[Qemu-devel] [PULL for-2.4 0/4] Block patches

[Qemu-devel] [PULL for-2.4 0/4] Block patches

[Stefan Hajnoczi]

The following changes since commit f73ca7363440240b7ee5ee7f7ddb1c64751efb54:

  Merge remote-tracking branch 'remotes/mst/tags/for_upstream' into staging (2015-07-20 13:25:28 +0100)

are available in the git repository at:

  git://github.com/stefanha/qemu.git tags/block-pull-request

for you to fetch changes up to 4fefb0d50cd0a803bba3cedaeb9adfb80cc61fed:

  AioContext: fix broken ctx->dispatching optimization (2015-07-20 17:14:22 +0100)

----------------------------------------------------------------

----------------------------------------------------------------

Fam Zheng (1):
  mirror: Speed up bitmap initial scanning

Paolo Bonzini (3):
  tests: remove irrelevant assertions from test-aio
  aio-win32: reorganize polling loop
  AioContext: fix broken ctx->dispatching optimization

 aio-posix.c             | 18 +++++-------
 aio-win32.c             | 41 ++++++++++++++------------
 async.c                 | 21 +++++---------
 block/mirror.c          | 14 ++++-----
 docs/aio_notify.promela | 77 +++++++++++++++++++++----------------------------
 include/block/aio.h     | 29 +++++++++++++++----
 tests/test-aio.c        | 26 +++--------------
 7 files changed, 103 insertions(+), 123 deletions(-)

-- 
2.4.3

[Qemu-devel] [PULL for-2.4 1/4] mirror: Speed up bitmap initial scanning

[Stefan Hajnoczi]

From: Fam Zheng <famz@redhat.com>

Limiting to sectors_per_chunk for each bdrv_is_allocated_above is slow,
because the underlying protocol driver would issue much more queries
than necessary. We should coalesce the query.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: <1436413678-7114-4-git-send-email-famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/mirror.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/block/mirror.c b/block/mirror.c
index 323f747..fc4d8f5 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -388,7 +388,7 @@ static void coroutine_fn mirror_run(void *opaque)
     MirrorBlockJob *s = opaque;
     MirrorExitData *data;
     BlockDriverState *bs = s->common.bs;
-    int64_t sector_num, end, sectors_per_chunk, length;
+    int64_t sector_num, end, length;
     uint64_t last_pause_ns;
     BlockDriverInfo bdi;
     char backing_filename[2]; /* we only need 2 characters because we are only
@@ -442,7 +442,6 @@ static void coroutine_fn mirror_run(void *opaque)
         goto immediate_exit;
     }
 
-    sectors_per_chunk = s->granularity >> BDRV_SECTOR_BITS;
     mirror_free_init(s);
 
     last_pause_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
@@ -450,7 +449,9 @@ static void coroutine_fn mirror_run(void *opaque)
         /* First part, loop on the sectors and initialize the dirty bitmap.  */
         BlockDriverState *base = s->base;
         for (sector_num = 0; sector_num < end; ) {
-            int64_t next = (sector_num | (sectors_per_chunk - 1)) + 1;
+            /* Just to make sure we are not exceeding int limit. */
+            int nb_sectors = MIN(INT_MAX >> BDRV_SECTOR_BITS,
+                                 end - sector_num);
             int64_t now = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
 
             if (now - last_pause_ns > SLICE_TIME) {
@@ -462,8 +463,7 @@ static void coroutine_fn mirror_run(void *opaque)
                 goto immediate_exit;
             }
 
-            ret = bdrv_is_allocated_above(bs, base,
-                                          sector_num, next - sector_num, &n);
+            ret = bdrv_is_allocated_above(bs, base, sector_num, nb_sectors, &n);
 
             if (ret < 0) {
                 goto immediate_exit;
@@ -472,10 +472,8 @@ static void coroutine_fn mirror_run(void *opaque)
             assert(n > 0);
             if (ret == 1) {
                 bdrv_set_dirty_bitmap(s->dirty_bitmap, sector_num, n);
-                sector_num = next;
-            } else {
-                sector_num += n;
             }
+            sector_num += n;
         }
     }
 
-- 
2.4.3

[Qemu-devel] [PULL for-2.4 2/4] tests: remove irrelevant assertions from test-aio

[Stefan Hajnoczi]

From: Paolo Bonzini <pbonzini@redhat.com>

In these tests, the purpose of the initial calls to aio_poll and
g_main_context_iteration is simply to put the AioContext in a
known state; the return value of the function does not really
matter.  The next patch will change those return values; change
the assertions to a while loop which expresses the intention
better.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1437250891-18715-2-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 tests/test-aio.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/tests/test-aio.c b/tests/test-aio.c
index a7cb5c9..e7bbb83 100644
--- a/tests/test-aio.c
+++ b/tests/test-aio.c
@@ -331,7 +331,7 @@ static void test_wait_event_notifier(void)
     EventNotifierTestData data = { .n = 0, .active = 1 };
     event_notifier_init(&data.e, false);
     aio_set_event_notifier(ctx, &data.e, event_ready_cb);
-    g_assert(!aio_poll(ctx, false));
+    while (aio_poll(ctx, false));
     g_assert_cmpint(data.n, ==, 0);
     g_assert_cmpint(data.active, ==, 1);
 
@@ -356,7 +356,7 @@ static void test_flush_event_notifier(void)
     EventNotifierTestData data = { .n = 0, .active = 10, .auto_set = true };
     event_notifier_init(&data.e, false);
     aio_set_event_notifier(ctx, &data.e, event_ready_cb);
-    g_assert(!aio_poll(ctx, false));
+    while (aio_poll(ctx, false));
     g_assert_cmpint(data.n, ==, 0);
     g_assert_cmpint(data.active, ==, 10);
 
@@ -669,7 +669,7 @@ static void test_source_wait_event_notifier(void)
     EventNotifierTestData data = { .n = 0, .active = 1 };
     event_notifier_init(&data.e, false);
     aio_set_event_notifier(ctx, &data.e, event_ready_cb);
-    g_assert(g_main_context_iteration(NULL, false));
+    while (g_main_context_iteration(NULL, false));
     g_assert_cmpint(data.n, ==, 0);
     g_assert_cmpint(data.active, ==, 1);
 
@@ -694,7 +694,7 @@ static void test_source_flush_event_notifier(void)
     EventNotifierTestData data = { .n = 0, .active = 10, .auto_set = true };
     event_notifier_init(&data.e, false);
     aio_set_event_notifier(ctx, &data.e, event_ready_cb);
-    g_assert(g_main_context_iteration(NULL, false));
+    while (g_main_context_iteration(NULL, false));
     g_assert_cmpint(data.n, ==, 0);
     g_assert_cmpint(data.active, ==, 10);
 
-- 
2.4.3

[Qemu-devel] [PULL for-2.4 3/4] aio-win32: reorganize polling loop

[Stefan Hajnoczi]

From: Paolo Bonzini <pbonzini@redhat.com>

Preparatory bugfixes and tweaks to the loop before the next patch:

- disable dispatch optimization during aio_prepare.  This fixes a bug.

- do not modify "blocking" until after the first WaitForMultipleObjects
call.  This is needed in the next patch.

- change the loop to do...while.  This makes it obvious that the loop
is always entered at least once.  In the next patch this is important
because the first iteration undoes the ctx->notify_me increment that
happened before entering the loop.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1437250891-18715-3-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 aio-win32.c | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/aio-win32.c b/aio-win32.c
index 233d8f5..9268b5c 100644
--- a/aio-win32.c
+++ b/aio-win32.c
@@ -284,11 +284,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
     int timeout;
 
     aio_context_acquire(ctx);
-    have_select_revents = aio_prepare(ctx);
-    if (have_select_revents) {
-        blocking = false;
-    }
-
     was_dispatching = ctx->dispatching;
     progress = false;
 
@@ -304,6 +299,8 @@ bool aio_poll(AioContext *ctx, bool blocking)
      */
     aio_set_dispatching(ctx, !blocking);
 
+    have_select_revents = aio_prepare(ctx);
+
     ctx->walking_handlers++;
 
     /* fill fd sets */
@@ -317,12 +314,18 @@ bool aio_poll(AioContext *ctx, bool blocking)
     ctx->walking_handlers--;
     first = true;
 
-    /* wait until next event */
-    while (count > 0) {
+    /* ctx->notifier is always registered.  */
+    assert(count > 0);
+
+    /* Multiple iterations, all of them non-blocking except the first,
+     * may be necessary to process all pending events.  After the first
+     * WaitForMultipleObjects call ctx->notify_me will be decremented.
+     */
+    do {
         HANDLE event;
         int ret;
 
-        timeout = blocking
+        timeout = blocking && !have_select_revents
             ? qemu_timeout_ns_to_ms(aio_compute_timeout(ctx)) : 0;
         if (timeout) {
             aio_context_release(ctx);
@@ -351,7 +354,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
         blocking = false;
 
         progress |= aio_dispatch_handlers(ctx, event);
-    }
+    } while (count > 0);
 
     progress |= timerlistgroup_run_timers(&ctx->tlg);
 
-- 
2.4.3

[Qemu-devel] [PULL for-2.4 4/4] AioContext: fix broken ctx->dispatching optimization

[Stefan Hajnoczi]

From: Paolo Bonzini <pbonzini@redhat.com>

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>
Message-id: 1437250891-18715-4-git-send-email-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 aio-posix.c             | 18 +++++-------
 aio-win32.c             | 20 ++++++-------
 async.c                 | 21 +++++---------
 docs/aio_notify.promela | 77 +++++++++++++++++++++----------------------------
 include/block/aio.h     | 29 +++++++++++++++----
 tests/test-aio.c        | 18 ------------
 6 files changed, 81 insertions(+), 102 deletions(-)

diff --git a/aio-posix.c b/aio-posix.c
index 4abec38..249889f 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 9268b5c..ea655b0 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 77d080d..a232192 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 ad3f6f0..fccc7ee 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);
+    do
+        :: true -> {
+            notify_me++;
+
+            if
+#ifndef BUG
+                :: (req > 0) -> skip;
+#endif
+                :: else ->
+                    // Wait for a nudge from the other side
+                    do
+                        :: event == 1 -> { event = 0; break; }
+                    od;
+            fi;
+
+            notify_me--;
 
-            // 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;
-
-            dispatching = 1;
-
-            // 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".
-
-            dispatching = 0;
         }
-        :: 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 b46103e..be91e3f 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 e7bbb83..217e337 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);
-- 
2.4.3

Re: [Qemu-devel] [PULL for-2.4 0/4] Block patches

[Peter Maydell]

On 20 July 2015 at 19:05, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> The following changes since commit f73ca7363440240b7ee5ee7f7ddb1c64751efb54:
>
>   Merge remote-tracking branch 'remotes/mst/tags/for_upstream' into staging (2015-07-20 13:25:28 +0100)
>
> are available in the git repository at:
>
>   git://github.com/stefanha/qemu.git tags/block-pull-request
>
> for you to fetch changes up to 4fefb0d50cd0a803bba3cedaeb9adfb80cc61fed:
>
>   AioContext: fix broken ctx->dispatching optimization (2015-07-20 17:14:22 +0100)
>
> ----------------------------------------------------------------
>
> ----------------------------------------------------------------
>
> Fam Zheng (1):
>   mirror: Speed up bitmap initial scanning
>
> Paolo Bonzini (3):
>   tests: remove irrelevant assertions from test-aio
>   aio-win32: reorganize polling loop
>   AioContext: fix broken ctx->dispatching optimization

I'm afraid this seems to cause 'make check' to hang on one
of the PPC tests:

QTEST_QEMU_BINARY=ppc-softmmu/qemu-system-ppc QTEST_QEMU_IMG=qemu-img
MALLOC_PERTURB_=${MALLOC_PERTURB_:-$((RANDOM % 255 + 1))} gtester -k
--verbose -m=quick tests/endianness-test tests/boot-order-test
tests/qom-test
TEST: tests/endianness-test... (pid=27216)
  /ppc/endianness/g3beige:                                             OK
  /ppc/endianness/split/g3beige:                                       OK
  /ppc/endianness/combine/g3beige:                                     OK
  /ppc/endianness/prep:                                                OK
  /ppc/endianness/split/prep:                                          OK
  /ppc/endianness/combine/prep:                                        OK
  /ppc/endianness/bamboo:                                              OK
  /ppc/endianness/split/bamboo:                                        OK
  /ppc/endianness/combine/bamboo:                                      OK
PASS: tests/endianness-test
TEST: tests/boot-order-test... (pid=27228)
  /ppc/boot-order/prep:                                                OK
  /ppc/boot-order/pmac_oldworld:

(and then it just sits there. Seen on x86-64/Linux and OSX hosts.)

Backtrace from connecting a gdb to it:
(gdb) thread apply all bt

Thread 3 (Thread 0x2ac8d2d5f700 (LWP 17911)):
#0  __lll_lock_wait () at
../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
#1  0x00002ac8c6310657 in _L_lock_909 () from
/lib/x86_64-linux-gnu/libpthread.so.0
#2  0x00002ac8c6310480 in __GI___pthread_mutex_lock
(mutex=0x2ac8be8dad00 <qemu_global_mutex>) at
../nptl/pthread_mutex_lock.c:79
#3  0x00002ac8be3a9f89 in qemu_mutex_lock
(mutex=mutex@entry=0x2ac8be8dad00 <qemu_global_mutex>) at
/home/petmay01/linaro/qemu-for-merges/util/qemu-thread-posix.c:73
#4  0x00002ac8be0554a4 in qemu_mutex_lock_iothread () at
/home/petmay01/linaro/qemu-for-merges/cpus.c:1164
#5  0x00002ac8be3ba33e in call_rcu_thread (opaque=<optimised out>) at
/home/petmay01/linaro/qemu-for-merges/util/rcu.c:241
#6  0x00002ac8c630e182 in start_thread (arg=0x2ac8d2d5f700) at
pthread_create.c:312
#7  0x00002ac8c661e47d in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Thread 2 (Thread 0x2ac8d33ac700 (LWP 17912)):
#0  0x00002ac8c63160d1 in do_sigwait (sig=0x2ac8d33ab99c,
set=<optimised out>) at
../nptl/sysdeps/unix/sysv/linux/../../../../../sysdeps/unix/sysv/linux/sigwait.c:60
#1  __sigwait (set=set@entry=0x2ac8d33ab9a0,
sig=sig@entry=0x2ac8d33ab99c) at
../nptl/sysdeps/unix/sysv/linux/../../../../../sysdeps/unix/sysv/linux/sigwait.c:97
#2  0x00002ac8be055937 in qemu_dummy_cpu_thread_fn
(arg=0x2ac8bed53450) at
/home/petmay01/linaro/qemu-for-merges/cpus.c:1016
#3  0x00002ac8c630e182 in start_thread (arg=0x2ac8d33ac700) at
pthread_create.c:312
#4  0x00002ac8c661e47d in clone () at
../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Thread 1 (Thread 0x2ac8be572680 (LWP 17910)):
#0  syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
#1  0x00002ac8be3aa5f2 in futex_wait (val=4294967295,
ev=0x2ac8be669708) at
/home/petmay01/linaro/qemu-for-merges/util/qemu-thread-posix.c:301
#2  qemu_event_wait (ev=ev@entry=0x2ac8be669708) at
/home/petmay01/linaro/qemu-for-merges/util/qemu-thread-posix.c:399
#3  0x00002ac8be325e41 in qemu_clock_enable
(type=type@entry=QEMU_CLOCK_VIRTUAL, enabled=enabled@entry=false) at
/home/petmay01/linaro/qemu-for-merges/qemu-timer.c:167
#4  0x00002ac8be055d22 in pause_all_vcpus () at
/home/petmay01/linaro/qemu-for-merges/cpus.c:1200
#5  0x00002ac8be01491d in main_loop_should_exit () at
/home/petmay01/linaro/qemu-for-merges/vl.c:1864
#6  main_loop () at /home/petmay01/linaro/qemu-for-merges/vl.c:1906
#7  main (argc=<optimised out>, argv=<optimised out>, envp=<optimised
out>) at /home/petmay01/linaro/qemu-for-merges/vl.c:4653

thanks
-- PMM

Re: [Qemu-devel] [PULL for-2.4 0/4] Block patches

[Paolo Bonzini]

On 21/07/2015 11:04, Peter Maydell wrote:
> On 20 July 2015 at 19:05, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>> The following changes since commit f73ca7363440240b7ee5ee7f7ddb1c64751efb54:
>>
>>   Merge remote-tracking branch 'remotes/mst/tags/for_upstream' into staging (2015-07-20 13:25:28 +0100)
>>
>> are available in the git repository at:
>>
>>   git://github.com/stefanha/qemu.git tags/block-pull-request
>>
>> for you to fetch changes up to 4fefb0d50cd0a803bba3cedaeb9adfb80cc61fed:
>>
>>   AioContext: fix broken ctx->dispatching optimization (2015-07-20 17:14:22 +0100)
>>
>> ----------------------------------------------------------------
>>
>> ----------------------------------------------------------------
>>
>> Fam Zheng (1):
>>   mirror: Speed up bitmap initial scanning
>>
>> Paolo Bonzini (3):
>>   tests: remove irrelevant assertions from test-aio
>>   aio-win32: reorganize polling loop
>>   AioContext: fix broken ctx->dispatching optimization
> 
> I'm afraid this seems to cause 'make check' to hang on one
> of the PPC tests:
> 
> QTEST_QEMU_BINARY=ppc-softmmu/qemu-system-ppc QTEST_QEMU_IMG=qemu-img
> MALLOC_PERTURB_=${MALLOC_PERTURB_:-$((RANDOM % 255 + 1))} gtester -k
> --verbose -m=quick tests/endianness-test tests/boot-order-test
> tests/qom-test
> TEST: tests/endianness-test... (pid=27216)
>   /ppc/endianness/g3beige:                                             OK
>   /ppc/endianness/split/g3beige:                                       OK
>   /ppc/endianness/combine/g3beige:                                     OK
>   /ppc/endianness/prep:                                                OK
>   /ppc/endianness/split/prep:                                          OK
>   /ppc/endianness/combine/prep:                                        OK
>   /ppc/endianness/bamboo:                                              OK
>   /ppc/endianness/split/bamboo:                                        OK
>   /ppc/endianness/combine/bamboo:                                      OK
> PASS: tests/endianness-test
> TEST: tests/boot-order-test... (pid=27228)
>   /ppc/boot-order/prep:                                                OK
>   /ppc/boot-order/pmac_oldworld:
> 
> (and then it just sits there. Seen on x86-64/Linux and OSX hosts.)
> 
> Backtrace from connecting a gdb to it:
> (gdb) thread apply all bt
> 
> Thread 3 (Thread 0x2ac8d2d5f700 (LWP 17911)):
> #0  __lll_lock_wait () at
> ../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:135
> #1  0x00002ac8c6310657 in _L_lock_909 () from
> /lib/x86_64-linux-gnu/libpthread.so.0
> #2  0x00002ac8c6310480 in __GI___pthread_mutex_lock
> (mutex=0x2ac8be8dad00 <qemu_global_mutex>) at
> ../nptl/pthread_mutex_lock.c:79
> #3  0x00002ac8be3a9f89 in qemu_mutex_lock
> (mutex=mutex@entry=0x2ac8be8dad00 <qemu_global_mutex>) at
> /home/petmay01/linaro/qemu-for-merges/util/qemu-thread-posix.c:73
> #4  0x00002ac8be0554a4 in qemu_mutex_lock_iothread () at
> /home/petmay01/linaro/qemu-for-merges/cpus.c:1164
> #5  0x00002ac8be3ba33e in call_rcu_thread (opaque=<optimised out>) at
> /home/petmay01/linaro/qemu-for-merges/util/rcu.c:241
> #6  0x00002ac8c630e182 in start_thread (arg=0x2ac8d2d5f700) at
> pthread_create.c:312
> #7  0x00002ac8c661e47d in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:111
> 
> Thread 2 (Thread 0x2ac8d33ac700 (LWP 17912)):
> #0  0x00002ac8c63160d1 in do_sigwait (sig=0x2ac8d33ab99c,
> set=<optimised out>) at
> ../nptl/sysdeps/unix/sysv/linux/../../../../../sysdeps/unix/sysv/linux/sigwait.c:60
> #1  __sigwait (set=set@entry=0x2ac8d33ab9a0,
> sig=sig@entry=0x2ac8d33ab99c) at
> ../nptl/sysdeps/unix/sysv/linux/../../../../../sysdeps/unix/sysv/linux/sigwait.c:97
> #2  0x00002ac8be055937 in qemu_dummy_cpu_thread_fn
> (arg=0x2ac8bed53450) at
> /home/petmay01/linaro/qemu-for-merges/cpus.c:1016
> #3  0x00002ac8c630e182 in start_thread (arg=0x2ac8d33ac700) at
> pthread_create.c:312
> #4  0x00002ac8c661e47d in clone () at
> ../sysdeps/unix/sysv/linux/x86_64/clone.S:111
> 
> Thread 1 (Thread 0x2ac8be572680 (LWP 17910)):
> #0  syscall () at ../sysdeps/unix/sysv/linux/x86_64/syscall.S:38
> #1  0x00002ac8be3aa5f2 in futex_wait (val=4294967295,
> ev=0x2ac8be669708) at
> /home/petmay01/linaro/qemu-for-merges/util/qemu-thread-posix.c:301
> #2  qemu_event_wait (ev=ev@entry=0x2ac8be669708) at
> /home/petmay01/linaro/qemu-for-merges/util/qemu-thread-posix.c:399
> #3  0x00002ac8be325e41 in qemu_clock_enable
> (type=type@entry=QEMU_CLOCK_VIRTUAL, enabled=enabled@entry=false) at
> /home/petmay01/linaro/qemu-for-merges/qemu-timer.c:167
> #4  0x00002ac8be055d22 in pause_all_vcpus () at
> /home/petmay01/linaro/qemu-for-merges/cpus.c:1200
> #5  0x00002ac8be01491d in main_loop_should_exit () at
> /home/petmay01/linaro/qemu-for-merges/vl.c:1864
> #6  main_loop () at /home/petmay01/linaro/qemu-for-merges/vl.c:1906
> #7  main (argc=<optimised out>, argv=<optimised out>, envp=<optimised
> out>) at /home/petmay01/linaro/qemu-for-merges/vl.c:4653
> 
> thanks
> -- PMM
> 

That's mine...

Paolo