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