Deadlock between devman and locsrv

[Ondra Hlavatý]

In yet undiscovered conditions, a deadlock while booting occurs. The kernel is still alive, but devman is stuck and the compositor won't start. It does happen quite frequently on "bad" builds, but "bad" build happens very sparsely. It seems to be a result of a race condition. Also, level of optimization is related - it happens more on optimized builds, never happened with -O0.

In the attached image, you can see the waiting IPC calls. We already discussed on this on IRC and mailing list, but then it happened once again, so here you have the bug report.

There is a cycle, which I haven't noticed at first. It is probably the cause of the deadlock, but I'm not able to identify the root cause myself.

Of course, there is a possibility that our xhci driver is causing it. But in every case, faulty driver should not block devman.

[Ondra Hlavatý]

IPC state in the deadlock

[Jakub Jermář]

With some effort, I was able to reproduce this issue. I get a similar picture of the waiting processes, see the attached log from kconsle for details.

[Jakub Jermář]

kconsole log with the output of the ipc command for the essential tasks

[Jakub Jermář]

List of all running tasks

[Jakub Jermář]

Console output up to the hang

[Jakub Jermář]

From the ipc log, I have partially reconstructed the wait graph of the system.

1. devman sends DRIVER_DEV_ADD(1024) to ps2mouse
2. ps2mouse sends DEVMAN_ADD_FUNCTION(1025) to devman
3. devman grabs device_tree.rwlock in fun_online()
4. devman enters loc_register_tree_function() and loc_service_register(); sends LOC_SERVICE_REGISTER(1027) to locsrv followed by IPC_M_DATA_WRITE(7)
5. locsrv for some reason does not reply to the LOC_SERVICE_REGISTER request; since there is no IPC on which LOC_SERVICE_REGISTER depends directly, this could mean that the request in locsrv is blocking on a synchronization primitive
6. locsrv itself has 4 active calls (method=1024) to ethip; had it been a single request, I would say this is LOC_EVENT_CAT_CHANGE(1024), but in this case I am not sure.
7. ethip is itself not answering any of the four 1024 calls from locsrv and sends IPC method 1029 to e1k
8. e1k sends HW_RES_ENABLE_INTERRUPT for IRQ 11 to pciintel
9. pciintel sends IPC_CONNECT_ME_TO(2) to ns which in turn forwards it to devman
10. devman tries to handle the call in devman_connection_device(), but gets blocked on the device_tree.rwlock which was locked in step 3 by fun_online()

Some notes about the things I don't know. Speculations follow:

If in step 6 the four messages sent by locsrv to ethip are indeed LOC_SERVICE_REGISTER, then they are sent from loc_category_change_event(), which takes the callback_sess_mutex. Currently, there can be IPC_MAX_ASYNC_CALLS=4 active calls per one phone. This means that if there are more than 4 callback sessions in the callback_sess_list all going to a single destination, the whole fibril will be blocked inside the callback_sess_mutex-protected critical section. This would correspond to the blocked nconfsrv which is waiting for LOC_CALLBACK_CREATE reply from locsrv. This message could not be answered at this point because callback_sess_mutex is held. I don't, however, see how ethip could have over 4 callbacks registered at locsrv.

I also don't know yet, what the communication between ethip and e1k represents. Any tips?

[Jakub Jermář]

So it looks like a driver registered itself in a category at locsrv. This resulted in a "category changed event" notification which transitively depended on devman and its ability to grab the device_tree.rwlock, but in the meantime devman started another request which grabbed device_tree.rwlock and was waiting for the locsrv connection fibril to handle its LOC_SERVICE_REGISTER request. locsrv was thus waiting for devman and devman was waiting for locsrv.

[Jakub Jermář]

I suspect this became reproducible when the interrupt controller drivers were DDFified. Before that pciintel would not go through devman when enabling the interrupt.

In order to fix this, three things are needed:

1. locsrv needs to be able to process multiple exchanges from devman in parallel, so the devman-to-locsrv exchange type will need to be upgraded to parallel from the current serial;
2. devman needs to be careful not to end exchanges with locsrv before it receives the answer, otherwise another request might reuse the old phone instead of using a new one, which would again lead to the present deadlock;
3. the locsrv connection fibril can get blocked even after answering the call from devman in loc_category_change_event() if the limit of maximum async calls per phone (currently 4, see IPC_MAX_ASYNC_CALLS) was reached; we basically need to make sure that when the call is answered the fibril will not block otherwise we will still be getting this deadlock.

As for 3., we might either change the code to return error instead of blocking the fibril if there are more than IPC_MAX_ASYNC_CALLS per phone or try to answer LOC_SERVICE_ADD_TO_CAT only after loc_category_change_event() returns.

[Jakub Jermář]

Fixed in mainline,2873, mainline,2874, mainline,2875, mainline,2876 and mainline,2877.