[PATCH] V1 1/2] ipc: let message queues use SIGEV_THREAD_ID with mq_notify

[Steven Stewart-Gallus <sstewartgallus00@mylangara.bc.ca>]

From: Steven Stewart-Gallus <sstewartgallus00@mylangara.bc.ca>

Currently the only thread-safe way of using mq_notify with message
queues is to use the SIGEV_THREAD option. Unfortunately, existing
wrappers around such functionality spawn a thread each time a
notification happens instead of caching threads which is slow and
inconvenient for debugging. This change lets message queues use
SIGEV_THREAD_ID with mq_notify and so target notifications to only one
thread at a time which is thread-safe.

Signed-off-by: Steven Stewart-Gallus <sstewartgallus00@mylangara.bc.ca>
---

Hello! This is a simple patch I made more for the purpose of learning
how the kernel works then for any serious purpose. Currently, in my
own program I now simply use a thread pool and poll for asynchronously
running message sends and receives but a thread-safe mq_notify would
have been useful for me and possibly saved me some effort. So, I won't
be seriously bothered if this patch is rejected but I think other
people might find it useful nonetheless. I also think this behaviour
seems quite natural and appropriate to support.

I am sending this patch for review but I am pretty sure it is mistaken
in some ways. In particular, I believe this patch is wrong because I
don't believe it filters out thread ids that belong to other processes
and I am not sure how to do that.

Thank you,
Steven Stewart-Gallus

diff --git a/man3/mq_notify.3 b/man3/mq_notify.3
index a71aac4..41a1b96 100644
--- a/man3/mq_notify.3
+++ b/man3/mq_notify.3
@@ -95,6 +95,16 @@ as if it were the start function of a new thread.
 See
 .BR sigevent (7)
 for details.
+.TP
+.BR SIGEV_THREAD_ID " (Linux-specific)"
+As for
+.BR SIGEV_SIGNAL ,
+but the signal is targeted at the thread whose ID is given in
+.IR sigev_notify_thread_id ,
+which must be a thread in the same process as the caller.
+See
+.BR sigevent (7)
+for general details.
 .PP
 Only one process can be registered to receive notification
 from a message queue.
diff --git a/man7/sigevent.7 b/man7/sigevent.7
index 9cec77e..15da9fe 100644
--- a/man7/sigevent.7
+++ b/man7/sigevent.7
@@ -125,8 +125,10 @@ structure that defines attributes for the new thread (see
 .TP
 .BR SIGEV_THREAD_ID " (Linux-specific)"
 .\" | SIGEV_SIGNAL vs not?
-Currently used only by POSIX timers; see
-.BR timer_create (2).
+Currently used only by POSIX timers and message queues; see
+.BR timer_create (2)
+and
+.BR mq_notify (2).
 .SH CONFORMING TO
 POSIX.1-2001.
 .SH SEE ALSO