| Message ID | 1512650481-1723-1-git-send-email-peter.maydell@linaro.org |
|---|---|
| State | Accepted |
| Headers | show |
| Series | linux-user: wrap fork() in a start/end exclusive section | expand |
Le 07/12/2017 à 13:41, Peter Maydell a écrit : > 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 <peter.maydell@linaro.org> > Reported-by: Stuart Monteith <stuart.monteith@linaro.org> > --- > Based-on: <1512397331-15238-1-git-send-email-peter.maydell@linaro.org> > (this applies on top of 'linux-user: Fix locking order in fork_start()') > > I think this should fix the deadlock that Stuart reports, but I > can't reproduce it, so testing welcome. > > linux-user/main.c | 5 +++++ > 1 file changed, 5 insertions(+) > > diff --git a/linux-user/main.c b/linux-user/main.c > index 146ee3e..ff116fe 100644 > --- a/linux-user/main.c > +++ b/linux-user/main.c > @@ -128,6 +128,7 @@ 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(); > @@ -148,9 +149,13 @@ 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(); > } > } > > Applied to my linux-user branch. Thanks, Laurent
diff --git a/linux-user/main.c b/linux-user/main.c index 146ee3e..ff116fe 100644 --- a/linux-user/main.c +++ b/linux-user/main.c @@ -128,6 +128,7 @@ 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(); @@ -148,9 +149,13 @@ 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(); } }
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 <peter.maydell@linaro.org> Reported-by: Stuart Monteith <stuart.monteith@linaro.org> --- Based-on: <1512397331-15238-1-git-send-email-peter.maydell@linaro.org> (this applies on top of 'linux-user: Fix locking order in fork_start()') I think this should fix the deadlock that Stuart reports, but I can't reproduce it, so testing welcome. linux-user/main.c | 5 +++++ 1 file changed, 5 insertions(+) -- 2.7.4