Add support for rw semaphore under PREEMPT_RT_FULL

[mirror_spl.git] / module / spl / spl-rwlock.c

diff --git a/module/spl/spl-rwlock.c b/module/spl/spl-rwlock.c
index 77f46f2d6fdf5119132424fe396de23378d4e644..9e96c4f27ec01593ccf8d348b590ee095a189dd7 100644 (file)
--- a/module/spl/spl-rwlock.c
+++ b/module/spl/spl-rwlock.c
@@ -32,7 +32,37 @@
 
 #define DEBUG_SUBSYSTEM S_RWLOCK
 
-#if defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
+#if defined(CONFIG_PREEMPT_RT_FULL)
+
+#include <linux/rtmutex.h>
+
+static int
+__rwsem_tryupgrade(struct rw_semaphore *rwsem)
+{
+       ASSERT(rt_mutex_owner(&rwsem->lock) == current);
+
+       /*
+        * Under the realtime patch series, rwsem is implemented as a
+        * single mutex held by readers and writers alike. However,
+        * this implementation would prevent a thread from taking a
+        * read lock twice, as the mutex would already be locked on
+        * the second attempt. Therefore the implementation allows a
+        * single thread to take a rwsem as read lock multiple times
+        * tracking that nesting as read_depth counter.
+        */
+       if (rwsem->read_depth <= 1) {
+               /*
+                * In case, the current thread has not taken the lock
+                * more than once as read lock, we can allow an
+                * upgrade to a write lock. rwsem_rt.h implements
+                * write locks as read_depth == 0.
+                */
+               rwsem->read_depth = 0;
+               return (1);
+       }
+       return (0);
+}
+#elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
 static int
 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 {