From 9750c7d9d35ac49262848a0996667f8e6c782dc8 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Fri, 22 Dec 2017 11:08:59 +0100 Subject: [PATCH 41/42] iothread: fix iothread_stop() race condition RH-Author: Stefan Hajnoczi Message-id: <20171222110900.24813-20-stefanha@redhat.com> Patchwork-id: 78501 O-Subject: [RHV7.5 qemu-kvm-rhev PATCH 19/20] iothread: fix iothread_stop() race condition Bugzilla: 1519721 RH-Acked-by: Paolo Bonzini RH-Acked-by: Jeffrey Cody RH-Acked-by: Miroslav Rezanina 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=, __fds=) at /usr/include/bits/poll2.h:77 #2 0x000055959992eac9 in qemu_poll_ns (fds=, nfds=, timeout=) 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=) at util/qemu-thread-posix.c:547 #2 0x00005595996808ae in iothread_stop (iothread=) at iothread.c:91 #3 0x000055959968094d in iothread_stop_iter (object=, opaque=) at iothread.c:102 #4 0x0000559599857d97 in do_object_child_foreach (obj=obj@entry=0x55959bdb8100, fn=fn@entry=0x559599680930 , 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 , opaque=opaque@entry=0x0) at qom/object.c:867 #6 0x0000559599680a6e in iothread_stop_all () at iothread.c:341 #7 0x000055959955b1d5 in main (argc=, argv=, envp=) 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 Reviewed-by: Eric Blake Message-id: 20171207201320.19284-6-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi (cherry picked from commit 2362a28ea11c145e1a13ae79342d76dc118a72a6) Signed-off-by: Stefan Hajnoczi Signed-off-by: Miroslav Rezanina --- include/sysemu/iothread.h | 3 ++- iothread.c | 20 +++++++++++++++----- 2 files changed, 17 insertions(+), 6 deletions(-) diff --git a/include/sysemu/iothread.h b/include/sysemu/iothread.h index 55de171..799614f 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 e7b93e0..d8b6c1f 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) { -- 1.8.3.1