[RFC][PATCH 4/5] locking: Remove smp_mb__before_spinlock()

[Peter Zijlstra]

Now that there are no users of smp_mb__before_spinlock() left, remove
it entirely.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 Documentation/memory-barriers.txt                    |    5 ---
 Documentation/translations/ko_KR/memory-barriers.txt |    5 ---
 arch/arm64/include/asm/spinlock.h                    |    9 ------
 arch/powerpc/include/asm/barrier.h                   |    2 -
 fs/userfaultfd.c                                     |   25 ++++++++-----------
 include/linux/spinlock.h                             |   13 ---------
 6 files changed, 13 insertions(+), 46 deletions(-)

--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1982,10 +1982,7 @@ In all cases there are variants on "ACQU
      ACQUIRE operation has completed.
 
      Memory operations issued before the ACQUIRE may be completed after
-     the ACQUIRE operation has completed.  An smp_mb__before_spinlock(),
-     combined with a following ACQUIRE, orders prior stores against
-     subsequent loads and stores.  Note that this is weaker than smp_mb()!
-     The smp_mb__before_spinlock() primitive is free on many architectures.
+     the ACQUIRE operation has completed.
 
  (2) RELEASE operation implication:
 
--- a/Documentation/translations/ko_KR/memory-barriers.txt
+++ b/Documentation/translations/ko_KR/memory-barriers.txt
@@ -1956,10 +1956,7 @@ MMIO ìê ëëì
      ëì ìëëëë.
 
      ACQUIRE ììì ììë ëëë ìíëììì ACQUIRE ìíëììì ìëë íì
-     ìëë ì ììëë.  smp_mb__before_spinlock() ëì ACQUIRE ê ìíëë
-     ìë ëëì ëë ìì ìíìë ëë ëì ëëì ìíìì ëí ìì
-     ëìëë.  ìê smp_mb() ëë ìíë êìì êìíìì!  ëì ìííììì
-     smp_mb__before_spinlock() ì ìì ìëìë íì ììëë.
+     ìëë ì ììëë.
 
  (2) RELEASE ìíëììì ìí:
 
--- a/arch/arm64/include/asm/spinlock.h
+++ b/arch/arm64/include/asm/spinlock.h
@@ -358,15 +358,6 @@ static inline int arch_read_trylock(arch
 #define arch_read_relax(lock)	cpu_relax()
 #define arch_write_relax(lock)	cpu_relax()
 
-/*
- * Accesses appearing in program order before a spin_lock() operation
- * can be reordered with accesses inside the critical section, by virtue
- * of arch_spin_lock being constructed using acquire semantics.
- *
- * In cases where this is problematic (e.g. try_to_wake_up), an
- * smp_mb__before_spinlock() can restore the required ordering.
- */
-#define smp_mb__before_spinlock()	smp_mb()
 /* See include/linux/spinlock.h */
 #define smp_mb__after_spinlock()	smp_mb()
 
--- a/arch/powerpc/include/asm/barrier.h
+++ b/arch/powerpc/include/asm/barrier.h
@@ -74,8 +74,6 @@ do {									\
 	___p1;								\
 })
 
-#define smp_mb__before_spinlock()   smp_mb()
-
 #include <asm-generic/barrier.h>
 
 #endif /* _ASM_POWERPC_BARRIER_H */
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -109,27 +109,24 @@ static int userfaultfd_wake_function(wai
 		goto out;
 	WRITE_ONCE(uwq->waken, true);
 	/*
-	 * The implicit smp_mb__before_spinlock in try_to_wake_up()
-	 * renders uwq->waken visible to other CPUs before the task is
-	 * waken.
+	 * The Program-Order guarantees provided by the scheduler
+	 * ensure uwq->waken is visible before the task is woken.
 	 */
 	ret = wake_up_state(wq->private, mode);
-	if (ret)
+	if (ret) {
 		/*
 		 * Wake only once, autoremove behavior.
 		 *
-		 * After the effect of list_del_init is visible to the
-		 * other CPUs, the waitqueue may disappear from under
-		 * us, see the !list_empty_careful() in
-		 * handle_userfault(). try_to_wake_up() has an
-		 * implicit smp_mb__before_spinlock, and the
-		 * wq->private is read before calling the extern
-		 * function "wake_up_state" (which in turns calls
-		 * try_to_wake_up). While the spin_lock;spin_unlock;
-		 * wouldn't be enough, the smp_mb__before_spinlock is
-		 * enough to avoid an explicit smp_mb() here.
+		 * After the effect of list_del_init is visible to the other
+		 * CPUs, the waitqueue may disappear from under us, see the
+		 * !list_empty_careful() in handle_userfault().
+		 *
+		 * try_to_wake_up() has an implicit smp_mb(), and the
+		 * wq->private is read before calling the extern function
+		 * "wake_up_state" (which in turns calls try_to_wake_up).
 		 */
 		list_del_init(&wq->entry);
+	}
 out:
 	return ret;
 }
--- a/include/linux/spinlock.h
+++ b/include/linux/spinlock.h
@@ -118,19 +118,6 @@ do {								\
 #endif
 
 /*
- * Despite its name it doesn't necessarily has to be a full barrier.
- * It should only guarantee that a STORE before the critical section
- * can not be reordered with LOADs and STOREs inside this section.
- * spin_lock() is the one-way barrier, this LOAD can not escape out
- * of the region. So the default implementation simply ensures that
- * a STORE can not move into the critical section, smp_wmb() should
- * serialize it with another STORE done by spin_lock().
- */
-#ifndef smp_mb__before_spinlock
-#define smp_mb__before_spinlock()	smp_wmb()
-#endif
-
-/*
  * This barrier must provide two things:
  *
  *   - it must guarantee a STORE before the spin_lock() is ordered against a