The UI Has Landed
The Plugin Hang UI
landed in mozilla-central in time for January’s merge. This means that it is
now available on both Nightly and Aurora.
While it’s great that this code is now available to a larger audience, there
are consequences to this. :-)
Telemetry and Pref Adjustments
The first (and more pleasant) consequence is that we are now receiving telemetry
about the UI’s usage patterns. This allows us to make some adjustments as the
Plugin Hang UI gets closer to the release channel. As happy as I am that this
feature will be putting users in the driver’s seat when dealing with hung
plugins, it’s also important to not annoy users.
Initially our telemetry suggested that the Plugin Hang UI frequently appeared
but then cancelled automatically because the plugin resumed execution. This
indicated to us that we should increase the default value of the
dom.ipc.plugins.hangUITimeoutSecs preference (bug 833560).
There has also been some discussion about scaling the Hang UI threshold depending
on hardware performance and user behaviour. This threshold is tricky to balance;
while we want users to be able to terminate a hanging plugin, we want to provide
that feature with minimal annoyance.
Another consequence of the feature landing is that we received reports from
Nightly and Aurora showing that the Plugin Hang UI was inducing crashes in the
browser itself. Unfortunately the only thing that the stack traces were telling
us was that the browser-side code that hosts out-of-process plugins was not
being cleaned up properly after forcibly terminating the plugin container. We
didn’t have any steps to reproduce. What broke this mystery wide open was when
our crash stats were finally able to show some correlations. I learned that
nearly 50% of the crashes happened on single-core CPUs.
Once I was able to test this out for myself, the issue revealed itself in short order.
Fortunately for me, even though the problem involved thread scheduling, I was still able
to reproduce it in a debugger. This patch
took care of the problem.
There’s a bit of nuance to what was happening with this crash. When
plugin-container.exe is forcibly terminated, cleanup of the browser-facing
plugin code may be executed in one of two different sequences:
Sequence 1 (Most common)
plugin-container.exe, the Plugin Hang UI posts a
CleanupFromTimeout work item to the main thread. Concurrently on the
I/O thread, Firefox’s
RPCChannel detects an error and posts a
OnNotifyMaybeChannelError work item.
OnNotifyMaybeChannelError executes and sets the channel’s state to
ChannelError. It also cleans up the
PluginModuleParent actor and its
actors for subprotocols.
CleanupFromTimeout runs and attempts to Close the channel. This
is effectively a no-op since the channel was already closed with an
error status by step 2.
Sequence 2 (Less common unless running on fewer cores)
Same as in Sequence 1.
CleanupFromTimeout runs before
work item attempts to do a regular
Close on the channel. Since the
channel’s state is still set to
ChannelConnected, the close operation
doesn’t realize that it needs to do additonal cleanup. It performs
a clean shutdown of the RPC channel without properly cleaning up the
OnMaybeNotifyChannelError runs, sees the channel is already closed
due to the activities in step 2, and does nothing.
A crash later occurs because the actors were never cleaned up properly.
Some Additional Analysis
Sequence 2 cannot crash if the
plugin-container.exe process is terminated
by the main thread. This is because
false in this case and the channel’s state is set to
the time that the
CleanupFromTimeout work item executes. This guarantees that
a full cleanup will be done by
With the Plugin Hang UI,
plugin-container.exe is not terminated by the main
thread, so the channel’s state must be explicitly updated after termination.
The patch modifies the
CleanupFromTimeout work item so that if the
plugin container was terminated outside the main thread, the channel is
explicitly closed with an error state. This ensures that the actors are
properly cleaned up.
I filed bug 834127
when QA discovered that sometimes the Plugin Hang UI was not being displayed.
I found out that Firefox was correctly spawning
it was not showing any UI.
This lead be back down the input queue rabbit hole that I briefly discussed
last time. I learned that on an intermittent basis, the Plugin Hang UI dialog
box was being hung up on Win32
SetFocus calls. What I did
know was that my attempts to explicitly detach the Plugin Hang UI’s thread
from the hung Firefox thread weren’t working as well as I would have liked.
After numerous attempts
at fixing these issues, I determined that for the time being we will need to
rescind the owner-owned relationship between Firefox and the Plugin Hang UI
dialog. If we didn’t do so, seemingly benign actions like calling
or passing a
WM_NCLBUTTONDOWN message to
DefWindowProc would be enough
to bring the Plugin Hang UI to a halt. Why is this, you ask? I decided to
peek into kernel mode to find out.
Adventures in Kernel Mode
Recall that the guts of
GDI32 were moved into kernel mode
win32k.sys in the Windows NT 3.5 timeframe. It follows that any
efforts to examine Win32 internals necessitates peering into kernel mode.
I didn’t need an elaborate remote debugging setup for my purposes, so I
used Sysinternals LiveKd
to take a snapshot of my kernel and debug it.
My workflow was essentially as follows:
- Plugin Hang UI gets stuck
- Attach a debugger (I use WinDbg) to
- Run LiveKd
!thread -t <tid> into the kernel debugger, where
<tid> is the
thread ID that is hung in the user-mode WinDbg
Every kernel-mode call stack that I examined on a hung thread usually ended up going through this code path:
Child-SP RetAddr : Args to Child : Call Site
fffff880`0def3ec0 fffff800`0327c652 : fffffa80`079c43c0 fffffa80`079c43c0 00000000`00000000 fffffa80`00000008 : nt!KiSwapContext+0x7a
fffff880`0def4000 fffff800`0328da9f : fffffd54`0000000c fffffd54`000002a0 000002ac`00000000 00000804`fffffd54 : nt!KiCommitThreadWait+0x1d2
fffff880`0def4090 fffff800`03278a14 : 00000000`00000000 00000000`00000005 00000000`00000000 fffff800`03279600 : nt!KeWaitForSingleObject+0x19f
fffff880`0def4130 fffff800`03279691 : fffffa80`079c43c0 fffffa80`079c4410 00000000`00000000 00000000`00000000 : nt!KiSuspendThread+0x54
fffff880`0def4170 fffff800`0327c85d : fffffa80`079c43c0 00000000`00000000 fffff800`032789c0 00000000`00000000 : nt!KiDeliverApc+0x201
fffff880`0def41f0 fffff800`0328da9f : fffffa80`0a7510e0 fffff800`0327c26f fffffa80`00000000 fffff800`03402e80 : nt!KiCommitThreadWait+0x3dd
fffff880`0def4280 fffff960`0010d457 : fffff900`c2255200 00000000`0000000d 00000000`00000001 00000000`00000000 : nt!KeWaitForSingleObject+0x19f
fffff880`0def4320 fffff960`0010d4f1 : fffff880`0def0000 fffff900`c08e0e20 00000000`00000000 00000000`00000000 : win32k!xxxRealSleepThread+0x257
fffff880`0def43c0 fffff960`000c46d3 : 00000000`00000000 fffff900`c093ee90 00000000`00000200 00000000`00000046 : win32k!xxxSleepThread+0x59
fffff880`0def43f0 fffff960`0010c53e : fffff900`c093ee90 fffff900`c08e0e20 00000000`00000000 fffff900`c25724b0 : win32k!xxxInterSendMsgEx+0x112a
fffff880`0def4500 fffff960`000d8ccf : fffff900`c25724b0 fffff900`c25724b0 00000000`003c031a fffff900`c0800b90 : win32k!xxxSendMessageTimeout+0x1de
fffff880`0def45b0 fffff960`000dae0b : fffff960`00000006 fffff880`0def47c0 fffff960`00000000 fffff900`00000000 : win32k!xxxCalcValidRects+0x1a3
fffff880`0def46f0 fffff960`000dac3a : fffff900`00000001 fffff900`c08e0e20 fffff880`0def49b8 fffff900`00000000 : win32k!xxxEndDeferWindowPosEx+0x18f
fffff880`0def47b0 fffff960`000d6e09 : 00000000`00000000 00000000`00000001 fffff900`00000000 fffff800`00000000 : win32k!xxxSetWindowPos+0x156
fffff880`0def4830 fffff960`0007c66d : fffff900`00000000 fffff900`0004366c fffff900`00000000 fffff900`c209f410 : win32k!xxxActivateThisWindow+0x441
The highlighted hexadecimal value above is a handle to a window with class
MSCTFIME UI (i.e. a Microsoft IME window) that was created by the main
UI thread in the hung Flash process. Despite my best efforts to try to
disconnect the Plugin Hang UI input queue from these threads, Windows is
insistent on sending a synchronous, inter-thread message to the IME window
on the hung Flash thread. This doesn’t happen if we omit setting Firefox’s
HWND as the Plugin Hang UI’s owner window.
Now that the Plugin Hang UI specifies a
NULL owner, Windows can no longer
guarantee that the Plugin Hang UI will always appear above the Firefox window
that was active at the time that the plugin hang occurred. Having said that,
our experiments have shown that Windows does not try to promote an unresponsive
window to the front of the Z-order.
In a multiple-window situation, Windows may move the other, inactive Firefox
windows ahead of the Plugin Hang UI in the Z-order, but the window that was
active when Firefox first hung does not move. I think that this is acceptable.
In fact, the same behavour would be observable if we had been able to specify
If you’re interested in helping to test this feature, please install Nightly
and try the repro from bug 834127.
When the Plugin Hang UI fires, try to move around the Plugin Hang UI as well
as the other windows on your desktop, including Firefox. If the Z-order changes
differently from the way that I have observed here, please try to generate
some steps to reproduce, drop me a line (aklotz on
#perf or MoCo email)
and let me know what’s going on. Thanks!