[PATCH V2 5/5] libxfs: add wrappers for kernel semaphores

[Chandan Babu R <chandan.babu@oracle.com>]

From: Dave Chinner <dchinner@redhat.com>

Implemented via pthread mutexes.

On Linux, fast pthread mutexes don't actaully check which thread
owns the lock on unlock, so can be used in situations where the
unlock occurs in a different thread to the lock. This is
non-portable behaviour, so if other platforms are supported, this
may need to be converted to posix semaphores.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
 include/Makefile     |  1 +
 include/libxfs.h     |  1 +
 include/sema.h       | 35 +++++++++++++++++++++++++++++++++++
 libxfs/libxfs_priv.h |  1 +
 4 files changed, 38 insertions(+)
 create mode 100644 include/sema.h

diff --git a/include/Makefile b/include/Makefile
index 98031e70..ce89d023 100644
--- a/include/Makefile
+++ b/include/Makefile
@@ -17,6 +17,7 @@ LIBHFILES = libxfs.h \
 	kmem.h \
 	list.h \
 	parent.h \
+	sema.h \
 	spinlock.h \
 	xfs_inode.h \
 	xfs_log_recover.h \
diff --git a/include/libxfs.h b/include/libxfs.h
index 61475347..ca5a21b0 100644
--- a/include/libxfs.h
+++ b/include/libxfs.h
@@ -20,6 +20,7 @@
 #include "atomic.h"
 #include "spinlock.h"
 #include "completion.h"
+#include "sema.h"
 
 #include "xfs_types.h"
 #include "xfs_fs.h"
diff --git a/include/sema.h b/include/sema.h
new file mode 100644
index 00000000..bcccb156
--- /dev/null
+++ b/include/sema.h
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2019-20 RedHat, Inc.
+ * All Rights Reserved.
+ */
+#ifndef __LIBXFS_SEMA_H__
+#define __LIBXFS_SEMA_H__
+
+/*
+ * This implements kernel compatible semaphore _exclusion_ semantics. It does
+ * not implement counting semaphore behaviour.
+ *
+ * This makes use of the fact that fast pthread mutexes on Linux don't check
+ * that the unlocker is the same thread that locked the mutex, and hence can be
+ * unlocked in a different thread safely.
+ *
+ * If this needs to be portable or we require counting semaphore behaviour in
+ * libxfs code, this requires re-implementation based on posix semaphores.
+ */
+struct semaphore {
+	pthread_mutex_t		lock;
+};
+
+#define sema_init(l, nolock)		\
+do {					\
+	pthread_mutex_init(&(l)->lock, NULL);	\
+	if (!nolock)			\
+		pthread_mutex_lock(&(l)->lock);	\
+} while (0)
+
+#define down(l)			pthread_mutex_lock(&(l)->lock)
+#define down_trylock(l)		pthread_mutex_trylock(&(l)->lock)
+#define up(l)			pthread_mutex_unlock(&(l)->lock)
+
+#endif /* __LIBXFS_SEMA_H__ */
diff --git a/libxfs/libxfs_priv.h b/libxfs/libxfs_priv.h
index 9f28fd90..1fc243cf 100644
--- a/libxfs/libxfs_priv.h
+++ b/libxfs/libxfs_priv.h
@@ -50,6 +50,7 @@
 #include "atomic.h"
 #include "spinlock.h"
 #include "completion.h"
+#include "sema.h"
 
 #include "xfs_types.h"
 #include "xfs_arch.h"
-- 
2.30.2