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authorPaolo Bonzini <pbonzini@redhat.com>2015-07-21 16:07:51 +0200
committerStefan Hajnoczi <stefanha@redhat.com>2015-07-22 12:41:40 +0100
commiteabc977973103527bbb8fed69c91cfaa6691f8ab (patch)
tree3e777c3809cc2982d6da2322364bd2f650f1c759
parent6493c975af75be5b8d9ade954239bdf5492b7911 (diff)
downloadqemu-arm-eabc977973103527bbb8fed69c91cfaa6691f8ab.tar.gz
AioContext: fix broken ctx->dispatching optimization
This patch rewrites the ctx->dispatching optimization, which was the cause of some mysterious hangs that could be reproduced on aarch64 KVM only. The hangs were indirectly caused by aio_poll() and in particular by flash memory updates's call to blk_write(), which invokes aio_poll(). Fun stuff: they had an extremely short race window, so much that adding all kind of tracing to either the kernel or QEMU made it go away (a single printf made it half as reproducible). On the plus side, the failure mode (a hang until the next keypress) made it very easy to examine the state of the process with a debugger. And there was a very nice reproducer from Laszlo, which failed pretty often (more than half of the time) on any version of QEMU with a non-debug kernel; it also failed fast, while still in the firmware. So, it could have been worse. For some unknown reason they happened only with virtio-scsi, but that's not important. It's more interesting that they disappeared with io=native, making thread-pool.c a likely suspect for where the bug arose. thread-pool.c is also one of the few places which use bottom halves across threads, by the way. I hope that no other similar bugs exist, but just in case :) I am going to describe how the successful debugging went... Since the likely culprit was the ctx->dispatching optimization, which mostly affects bottom halves, the first observation was that there are two qemu_bh_schedule() invocations in the thread pool: the one in the aio worker and the one in thread_pool_completion_bh. The latter always causes the optimization to trigger, the former may or may not. In order to restrict the possibilities, I introduced new functions qemu_bh_schedule_slow() and qemu_bh_schedule_fast(): /* qemu_bh_schedule_slow: */ ctx = bh->ctx; bh->idle = 0; if (atomic_xchg(&bh->scheduled, 1) == 0) { event_notifier_set(&ctx->notifier); } /* qemu_bh_schedule_fast: */ ctx = bh->ctx; bh->idle = 0; assert(ctx->dispatching); atomic_xchg(&bh->scheduled, 1); Notice how the atomic_xchg is still in qemu_bh_schedule_slow(). This was already debated a few months ago, so I assumed it to be correct. In retrospect this was a very good idea, as you'll see later. Changing thread_pool_completion_bh() to qemu_bh_schedule_fast() didn't trigger the assertion (as expected). Changing the worker's invocation to qemu_bh_schedule_slow() didn't hide the bug (another assumption which luckily held). This already limited heavily the amount of interaction between the threads, hinting that the problematic events must have triggered around thread_pool_completion_bh(). As mentioned early, invoking a debugger to examine the state of a hung process was pretty easy; the iothread was always waiting on a poll(..., -1) system call. Infinite timeouts are much rarer on x86, and this could be the reason why the bug was never observed there. With the buggy sequence more or less resolved to an interaction between thread_pool_completion_bh() and poll(..., -1), my "tracing" strategy was to just add a few qemu_clock_get_ns(QEMU_CLOCK_REALTIME) calls, hoping that the ordering of aio_ctx_prepare(), aio_ctx_dispatch, poll() and qemu_bh_schedule_fast() would provide some hint. The output was: (gdb) p last_prepare $3 = 103885451 (gdb) p last_dispatch $4 = 103876492 (gdb) p last_poll $5 = 115909333 (gdb) p last_schedule $6 = 115925212 Notice how the last call to qemu_poll_ns() came after aio_ctx_dispatch(). This makes little sense unless there is an aio_poll() call involved, and indeed with a slightly different instrumentation you can see that there is one: (gdb) p last_prepare $3 = 107569679 (gdb) p last_dispatch $4 = 107561600 (gdb) p last_aio_poll $5 = 110671400 (gdb) p last_schedule $6 = 110698917 So the scenario becomes clearer: iothread VCPU thread -------------------------------------------------------------------------- aio_ctx_prepare aio_ctx_check qemu_poll_ns(timeout=-1) aio_poll aio_dispatch thread_pool_completion_bh qemu_bh_schedule() At this point bh->scheduled = 1 and the iothread has not been woken up. The solution must be close, but this alone should not be a problem, because the bottom half is only rescheduled to account for rare situations (see commit 3c80ca1, thread-pool: avoid deadlock in nested aio_poll() calls, 2014-07-15). Introducing a third thread---a thread pool worker thread, which also does qemu_bh_schedule()---does bring out the problematic case. The third thread must be awakened *after* the callback is complete and thread_pool_completion_bh has redone the whole loop, explaining the short race window. And then this is what happens: thread pool worker -------------------------------------------------------------------------- <I/O completes> qemu_bh_schedule() Tada, bh->scheduled is already 1, so qemu_bh_schedule() does nothing and the iothread is never woken up. This is where the bh->scheduled optimization comes into play---it is correct, but removing it would have masked the bug. So, what is the bug? Well, the question asked by the ctx->dispatching optimization ("is any active aio_poll dispatching?") was wrong. The right question to ask instead is "is any active aio_poll *not* dispatching", i.e. in the prepare or poll phases? In that case, the aio_poll is sleeping or might go to sleep anytime soon, and the EventNotifier must be invoked to wake it up. In any other case (including if there is *no* active aio_poll at all!) we can just wait for the next prepare phase to pick up the event (e.g. a bottom half); the prepare phase will avoid the blocking and service the bottom half. Expressing the invariant with a logic formula, the broken one looked like: !(exists(thread): in_dispatching(thread)) => !optimize or equivalently: !(exists(thread): in_aio_poll(thread) && in_dispatching(thread)) => !optimize In the correct one, the negation is in a slightly different place: (exists(thread): in_aio_poll(thread) && !in_dispatching(thread)) => !optimize or equivalently: (exists(thread): in_prepare_or_poll(thread)) => !optimize Even if the difference boils down to moving an exclamation mark :) the implementation is quite different. However, I think the new one is simpler to understand. In the old implementation, the "exists" was implemented with a boolean value. This didn't really support well the case of multiple concurrent event loops, but I thought that this was okay: aio_poll holds the AioContext lock so there cannot be concurrent aio_poll invocations, and I was just considering nested event loops. However, aio_poll _could_ indeed be concurrent with the GSource. This is why I came up with the wrong invariant. In the new implementation, "exists" is computed simply by counting how many threads are in the prepare or poll phases. There are some interesting points to consider, but the gist of the idea remains: 1) AioContext can be used through GSource as well; as mentioned in the patch, bit 0 of the counter is reserved for the GSource. 2) the counter need not be updated for a non-blocking aio_poll, because it won't sleep forever anyway. This is just a matter of checking the "blocking" variable. This requires some changes to the win32 implementation, but is otherwise not too complicated. 3) as mentioned above, the new implementation will not call aio_notify when there is *no* active aio_poll at all. The tests have to be adjusted for this change. The calls to aio_notify in async.c are fine; they only want to kick aio_poll out of a blocking wait, but need not do anything if aio_poll is not running. 4) nested aio_poll: these just work with the new implementation; when a nested event loop is invoked, the outer event loop is never in the prepare or poll phases. The outer event loop thus has already decremented the counter. Reported-by: Richard W. M. Jones <rjones@redhat.com> Reported-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-5-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
-rw-r--r--aio-posix.c18
-rw-r--r--aio-win32.c20
-rw-r--r--async.c21
-rw-r--r--docs/aio_notify.promela77
-rw-r--r--include/block/aio.h29
-rw-r--r--tests/test-aio.c18
6 files changed, 81 insertions, 102 deletions
diff --git a/aio-posix.c b/aio-posix.c
index 4abec38866..249889f14c 100644
--- a/aio-posix.c
+++ b/aio-posix.c
@@ -233,26 +233,23 @@ static void add_pollfd(AioHandler *node)
bool aio_poll(AioContext *ctx, bool blocking)
{
AioHandler *node;
- bool was_dispatching;
int i, ret;
bool progress;
int64_t timeout;
aio_context_acquire(ctx);
- was_dispatching = ctx->dispatching;
progress = false;
/* aio_notify can avoid the expensive event_notifier_set if
* everything (file descriptors, bottom halves, timers) will
* be re-evaluated before the next blocking poll(). This is
* already true when aio_poll is called with blocking == false;
- * if blocking == true, it is only true after poll() returns.
- *
- * If we're in a nested event loop, ctx->dispatching might be true.
- * In that case we can restore it just before returning, but we
- * have to clear it now.
+ * if blocking == true, it is only true after poll() returns,
+ * so disable the optimization now.
*/
- aio_set_dispatching(ctx, !blocking);
+ if (blocking) {
+ atomic_add(&ctx->notify_me, 2);
+ }
ctx->walking_handlers++;
@@ -272,6 +269,9 @@ bool aio_poll(AioContext *ctx, bool blocking)
aio_context_release(ctx);
}
ret = qemu_poll_ns((GPollFD *)pollfds, npfd, timeout);
+ if (blocking) {
+ atomic_sub(&ctx->notify_me, 2);
+ }
if (timeout) {
aio_context_acquire(ctx);
}
@@ -287,12 +287,10 @@ bool aio_poll(AioContext *ctx, bool blocking)
ctx->walking_handlers--;
/* Run dispatch even if there were no readable fds to run timers */
- aio_set_dispatching(ctx, true);
if (aio_dispatch(ctx)) {
progress = true;
}
- aio_set_dispatching(ctx, was_dispatching);
aio_context_release(ctx);
return progress;
diff --git a/aio-win32.c b/aio-win32.c
index 9268b5c9fc..ea655b0935 100644
--- a/aio-win32.c
+++ b/aio-win32.c
@@ -279,25 +279,23 @@ bool aio_poll(AioContext *ctx, bool blocking)
{
AioHandler *node;
HANDLE events[MAXIMUM_WAIT_OBJECTS + 1];
- bool was_dispatching, progress, have_select_revents, first;
+ bool progress, have_select_revents, first;
int count;
int timeout;
aio_context_acquire(ctx);
- was_dispatching = ctx->dispatching;
progress = false;
/* aio_notify can avoid the expensive event_notifier_set if
* everything (file descriptors, bottom halves, timers) will
* be re-evaluated before the next blocking poll(). This is
* already true when aio_poll is called with blocking == false;
- * if blocking == true, it is only true after poll() returns.
- *
- * If we're in a nested event loop, ctx->dispatching might be true.
- * In that case we can restore it just before returning, but we
- * have to clear it now.
+ * if blocking == true, it is only true after poll() returns,
+ * so disable the optimization now.
*/
- aio_set_dispatching(ctx, !blocking);
+ if (blocking) {
+ atomic_add(&ctx->notify_me, 2);
+ }
have_select_revents = aio_prepare(ctx);
@@ -331,10 +329,13 @@ bool aio_poll(AioContext *ctx, bool blocking)
aio_context_release(ctx);
}
ret = WaitForMultipleObjects(count, events, FALSE, timeout);
+ if (blocking) {
+ assert(first);
+ atomic_sub(&ctx->notify_me, 2);
+ }
if (timeout) {
aio_context_acquire(ctx);
}
- aio_set_dispatching(ctx, true);
if (first && aio_bh_poll(ctx)) {
progress = true;
@@ -358,7 +359,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
progress |= timerlistgroup_run_timers(&ctx->tlg);
- aio_set_dispatching(ctx, was_dispatching);
aio_context_release(ctx);
return progress;
}
diff --git a/async.c b/async.c
index 77d080d6f5..a232192148 100644
--- a/async.c
+++ b/async.c
@@ -184,6 +184,8 @@ aio_ctx_prepare(GSource *source, gint *timeout)
{
AioContext *ctx = (AioContext *) source;
+ atomic_or(&ctx->notify_me, 1);
+
/* We assume there is no timeout already supplied */
*timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));
@@ -200,6 +202,7 @@ aio_ctx_check(GSource *source)
AioContext *ctx = (AioContext *) source;
QEMUBH *bh;
+ atomic_and(&ctx->notify_me, ~1);
for (bh = ctx->first_bh; bh; bh = bh->next) {
if (!bh->deleted && bh->scheduled) {
return true;
@@ -254,23 +257,13 @@ ThreadPool *aio_get_thread_pool(AioContext *ctx)
return ctx->thread_pool;
}
-void aio_set_dispatching(AioContext *ctx, bool dispatching)
-{
- ctx->dispatching = dispatching;
- if (!dispatching) {
- /* Write ctx->dispatching before reading e.g. bh->scheduled.
- * Optimization: this is only needed when we're entering the "unsafe"
- * phase where other threads must call event_notifier_set.
- */
- smp_mb();
- }
-}
-
void aio_notify(AioContext *ctx)
{
- /* Write e.g. bh->scheduled before reading ctx->dispatching. */
+ /* Write e.g. bh->scheduled before reading ctx->notify_me. Pairs
+ * with atomic_or in aio_ctx_prepare or atomic_add in aio_poll.
+ */
smp_mb();
- if (!ctx->dispatching) {
+ if (ctx->notify_me) {
event_notifier_set(&ctx->notifier);
}
}
diff --git a/docs/aio_notify.promela b/docs/aio_notify.promela
index ad3f6f08b0..fccc7ee1c3 100644
--- a/docs/aio_notify.promela
+++ b/docs/aio_notify.promela
@@ -1,5 +1,5 @@
/*
- * This model describes the interaction between aio_set_dispatching()
+ * This model describes the interaction between ctx->notify_me
* and aio_notify().
*
* Author: Paolo Bonzini <pbonzini@redhat.com>
@@ -14,57 +14,53 @@
* spin -a docs/aio_notify.promela
* gcc -O2 pan.c
* ./a.out -a
+ *
+ * To verify it (with a bug planted in the model):
+ * spin -a -DBUG docs/aio_notify.promela
+ * gcc -O2 pan.c
+ * ./a.out -a
*/
#define MAX 4
#define LAST (1 << (MAX - 1))
#define FINAL ((LAST << 1) - 1)
-bool dispatching;
+bool notify_me;
bool event;
-int req, done;
+int req;
+int done;
active proctype waiter()
{
- int fetch, blocking;
+ int fetch;
- do
- :: done != FINAL -> {
- // Computing "blocking" is separate from execution of the
- // "bottom half"
- blocking = (req == 0);
-
- // This is our "bottom half"
- atomic { fetch = req; req = 0; }
- done = done | fetch;
-
- // Wait for a nudge from the other side
- do
- :: event == 1 -> { event = 0; break; }
- :: !blocking -> break;
- od;
+ do
+ :: true -> {
+ notify_me++;
- dispatching = 1;
+ if
+#ifndef BUG
+ :: (req > 0) -> skip;
+#endif
+ :: else ->
+ // Wait for a nudge from the other side
+ do
+ :: event == 1 -> { event = 0; break; }
+ od;
+ fi;
- // If you are simulating this model, you may want to add
- // something like this here:
- //
- // int foo; foo++; foo++; foo++;
- //
- // This only wastes some time and makes it more likely
- // that the notifier process hits the "fast path".
+ notify_me--;
- dispatching = 0;
+ atomic { fetch = req; req = 0; }
+ done = done | fetch;
}
- :: else -> break;
od
}
active proctype notifier()
{
int next = 1;
- int sets = 0;
do
:: next <= LAST -> {
@@ -74,8 +70,8 @@ active proctype notifier()
// aio_notify
if
- :: dispatching == 0 -> sets++; event = 1;
- :: else -> skip;
+ :: notify_me == 1 -> event = 1;
+ :: else -> printf("Skipped event_notifier_set\n"); skip;
fi;
// Test both synchronous and asynchronous delivery
@@ -86,19 +82,12 @@ active proctype notifier()
:: 1 -> skip;
fi;
}
- :: else -> break;
od;
- printf("Skipped %d event_notifier_set\n", MAX - sets);
}
-#define p (done == FINAL)
-
-never {
- do
- :: 1 // after an arbitrarily long prefix
- :: p -> break // p becomes true
- od;
- do
- :: !p -> accept: break // it then must remains true forever after
- od
+never { /* [] done < FINAL */
+accept_init:
+ do
+ :: done < FINAL -> skip;
+ od;
}
diff --git a/include/block/aio.h b/include/block/aio.h
index b46103ece7..be91e3f701 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -63,10 +63,30 @@ struct AioContext {
*/
int walking_handlers;
- /* Used to avoid unnecessary event_notifier_set calls in aio_notify.
- * Writes protected by lock or BQL, reads are lockless.
+ /* Used to avoid unnecessary event_notifier_set calls in aio_notify;
+ * accessed with atomic primitives. 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. If it is non-zero, you may need to wake up a
+ * concurrent aio_poll or the glib main event loop, making
+ * event_notifier_set necessary.
+ *
+ * Bit 0 is reserved for GSource usage of the AioContext, and is 1
+ * between a call to aio_ctx_check and the next call to aio_ctx_dispatch.
+ * Bits 1-31 simply count the number of active calls to aio_poll
+ * that are in the prepare or poll phase.
+ *
+ * The GSource and aio_poll must use a different mechanism because
+ * there is no certainty that a call to GSource's prepare callback
+ * (via g_main_context_prepare) is indeed followed by check and
+ * dispatch. It's not clear whether this would be a bug, but let's
+ * play safe and allow it---it will just cause extra calls to
+ * event_notifier_set until the next call to dispatch.
+ *
+ * Instead, the aio_poll calls include both the prepare and the
+ * dispatch phase, hence a simple counter is enough for them.
*/
- bool dispatching;
+ uint32_t notify_me;
/* lock to protect between bh's adders and deleter */
QemuMutex bh_lock;
@@ -89,9 +109,6 @@ struct AioContext {
QEMUTimerListGroup tlg;
};
-/* Used internally to synchronize aio_poll against qemu_bh_schedule. */
-void aio_set_dispatching(AioContext *ctx, bool dispatching);
-
/**
* aio_context_new: Allocate a new AioContext.
*
diff --git a/tests/test-aio.c b/tests/test-aio.c
index e7bbb8345a..217e33772e 100644
--- a/tests/test-aio.c
+++ b/tests/test-aio.c
@@ -97,14 +97,6 @@ static void event_ready_cb(EventNotifier *e)
/* Tests using aio_*. */
-static void test_notify(void)
-{
- g_assert(!aio_poll(ctx, false));
- aio_notify(ctx);
- g_assert(!aio_poll(ctx, true));
- g_assert(!aio_poll(ctx, false));
-}
-
typedef struct {
QemuMutex start_lock;
bool thread_acquired;
@@ -494,14 +486,6 @@ static void test_timer_schedule(void)
* works well, and that's what I am using.
*/
-static void test_source_notify(void)
-{
- while (g_main_context_iteration(NULL, false));
- aio_notify(ctx);
- g_assert(g_main_context_iteration(NULL, true));
- g_assert(!g_main_context_iteration(NULL, false));
-}
-
static void test_source_flush(void)
{
g_assert(!g_main_context_iteration(NULL, false));
@@ -830,7 +814,6 @@ int main(int argc, char **argv)
while (g_main_context_iteration(NULL, false));
g_test_init(&argc, &argv, NULL);
- g_test_add_func("/aio/notify", test_notify);
g_test_add_func("/aio/acquire", test_acquire);
g_test_add_func("/aio/bh/schedule", test_bh_schedule);
g_test_add_func("/aio/bh/schedule10", test_bh_schedule10);
@@ -845,7 +828,6 @@ int main(int argc, char **argv)
g_test_add_func("/aio/event/flush", test_flush_event_notifier);
g_test_add_func("/aio/timer/schedule", test_timer_schedule);
- g_test_add_func("/aio-gsource/notify", test_source_notify);
g_test_add_func("/aio-gsource/flush", test_source_flush);
g_test_add_func("/aio-gsource/bh/schedule", test_source_bh_schedule);
g_test_add_func("/aio-gsource/bh/schedule10", test_source_bh_schedule10);