Re: [Qemu-devel] [PATCH v2 0/3] AioContext: ctx->dispatching is dead, all hail ctx->notify_me

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/07/2015 06:44, Paolo Bonzini wrote:
> 
> 
> On 16/07/2015 21:05, Richard W.M. Jones wrote:
>>
>> Sorry to spoil things, but I'm still seeing this bug, although it is
>> now a lot less frequent with your patch.  I would estimate it happens
>> more often than 1 in 5 runs with qemu.git, and probably 1 in 200 runs
>> with qemu.git + the v2 patch series.
>>
>> It's the exact same hang in both cases.
>>
>> Is it possible that this patch doesn't completely close any race?
>>
>> Still, it is an improvement, so there is that.
> 
> Would seem at first glance like a different bug.
> 
> Interestingly, adding some "tracing" (qemu_clock_get_ns) makes the bug
> more likely: now it reproduces in about 10 tries.  Of course :) adding
> other kinds of tracing instead make it go away again (>50 tries).
> 
> Perhaps this:
> 
>    i/o thread         vcpu thread                   worker thread
>    ---------------------------------------------------------------------
>    lock_iothread
>    notify_me = 1
>    ...
>    unlock_iothread
>                       lock_iothread
>                       notify_me = 3
>                       ppoll
>                       notify_me = 1
>                                                      bh->scheduled = 1
>                                                      event_notifier_set
>                       event_notifier_test_and_clear
>    ppoll
>     ^^ hang
> 	
> In the exact shape above, it doesn't seem too likely to happen, but
> perhaps there's another simpler case.  Still, the bug exists.
> 
> The above is not really related to notify_me.  Here the notification is
> not being optimized away!  So I wonder if this one has been there forever.
> 
> Fam suggested putting the event_notifier_test_and_clear before
> aio_bh_poll(), but it does not work.  I'll look more close
> 
> However, an unconditional event_notifier_test_and_clear is pretty
> expensive.  On one hand, obviously correctness comes first.  On the
> other hand, an expensive operation at the wrong place can mask the race
> very easily; I'll let the fix run for a while, but I'm not sure if a
> successful test really says anything useful.

So it may not be useful, but still successful test is successful. :)
The following patch, which also includes the delta between v2 and v3
of this series, survived 674 reboots before hitting a definitely
unrelated problem:

error: kvm run failed Function not implemented
PC=00000000bf671210  SP=00000000c00001f0
X00=000000000a003e70 X01=0000000000000000 X02=00000000bf680548 X03=0000000000000030
X04=00000000bbb5fc18 X05=00000000004b7770 X06=00000000bf721930 X07=000000000000009a
X08=00000000bf716858 X09=0000000000000090 X10=0000000000000000 X11=0000000000000046
X12=00000000a007e03a X13=0000000000000000 X14=0000000000000000 X15=0000000000000000
X16=00000000bf716df0 X17=0000000000000000 X18=0000000000000000 X19=00000000bf6f5f18
X20=0000000000000000 X21=0000000000000000 X22=0000000000000000 X23=0000000000000000
X24=0000000000000000 X25=0000000000000000 X26=0000000000000000 X27=0000000000000000
X28=0000000000000000 X29=0000000000000000 X30=0000000000000000 PSTATE=60000305 (flags -ZC-)

For the record, this is the kvm_run struct:

$6 = {request_interrupt_window = 0 '\000', padding1 = "\000\000\000\000\000\000", exit_reason = 0, 
  ready_for_interrupt_injection = 0 '\000', if_flag = 0 '\000', flags = 0, cr8 = 0, apic_base = 0, {hw = {
      hardware_exit_reason = 150994968}, fail_entry = {hardware_entry_failure_reason = 150994968}, ex = {
      exception = 150994968, error_code = 0}, io = {direction = 24 '\030', size = 0 '\000', port = 2304, 
      count = 0, data_offset = 144}, debug = {arch = {<No data fields>}}, mmio = {phys_addr = 150994968, 
      data = "\220\000\000\000\000\000\000", len = 4, is_write = 0 '\000'}, hypercall = {nr = 150994968, 
      args = {144, 4, 0, 0, 0, 0}, ret = 0, longmode = 0, pad = 0}, tpr_access = {rip = 150994968, 
      is_write = 144, pad = 0}, s390_sieic = {icptcode = 24 '\030', ipa = 2304, ipb = 0}, 
    s390_reset_flags = 150994968, s390_ucontrol = {trans_exc_code = 150994968, pgm_code = 144}, dcr = {
      dcrn = 150994968, data = 0, is_write = 144 '\220'}, internal = {suberror = 150994968, ndata = 0, 
      data = {144, 4, 0 <repeats 14 times>}}, osi = {gprs = {150994968, 144, 4, 0 <repeats 29 times>}}, 
    papr_hcall = {nr = 150994968, ret = 144, args = {4, 0, 0, 0, 0, 0, 0, 0, 0}}, s390_tsch = {
      subchannel_id = 24, subchannel_nr = 2304, io_int_parm = 0, io_int_word = 144, ipb = 0, 
      dequeued = 4 '\004'}, epr = {epr = 150994968}, system_event = {type = 150994968, flags = 144}, 
    s390_stsi = {addr = 150994968, ar = 144 '\220', reserved = 0 '\000', fc = 0 '\000', sel1 = 0 '\000', 
      sel2 = 0}, 
    padding = "\030\000\000\t\000\000\000\000\220\000\000\000\000\000\000\000\004", '\000' <repeats 238 times>}, kvm_valid_regs = 0, kvm_dirty_regs = 0, s = {regs = {<No data fields>}, 
    padding = '\000' <repeats 2047 times>}}

Marc, does it ring any bell?

Paolo

diff --git a/aio-posix.c b/aio-posix.c
index 268d14d..d2011d0 100644
--- a/aio-posix.c
+++ b/aio-posix.c
@@ -273,6 +273,13 @@ bool aio_poll(AioContext *ctx, bool blocking)
         aio_context_acquire(ctx);
     }
 
+    /* This should be optimized... */
+    event_notifier_test_and_clear(&ctx->notifier);
+
+    if (blocking) {
+        atomic_sub(&ctx->notify_me, 2);
+    }
+
     /* if we have any readable fds, dispatch event */
     if (ret > 0) {
         for (i = 0; i < npfd; i++) {
@@ -283,10 +290,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
     npfd = 0;
     ctx->walking_handlers--;
 
-    if (blocking) {
-        atomic_sub(&ctx->notify_me, 2);
-    }
-
     /* Run dispatch even if there were no readable fds to run timers */
     if (aio_dispatch(ctx)) {
         progress = true;
diff --git a/aio-win32.c b/aio-win32.c
index 2bfd5f8..33809fd 100644
--- a/aio-win32.c
+++ b/aio-win32.c
@@ -326,6 +326,10 @@ bool aio_poll(AioContext *ctx, bool blocking)
         if (timeout) {
             aio_context_acquire(ctx);
         }
+
+        /* This should be optimized... */
+        event_notifier_test_and_clear(&ctx->notifier);
+
         if (blocking) {
             assert(first);
             atomic_sub(&ctx->notify_me, 2);
diff --git a/async.c b/async.c
index 9204907..120e183 100644
--- a/async.c
+++ b/async.c
@@ -202,6 +202,9 @@ aio_ctx_check(GSource *source)
     AioContext *ctx = (AioContext *) source;
     QEMUBH *bh;
 
+    /* This should be optimized... */
+    event_notifier_test_and_clear(&ctx->notifier);
+
     atomic_and(&ctx->notify_me, ~1);
     for (bh = ctx->first_bh; bh; bh = bh->next) {
         if (!bh->deleted && bh->scheduled) {
@@ -280,6 +280,10 @@ static void aio_rfifolock_cb(void *opaque)
     aio_notify(opaque);
 }
 
+static void event_notifier_dummy_cb(EventNotifier *e)
+{
+}
+
 AioContext *aio_context_new(Error **errp)
 {
     int ret;
@@ -292,7 +296,7 @@ AioContext *aio_context_new(Error **errp)
         return NULL;
     }
     g_source_set_can_recurse(&ctx->source, true);
-    aio_set_event_notifier(ctx, &ctx->notifier, NULL);
+    aio_set_event_notifier(ctx, &ctx->notifier, event_notifier_dummy_cb);
     ctx->thread_pool = NULL;
     qemu_mutex_init(&ctx->bh_lock);
     rfifolock_init(&ctx->lock, aio_rfifolock_cb, ctx);