From fb8c531ddeef55c8581d3882bd1b1f501d980a30 Mon Sep 17 00:00:00 2001
From: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Mon, 15 Dec 2014 11:53:26 +0100
Subject: libports: avoid acquiring global lock in message dispatch

* libports/interrupt-operation.c (ports_S_interrupt_operation): Update
`cancel_threshold' using atomic operations.
* libports/manage-multithread.c (internal_demuxer): Avoid taking the lock.
* libports/ports.h (struct port_info): Mention that one needs atomic
operations to access `cancel_threshold'.
---
 libports/interrupt-operation.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

(limited to 'libports/interrupt-operation.c')

diff --git a/libports/interrupt-operation.c b/libports/interrupt-operation.c
index 943bd4f4..5d4b0b7a 100644
--- a/libports/interrupt-operation.c
+++ b/libports/interrupt-operation.c
@@ -27,12 +27,18 @@ kern_return_t
 ports_S_interrupt_operation (struct port_info *pi,
 			     mach_port_seqno_t seqno)
 {
+  mach_port_seqno_t old;
+
   if (!pi)
     return EOPNOTSUPP;
-  pthread_mutex_lock (&_ports_lock);
-  if (pi->cancel_threshold < seqno)
-    pi->cancel_threshold = seqno;
-  pthread_mutex_unlock (&_ports_lock);
+
+ retry:
+  old = __atomic_load_n (&pi->cancel_threshold, __ATOMIC_SEQ_CST);
+  if (old < seqno
+      && ! __atomic_compare_exchange_n (&pi->cancel_threshold, &old, seqno,
+					0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST))
+	goto retry;
+
   ports_interrupt_rpcs (pi);
   return 0;
 }
-- 
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