From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: [RFC PATCH Userspace RCU] Discourage use of pthread_atfork() for call_rcu handlers Date: Wed, 26 Dec 2012 12:33:18 -0500 Message-ID: <20121226173318.GA22298@Krystal> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline List-Unsubscribe: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: libc-alpha-owner@sourceware.org To: "Paul E. McKenney" , Lai Jiangshan , Alan Stern , Stephen Hemminger Cc: linux-kernel@vger.kernel.org, lttng-dev@lists.lttng.org, rp@svcs.cs.pdx.edu, libc-alpha@sourceware.org List-Id: lttng-dev@lists.lttng.org Discourage use of glibc pthread_atfork() for call_rcu handlers due to its inappropriate assumptions about single-threadedness while pthread atfork handlers are executing. This results in hangs within the glibc memory allocator. Signed-off-by: Mathieu Desnoyers --- diff --git a/README b/README index 83330ea..2d29c1d 100644 --- a/README +++ b/README @@ -273,7 +273,20 @@ Interaction with fork() must invoke call_rcu_before_fork() before the fork() and 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. Thread Local Storage (TLS) -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com