Revert "rwsem-rt: Do not allow readers to nest"

This behaviour is required by cpufreq and its logic is "okay": It does a
read_lock followed by a try_read_lock.
Lockdep warns if one try a read_lock twice in -RT and vanilla so it
should be good. We still only allow multiple readers as long as it is in
the same process.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

diff --git a/include/linux/rwsem_rt.h b/include/linux/rwsem_rt.h
index 0065b08fbb7a671195b5f5ec707f3d0a1aa88ad5..924c2d274ab5d6593427e1d98e302ace2fe04642 100644 (file)
--- a/include/linux/rwsem_rt.h
+++ b/include/linux/rwsem_rt.h
@@ -20,6 +20,7 @@
 
 struct rw_semaphore {
        struct rt_mutex         lock;
+       int                     read_depth;
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
        struct lockdep_map      dep_map;
 #endif

diff --git a/kernel/locking/rt.c b/kernel/locking/rt.c
index 307983c639c30fb4de0333b66a96b75c18a12e01..2852623d46a756064c0d85d3d8c66054d4a4a014 100644 (file)
--- a/kernel/locking/rt.c
+++ b/kernel/locking/rt.c
@@ -316,7 +316,8 @@ EXPORT_SYMBOL(rt_up_write);
 void  rt_up_read(struct rw_semaphore *rwsem)
 {
        rwsem_release(&rwsem->dep_map, 1, _RET_IP_);
-       rt_mutex_unlock(&rwsem->lock);
+       if (--rwsem->read_depth == 0)
+               rt_mutex_unlock(&rwsem->lock);
 }
 EXPORT_SYMBOL(rt_up_read);
 
@@ -327,6 +328,7 @@ EXPORT_SYMBOL(rt_up_read);
 void  rt_downgrade_write(struct rw_semaphore *rwsem)
 {
        BUG_ON(rt_mutex_owner(&rwsem->lock) != current);
+       rwsem->read_depth = 1;
 }
 EXPORT_SYMBOL(rt_downgrade_write);
 
@@ -356,12 +358,23 @@ EXPORT_SYMBOL(rt_down_write_nested);
 
 int  rt_down_read_trylock(struct rw_semaphore *rwsem)
 {
-       int ret;
+       struct rt_mutex *lock = &rwsem->lock;
+       int ret = 1;
 
-       ret = rt_mutex_trylock(&rwsem->lock);
-       if (ret)
-               rwsem_acquire(&rwsem->dep_map, 0, 1, _RET_IP_);
+       /*
+        * recursive read locks succeed when current owns the rwsem,
+        * but not when read_depth == 0 which means that the rwsem is
+        * write locked.
+        */
+       if (rt_mutex_owner(lock) != current)
+               ret = rt_mutex_trylock(&rwsem->lock);
+       else if (!rwsem->read_depth)
+               ret = 0;
 
+       if (ret) {
+               rwsem->read_depth++;
+               rwsem_acquire(&rwsem->dep_map, 0, 1, _RET_IP_);
+       }
        return ret;
 }
 EXPORT_SYMBOL(rt_down_read_trylock);
@@ -371,7 +384,10 @@ static void __rt_down_read(struct rw_semaphore *rwsem, int subclass)
        struct rt_mutex *lock = &rwsem->lock;
 
        rwsem_acquire_read(&rwsem->dep_map, subclass, 0, _RET_IP_);
-       rt_mutex_lock(&rwsem->lock);
+
+       if (rt_mutex_owner(lock) != current)
+               rt_mutex_lock(&rwsem->lock);
+       rwsem->read_depth++;
 }
 
 void  rt_down_read(struct rw_semaphore *rwsem)
@@ -396,6 +412,7 @@ void  __rt_rwsem_init(struct rw_semaphore *rwsem, const char *name,
        debug_check_no_locks_freed((void *)rwsem, sizeof(*rwsem));
        lockdep_init_map(&rwsem->dep_map, name, key, 0);
 #endif
+       rwsem->read_depth = 0;
        rwsem->lock.save_state = 0;
 }
 EXPORT_SYMBOL(__rt_rwsem_init);