From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55489) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ddfFb-00046X-Ns for qemu-devel@nongnu.org; Fri, 04 Aug 2017 12:15:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ddfFY-0002Yh-Ds for qemu-devel@nongnu.org; Fri, 04 Aug 2017 12:14:59 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59466) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1ddfFY-0002YE-4L for qemu-devel@nongnu.org; Fri, 04 Aug 2017 12:14:56 -0400 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 317BA20A9B for ; Fri, 4 Aug 2017 16:14:55 +0000 (UTC) From: Paolo Bonzini Date: Fri, 4 Aug 2017 18:14:51 +0200 Message-Id: <20170804161452.17413-2-pbonzini@redhat.com> In-Reply-To: <20170804161452.17413-1-pbonzini@redhat.com> References: <20170804161452.17413-1-pbonzini@redhat.com> Subject: [Qemu-devel] [PATCH 1/2] rcu: completely disable pthread_atfork callbacks as soon as possible List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: dgilbert@redhat.com Because of -daemonize, system mode QEMU sometimes needs to fork() and keep RCU enabled in the child. However, there is a possible deadlock with synchronize_rcu: - the CPU thread is inside a RCU critical section and wants to take the BQL in order to do MMIO - the monitor thread, which is owning the BQL, calls rcu_init_lock which tries to take the rcu_sync_lock - the call_rcu thread has taken rcu_sync_lock in synchronize_rcu, but synchronize_rcu needs the CPU thread to end the critical section before returning. This cannot happen for user-mode emulation, because it does not have a BQL. To fix it, assume that system mode QEMU only forks in preparation for exec (except when daemonizing) and disable pthread_atfork as soon as the double fork has happened. Reported-by: David Gilbert Signed-off-by: Paolo Bonzini --- include/qemu/rcu.h | 6 ++++++ util/rcu.c | 20 ++++++++++++++++++++ vl.c | 1 + 3 files changed, 27 insertions(+) diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h index 83ae2808be..c0da9907e8 100644 --- a/include/qemu/rcu.h +++ b/include/qemu/rcu.h @@ -105,6 +105,12 @@ extern void synchronize_rcu(void); */ extern void rcu_register_thread(void); extern void rcu_unregister_thread(void); + +/* + * Support for fork(). fork() support is enabled at startup. + */ +extern void rcu_enable_atfork(void); +extern void rcu_disable_atfork(void); extern void rcu_after_fork(void); struct rcu_head; diff --git a/util/rcu.c b/util/rcu.c index 9adc5e4a36..2142ddd93b 100644 --- a/util/rcu.c +++ b/util/rcu.c @@ -318,15 +318,35 @@ static void rcu_init_complete(void) rcu_register_thread(); } +static int atfork_depth = 1; + +void rcu_enable_atfork(void) +{ + atfork_depth++; +} + +void rcu_disable_atfork(void) +{ + atfork_depth--; +} + #ifdef CONFIG_POSIX static void rcu_init_lock(void) { + if (atfork_depth < 1) { + return; + } + qemu_mutex_lock(&rcu_sync_lock); qemu_mutex_lock(&rcu_registry_lock); } static void rcu_init_unlock(void) { + if (atfork_depth < 1) { + return; + } + qemu_mutex_unlock(&rcu_registry_lock); qemu_mutex_unlock(&rcu_sync_lock); } diff --git a/vl.c b/vl.c index 99fcfa0442..8967115514 100644 --- a/vl.c +++ b/vl.c @@ -4121,6 +4121,7 @@ int main(int argc, char **argv, char **envp) set_memory_options(&ram_slots, &maxram_size, machine_class); os_daemonize(); + rcu_disable_atfork(); if (pid_file && qemu_create_pidfile(pid_file) != 0) { error_report("could not acquire pid file: %s", strerror(errno)); -- 2.13.3