From 069a8a3decf5981cd420014384ead470eac8561c Mon Sep 17 00:00:00 2001
From: Justus Winter <4winter@informatik.uni-hamburg.de>
Date: Tue, 18 Aug 2015 12:07:26 +0200
Subject: kern: provide a way to steal a lock

During the message transport, the sender acquires a lock that is
released by the sender.  By making this explicit, we can provide
stronger guarantees about the locking.

* ipc/ipc_mqueue.c (ipc_mqueue_receive): Steal lock.
* ipc/ipc_object.h (io_lock_steal): New macro.
* ipc/ipc_port.h (ip_lock_steal): Likewise.
* ipc/mach_msg.c (mach_msg_trap): Steal lock in the inlined version of
`ipc_mqueue_receive'.
* kern/lock.c (lock_done): Assert that the current thread holds the lock.
* kern/lock.h (lock_write_steal): New macro.
---
 kern/lock.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

(limited to 'kern/lock.h')

diff --git a/kern/lock.h b/kern/lock.h
index 74cd933..c8af883 100644
--- a/kern/lock.h
+++ b/kern/lock.h
@@ -211,10 +211,21 @@ extern void		lock_clear_recursive(lock_t);
 #if	! MACH_LDEBUG
 #define have_read_lock(l)	1
 #define have_write_lock(l)	1
+#define lock_write_surrender(l)
+#define lock_write_steal(l)
 #else	/* MACH_LDEBUG */
 /* XXX: We don't keep track of readers, so this is an approximation.  */
 #define have_read_lock(l)	((l)->read_count > 0)
 #define have_write_lock(l)	((l)->writer == current_thread())
+#define THREAD_SURRENDERED	((thread_t) ~0U)
+#define lock_write_surrender(l)	({					\
+			assert(have_write_lock(l));			\
+			(l)->writer = THREAD_SURRENDERED;		\
+		})
+#define lock_write_steal(l)	({					\
+			assert((l)->writer == THREAD_SURRENDERED);	\
+			(l)->writer = current_thread();			\
+		})
 #endif	/* MACH_LDEBUG */
 #define have_lock(l)		(have_read_lock(l) || have_write_lock(l))
 
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