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authorStefan Hajnoczi <stefanha@redhat.com>2017-12-07 20:13:19 +0000
committerStefan Hajnoczi <stefanha@redhat.com>2017-12-19 10:25:09 +0000
commit2362a28ea11c145e1a13ae79342d76dc118a72a6 (patch)
treed0c86376a8eb52407d77b45ac3e988a6f6abfeef
parentccc15f7dafff63cbcc3f6a7c51e380025d67f7f4 (diff)
iothread: fix iothread_stop() race condition
There is a small chance that iothread_stop() hangs as follows: Thread 3 (Thread 0x7f63eba5f700 (LWP 16105)): #0 0x00007f64012c09b6 in ppoll () at /lib64/libc.so.6 #1 0x000055959992eac9 in ppoll (__ss=0x0, __timeout=0x0, __nfds=<optimized out>, __fds=<optimized out>) at /usr/include/bits/poll2.h:77 #2 0x000055959992eac9 in qemu_poll_ns (fds=<optimized out>, nfds=<optimized out>, timeout=<optimized out>) at util/qemu-timer.c:322 #3 0x0000559599930711 in aio_poll (ctx=0x55959bdb83c0, blocking=blocking@entry=true) at util/aio-posix.c:629 #4 0x00005595996806fe in iothread_run (opaque=0x55959bd78400) at iothread.c:59 #5 0x00007f640159f609 in start_thread () at /lib64/libpthread.so.0 #6 0x00007f64012cce6f in clone () at /lib64/libc.so.6 Thread 1 (Thread 0x7f640b45b280 (LWP 16103)): #0 0x00007f64015a0b6d in pthread_join () at /lib64/libpthread.so.0 #1 0x00005595999332ef in qemu_thread_join (thread=<optimized out>) at util/qemu-thread-posix.c:547 #2 0x00005595996808ae in iothread_stop (iothread=<optimized out>) at iothread.c:91 #3 0x000055959968094d in iothread_stop_iter (object=<optimized out>, opaque=<optimized out>) at iothread.c:102 #4 0x0000559599857d97 in do_object_child_foreach (obj=obj@entry=0x55959bdb8100, fn=fn@entry=0x559599680930 <iothread_stop_iter>, opaque=opaque@entry=0x0, recurse=recurse@entry=false) at qom/object.c:852 #5 0x0000559599859477 in object_child_foreach (obj=obj@entry=0x55959bdb8100, fn=fn@entry=0x559599680930 <iothread_stop_iter>, opaque=opaque@entry=0x0) at qom/object.c:867 #6 0x0000559599680a6e in iothread_stop_all () at iothread.c:341 #7 0x000055959955b1d5 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4913 The relevant code from iothread_run() is: while (!atomic_read(&iothread->stopping)) { aio_poll(iothread->ctx, true); and iothread_stop(): iothread->stopping = true; aio_notify(iothread->ctx); ... qemu_thread_join(&iothread->thread); The following scenario can occur: 1. IOThread: while (!atomic_read(&iothread->stopping)) -> stopping=false 2. Main loop: iothread->stopping = true; aio_notify(iothread->ctx); 3. IOThread: aio_poll(iothread->ctx, true); -> hang The bug is explained by the AioContext->notify_me doc comments: "If this field is 0, everything (file descriptors, bottom halves, timers) will be re-evaluated before the next blocking poll(), thus the event_notifier_set call can be skipped." The problem is that "everything" does not include checking iothread->stopping. This means iothread_run() will block in aio_poll() if aio_notify() was called just before aio_poll(). This patch fixes the hang by replacing aio_notify() with aio_bh_schedule_oneshot(). This makes aio_poll() or g_main_loop_run() to return. Implementing this properly required a new bool running flag. The new flag prevents races that are tricky if we try to use iothread->stopping. Now iothread->stopping is purely for iothread_stop() and iothread->running is purely for the iothread_run() thread. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Message-id: 20171207201320.19284-6-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
-rw-r--r--include/sysemu/iothread.h3
-rw-r--r--iothread.c20
2 files changed, 17 insertions, 6 deletions
diff --git a/include/sysemu/iothread.h b/include/sysemu/iothread.h
index 55de1715c7..799614ffd2 100644
--- a/include/sysemu/iothread.h
+++ b/include/sysemu/iothread.h
@@ -29,7 +29,8 @@ typedef struct {
GOnce once;
QemuMutex init_done_lock;
QemuCond init_done_cond; /* is thread initialization done? */
- bool stopping;
+ bool stopping; /* has iothread_stop() been called? */
+ bool running; /* should iothread_run() continue? */
int thread_id;
/* AioContext poll parameters */
diff --git a/iothread.c b/iothread.c
index e7b93e02a3..d8b6c1fb27 100644
--- a/iothread.c
+++ b/iothread.c
@@ -55,7 +55,7 @@ static void *iothread_run(void *opaque)
qemu_cond_signal(&iothread->init_done_cond);
qemu_mutex_unlock(&iothread->init_done_lock);
- while (!atomic_read(&iothread->stopping)) {
+ while (iothread->running) {
aio_poll(iothread->ctx, true);
if (atomic_read(&iothread->worker_context)) {
@@ -78,16 +78,25 @@ static void *iothread_run(void *opaque)
return NULL;
}
+/* Runs in iothread_run() thread */
+static void iothread_stop_bh(void *opaque)
+{
+ IOThread *iothread = opaque;
+
+ iothread->running = false; /* stop iothread_run() */
+
+ if (iothread->main_loop) {
+ g_main_loop_quit(iothread->main_loop);
+ }
+}
+
void iothread_stop(IOThread *iothread)
{
if (!iothread->ctx || iothread->stopping) {
return;
}
iothread->stopping = true;
- aio_notify(iothread->ctx);
- if (atomic_read(&iothread->main_loop)) {
- g_main_loop_quit(iothread->main_loop);
- }
+ aio_bh_schedule_oneshot(iothread->ctx, iothread_stop_bh, iothread);
qemu_thread_join(&iothread->thread);
}
@@ -134,6 +143,7 @@ static void iothread_complete(UserCreatable *obj, Error **errp)
char *name, *thread_name;
iothread->stopping = false;
+ iothread->running = true;
iothread->thread_id = -1;
iothread->ctx = aio_context_new(&local_error);
if (!iothread->ctx) {