Add new 'cond_resched_bkl()' helper function

It acts exactly like a regular 'cond_resched()', but will not get
optimized away when CONFIG_PREEMPT is set.

Normal kernel code is already preemptable in the presense of
CONFIG_PREEMPT, so cond_resched() is optimized away (see commit
02b67cc "sched: do not do
cond_resched() when CONFIG_PREEMPT").

But when wanting to conditionally reschedule while holding a lock, you
need to use "cond_sched_lock(lock)", and the new function is the BKL
equivalent of that.

Also make fs/locks.c use it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/locks.c

@@ -773,7 +773,7 @@ static int flock_lock_file(struct file *filp, struct file_lock *request)
 	 * give it the opportunity to lock the file.
 	 */
 	if (found)
-		cond_resched();
+		cond_resched_bkl();
 
 find_conflict:
 	for_each_lock(inode, before) {

include/linux/sched.h

@@ -2037,20 +2037,24 @@ static inline int need_resched(void)
  * cond_resched_lock() will drop the spinlock before scheduling,
  * cond_resched_softirq() will enable bhs before scheduling.
  */
+extern int _cond_resched(void);
 #ifdef CONFIG_PREEMPT
 static inline int cond_resched(void)
 {
 	return 0;
 }
 #else
-extern int _cond_resched(void);
 static inline int cond_resched(void)
 {
 	return _cond_resched();
 }
 #endif
 extern int cond_resched_lock(spinlock_t * lock);
 extern int cond_resched_softirq(void);
+static inline int cond_resched_bkl(void)
+{
+	return _cond_resched();
+}
 
 /*
  * Does a critical section need to be broken due to another

kernel/sched.c

@@ -5525,7 +5525,6 @@ static void __cond_resched(void)
 	} while (need_resched());
 }
 
-#if !defined(CONFIG_PREEMPT) || defined(CONFIG_PREEMPT_VOLUNTARY)
 int __sched _cond_resched(void)
 {
 	if (need_resched() && !(preempt_count() & PREEMPT_ACTIVE) &&
@@ -5536,7 +5535,6 @@ int __sched _cond_resched(void)
 	return 0;
 }
 EXPORT_SYMBOL(_cond_resched);
-#endif
 
 /*
  * cond_resched_lock() - if a reschedule is pending, drop the given lock,