Re: [Qemu-devel] [PATCH v2] qemu-thread: fix qemu_thread_set_name() race in qemu_thread_create()

[Paolo Bonzini <pbonzini@redhat.com>]

On 04/01/2017 11:32, Daniel P. Berrange wrote:
> On Wed, Jan 04, 2017 at 09:32:01AM +0800, zhanghailiang wrote:
>> From: Caoxinhua <caoxinhua@huawei.com>
>>
>> QEMU will crash with the follow backtrace if the new created thread exited before
>> we call qemu_thread_set_name() for it.
>>
>>   (gdb) bt
>>   #0 0x00007f9a68b095d7 in __GI_raise (sig=sig@entry=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
>>   #1 0x00007f9a68b0acc8 in __GI_abort () at abort.c:90
>>   #2 0x00007f9a69cda389 in PAT_abort () from /usr/lib64/libuvpuserhotfix.so
>>   #3 0x00007f9a69cdda0d in patchIllInsHandler () from /usr/lib64/libuvpuserhotfix.so
>>   #4 <signal handler called>
>>   #5 pthread_setname_np (th=140298470549248, name=name@entry=0x8cc74a "io-task-worker") at ../nptl/sysdeps/unix/sysv/linux/pthread_setname.c:49
>>   #6 0x00000000007f5f20 in qemu_thread_set_name (thread=thread@entry=0x7ffd2ac09680, name=name@entry=0x8cc74a "io-task-worker") at util/qemu_thread_posix.c:459
>>   #7 0x00000000007f679e in qemu_thread_create (thread=thread@entry=0x7ffd2ac09680, name=name@entry=0x8cc74a "io-task-worker",start_routine=start_routine@entry=0x7c1300 <qio_task_thread_worker>, arg=arg@entry=0x7f99b8001720, mode=mode@entry=1) at util/qemu_thread_posix.c:498
>>   #8 0x00000000007c15b6 in qio_task_run_in_thread (task=task@entry=0x7f99b80033d0, worker=worker@entry=0x7bd920 <qio_channel_socket_connect_worker>, opaque=0x7f99b8003370, destroy=0x7c6220 <qapi_free_SocketAddress>) at io/task.c:133
>>   #9 0x00000000007bda04 in qio_channel_socket_connect_async (ioc=0x7f99b80014c0, addr=0x37235d0, callback=callback@entry=0x54ad00 <qemu_chr_socket_connected>, opaque=opaque@entry=0x38118b0, destroy=destroy@entry=0x0) at io/channel_socket.c:191
>>   #10 0x00000000005487f6 in socket_reconnect_timeout (opaque=0x38118b0) at qemu_char.c:4402
>>   #11 0x00007f9a6a1533b3 in g_timeout_dispatch () from /usr/lib64/libglib-2.0.so.0
>>   #12 0x00007f9a6a15299a in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
>>   #13 0x0000000000747386 in glib_pollfds_poll () at main_loop.c:227
>>   #14 0x0000000000747424 in os_host_main_loop_wait (timeout=404000000) at main_loop.c:272
>>   #15 0x0000000000747575 in main_loop_wait (nonblocking=nonblocking@entry=0) at main_loop.c:520
>>   #16 0x0000000000557d31 in main_loop () at vl.c:2170
>>   #17 0x000000000041c8b7 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:5083
>>
>> Let's detach the new thread after calling qemu_thread_set_name().
>>
>> Signed-off-by: Caoxinhua <caoxinhua@huawei.com>
>> ---
>> v2:
>>  Fix missing title
>> ---
>>  util/qemu-thread-posix.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/util/qemu-thread-posix.c b/util/qemu-thread-posix.c
>> index d20cdde..d31793d 100644
>> --- a/util/qemu-thread-posix.c
>> +++ b/util/qemu-thread-posix.c
>> @@ -481,12 +481,6 @@ void qemu_thread_create(QemuThread *thread, const char *name,
>>      if (err) {
>>          error_exit(err, __func__);
>>      }
>> -    if (mode == QEMU_THREAD_DETACHED) {
>> -        err = pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
>> -        if (err) {
>> -            error_exit(err, __func__);
>> -        }
>> -    }
>>  
>>      /* Leave signal handling to the iothread.  */
>>      sigfillset(&set);
>> @@ -499,6 +493,12 @@ void qemu_thread_create(QemuThread *thread, const char *name,
>>          qemu_thread_set_name(thread, name);
>>      }
>>  
>> +    if (mode == QEMU_THREAD_DETACHED) {
>> +        err = pthread_detach(thread->thread);
>> +        if (err) {
>> +            error_exit(err, __func__);
>> +        }
>> +    }
> 
> Is it permitted to be calling pthread_detach(), if there is a chance that
> the thread has already exited ? It seems reasonable, since a non-detached
> thread shouldl remain in a zombie state waiting to be join'd, but the man
> page is unclear on behaviour.

For what it's worth, all of glibc, musl and openbsd cater for the
possibility that the thread has already exited.  The worse weird/racy
cases would be when you have pthread_detach in a thread, pthread_join in
another, and the thread terminates in the meanwhile---but that cannot
happen here.

Paolo