doc: add IPC callback limitations

For asynchronous requests, user callback may be triggered either from
IPC thread or from interrupt thread. Because of this, delivery of
other interrupt-based events such as alarms may not be possible inside
the asynchronous IPC request callback handler. Document this
limitation.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>

diff --git a/doc/guides/prog_guide/multi_proc_support.rst b/doc/guides/prog_guide/multi_proc_support.rst
index 46a00ec..1384fe3 100644 (file)
--- a/doc/guides/prog_guide/multi_proc_support.rst
+++ b/doc/guides/prog_guide/multi_proc_support.rst
@@ -220,8 +220,8 @@ way communication mechanism, with the requester expecting a response from the
 other side.
 
 Both messages and requests will trigger a named callback on the receiver side.
-These callbacks will be called from within a dedicated IPC thread that is not
-part of EAL lcore threads.
+These callbacks will be called from within a dedicated IPC or interrupt thread
+that are not part of EAL lcore threads.
 
 Registering for incoming messages
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -280,6 +280,13 @@ For asynchronous requests, a function pointer to the callback function must be
 provided instead. This callback will be called when the request either has timed
 out, or will have received a response to all the messages that were sent.
 
+.. warning::
+
+    When an asynchronous request times out, the callback will be called not by
+    a dedicated IPC thread, but rather from EAL interrupt thread. Because of
+    this, it may not be possible for DPDK to trigger another interrupt-based
+    event (such as an alarm) while handling asynchronous IPC callback.
+
 When the callback is called, the original request descriptor will be provided
 (so that it would be possible to determine for which sent message this is a
 callback to), along with a response descriptor like the one described above.
@@ -311,6 +318,12 @@ supported. However, since sending messages (not requests) does not involve an
 IPC thread, sending messages while processing another message or request is
 supported.
 
+Asynchronous request callbacks may be triggered either from IPC thread or from
+interrupt thread, depending on whether the request has timed out. It is
+therefore suggested to avoid waiting for interrupt-based events (such as alarms)
+inside asynchronous IPC request callbacks. This limitation does not apply to
+messages or synchronous requests.
+
 If callbacks spend a long time processing the incoming requests, the requestor
 might time out, so setting the right timeout value on the requestor side is
 imperative.