mirror of
git://projects.qi-hardware.com/wernermisc.git
synced 2024-12-27 02:39:53 +02:00
99 lines
3.9 KiB
Diff
99 lines
3.9 KiB
Diff
This issue is under review:
|
|
https://www.rtems.org/bugzilla/show_bug.cgi?id=1961
|
|
|
|
If it's permissible to call rtems_message_queue_send from an
|
|
interrupt, then there is at least one race condition in the core
|
|
message subsystem.
|
|
|
|
This created the MIDI/mouse hang we love so much on M1.
|
|
|
|
The problem is as follows: RTEMS queues use pre-allocated message
|
|
buffers that are kept on an "inactive" (free) list. When enqueuing
|
|
a message, a buffer is first removed from the inactive list, data
|
|
it copied to it, and it is then added to the pending list.
|
|
|
|
The reverse happens when dequeuing. Besides these two queues, there
|
|
is also a counter called number_of_pending_messages keeping track,
|
|
as the name suggests, of the number of pending messages. It is
|
|
updated atomically together with changes to the pending buffers
|
|
list.
|
|
|
|
From the above it is clear that the counter will be out of sync with
|
|
the inactive list between the beginning and the end of an enqueue or
|
|
dequeue operation.
|
|
|
|
In order to minimize interrupt latency, RTEMS disables interrupts
|
|
only when adding and removing buffers from lists, but not throughout
|
|
the whole enqueuing/dequeuing operation. Instead, it disables the
|
|
scheduler during these operations, but this doesn't prevent
|
|
interrupts.
|
|
|
|
This means that the inconsistency between number_of_pending_messages
|
|
and the inactive list can be observed from an interrupt handler if
|
|
enqueuing or dequeuing is in progress.
|
|
|
|
_CORE_message_queue_Submit checks whether there is still room in the
|
|
queue by reading number_of_pending_messages. If there is room, it
|
|
then calls _CORE_message_queue_Allocate_message_buffer to obtain a
|
|
free buffer.
|
|
|
|
Given that number_of_pending_messages and the list of inactive
|
|
buffers can disagree, e.g., if _CORE_message_queue_Seize or another
|
|
_CORE_message_queue_Submit is executing concurrently,
|
|
_CORE_message_queue_Allocate_message_buffer may fail to obtain a
|
|
free buffer despite the prior test.
|
|
|
|
_CORE_message_queue_Allocate_message_buffer can detect a lack of
|
|
free buffers and indicates it by returning a NULL pointer. Checking
|
|
whether NULL has been returned instead of a buffer is optional and
|
|
depends on RTEMS_DEBUG.
|
|
|
|
If no check is performed, _CORE_message_queue_Submit will then try
|
|
to use the buffer. In the absence of hardware detecting the
|
|
de-referencing of NULL pointers, the wounded system will limp on a
|
|
little further until, at least in the case of M1, it finally hangs
|
|
somewhere.
|
|
|
|
The patch below avoids the problem in the scenario described above
|
|
by not using number_of_pending_messages as an indicator of whether
|
|
free buffers are available, but by simply trying to get a buffer,
|
|
and handling the result of failure.
|
|
|
|
This is similar to how _CORE_message_queue_Seize works.
|
|
|
|
Another possibility would be to make testing of the_message no
|
|
longer optional. But then, there would basically be two tests for
|
|
the same condition, which is ugly.
|
|
|
|
- Werner
|
|
|
|
Index: rtems/cpukit/score/src/coremsgsubmit.c
|
|
===================================================================
|
|
--- rtems.orig/cpukit/score/src/coremsgsubmit.c 2011-11-12 09:15:12.000000000 -0300
|
|
+++ rtems/cpukit/score/src/coremsgsubmit.c 2011-11-12 09:15:17.000000000 -0300
|
|
@@ -101,21 +101,9 @@
|
|
* No one waiting on the message queue at this time, so attempt to
|
|
* queue the message up for a future receive.
|
|
*/
|
|
- if ( the_message_queue->number_of_pending_messages <
|
|
- the_message_queue->maximum_pending_messages ) {
|
|
-
|
|
- the_message =
|
|
- _CORE_message_queue_Allocate_message_buffer( the_message_queue );
|
|
-
|
|
- #if defined(RTEMS_DEBUG)
|
|
- /*
|
|
- * NOTE: If the system is consistent, this error should never occur.
|
|
- */
|
|
-
|
|
- if ( !the_message )
|
|
- return CORE_MESSAGE_QUEUE_STATUS_UNSATISFIED;
|
|
- #endif
|
|
-
|
|
+ the_message =
|
|
+ _CORE_message_queue_Allocate_message_buffer( the_message_queue );
|
|
+ if ( the_message ) {
|
|
_CORE_message_queue_Copy_buffer(
|
|
buffer,
|
|
the_message->Contents.buffer,
|