocator from concurrent threads
+       while the pthread_atfork() handlers are executing.
+       Combining e.g.:
+       * call to free() from callbacks executed within call_rcu worker
+         threads,
+       * executing call_rcu atfork handlers within the glibc pthread
+         atfork mechanism,
+       will sometimes trigger interesting process hangs. This usually
+       hangs on a memory allocator lock within glibc.