From 06065c451f10c7ef62cfb575a87f323a70ae1c9e Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Thu, 7 Dec 2017 12:41:21 +0000 Subject: linux-user: wrap fork() in a start/end exclusive section When we do a fork() in usermode emulation, we need to be in a start/end exclusive section, so that we can ensure that no other thread is in an RCU section. Otherwise you can get this deadlock: - fork thread: has mmap_lock, waits for rcu_sync_lock (because rcu_init_lock() is registered as a pthread_atfork() hook) - RCU thread: has rcu_sync_lock, waits for rcu_read_(un)lock - another CPU thread: in RCU critical section, waits for mmap_lock This can show up if you have a heavily multithreaded guest program that does a fork(). Signed-off-by: Peter Maydell Reported-by: Stuart Monteith Message-Id: <1512650481-1723-1-git-send-email-peter.maydell@linaro.org> Signed-off-by: Laurent Vivier --- linux-user/main.c | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'linux-user') diff --git a/linux-user/main.c b/linux-user/main.c index e840691..2140465 100644 --- a/linux-user/main.c +++ b/linux-user/main.c @@ -127,6 +127,7 @@ int cpu_get_pic_interrupt(CPUX86State *env) /* Make sure everything is in a consistent state for calling fork(). */ void fork_start(void) { + start_exclusive(); mmap_fork_start(); qemu_mutex_lock(&tb_ctx.tb_lock); cpu_list_lock(); @@ -147,9 +148,13 @@ void fork_end(int child) qemu_mutex_init(&tb_ctx.tb_lock); qemu_init_cpu_list(); gdbserver_fork(thread_cpu); + /* qemu_init_cpu_list() takes care of reinitializing the + * exclusive state, so we don't need to end_exclusive() here. + */ } else { qemu_mutex_unlock(&tb_ctx.tb_lock); cpu_list_unlock(); + end_exclusive(); } } -- cgit v1.1