X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=kernel%2Ffutex.c;h=5f589279e4626c47b75b8a1d193d089f24a2f7b9;hb=c10dddc1d5055467f67d50724f5297fe23b69cc2;hp=08ec814ad9d2fe95ae4303085cb1967265bd3758;hpb=bfef92bb97b48d0ab5a543e2e7de09b6b014603e;p=firefly-linux-kernel-4.4.55.git

diff --git a/kernel/futex.c b/kernel/futex.c
index 08ec814ad9d2..5f589279e462 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -70,7 +70,10 @@
 #include "locking/rtmutex_common.h"
 
 /*
- * Basic futex operation and ordering guarantees:
+ * READ this before attempting to hack on futexes!
+ *
+ * Basic futex operation and ordering guarantees
+ * =============================================
  *
  * The waiter reads the futex value in user space and calls
  * futex_wait(). This function computes the hash bucket and acquires
@@ -119,7 +122,7 @@
  * sys_futex(WAIT, futex, val);
  *   futex_wait(futex, val);
  *
- *   waiters++;
+ *   waiters++; (a)
  *   mb(); (A) <-- paired with -.
  *                              |
  *   lock(hash_bucket(futex));  |
@@ -135,14 +138,14 @@
  *     unlock(hash_bucket(futex));
  *     schedule();                         if (waiters)
  *                                           lock(hash_bucket(futex));
- *                                           wake_waiters(futex);
- *                                           unlock(hash_bucket(futex));
+ *   else                                    wake_waiters(futex);
+ *     waiters--; (b)                        unlock(hash_bucket(futex));
  *
- * Where (A) orders the waiters increment and the futex value read -- this
- * is guaranteed by the head counter in the hb spinlock; and where (B)
- * orders the write to futex and the waiters read -- this is done by the
- * barriers in get_futex_key_refs(), through either ihold or atomic_inc,
- * depending on the futex type.
+ * Where (A) orders the waiters increment and the futex value read through
+ * atomic operations (see hb_waiters_inc) and where (B) orders the write
+ * to futex and the waiters read -- this is done by the barriers in
+ * get_futex_key_refs(), through either ihold or atomic_inc, depending on the
+ * futex type.
  *
  * This yields the following case (where X:=waiters, Y:=futex):
  *
@@ -155,9 +158,22 @@
  * Which guarantees that x==0 && y==0 is impossible; which translates back into
  * the guarantee that we cannot both miss the futex variable change and the
  * enqueue.
+ *
+ * Note that a new waiter is accounted for in (a) even when it is possible that
+ * the wait call can return error, in which case we backtrack from it in (b).
+ * Refer to the comment in queue_lock().
+ *
+ * Similarly, in order to account for waiters being requeued on another
+ * address we always increment the waiters for the destination bucket before
+ * acquiring the lock. It then decrements them again  after releasing it -
+ * the code that actually moves the futex(es) between hash buckets (requeue_futex)
+ * will do the additional required waiter count housekeeping. This is done for
+ * double_lock_hb() and double_unlock_hb(), respectively.
  */
 
+#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
 int __read_mostly futex_cmpxchg_enabled;
+#endif
 
 /*
  * Futex flags used to encode options to functions and preserve them across
@@ -1450,6 +1466,7 @@ retry:
 	hb2 = hash_futex(&key2);
 
 retry_private:
+	hb_waiters_inc(hb2);
 	double_lock_hb(hb1, hb2);
 
 	if (likely(cmpval != NULL)) {
@@ -1459,6 +1476,7 @@ retry_private:
 
 		if (unlikely(ret)) {
 			double_unlock_hb(hb1, hb2);
+			hb_waiters_dec(hb2);
 
 			ret = get_user(curval, uaddr1);
 			if (ret)
@@ -1508,6 +1526,7 @@ retry_private:
 			break;
 		case -EFAULT:
 			double_unlock_hb(hb1, hb2);
+			hb_waiters_dec(hb2);
 			put_futex_key(&key2);
 			put_futex_key(&key1);
 			ret = fault_in_user_writeable(uaddr2);
@@ -1517,6 +1536,7 @@ retry_private:
 		case -EAGAIN:
 			/* The owner was exiting, try again. */
 			double_unlock_hb(hb1, hb2);
+			hb_waiters_dec(hb2);
 			put_futex_key(&key2);
 			put_futex_key(&key1);
 			cond_resched();
@@ -1592,6 +1612,7 @@ retry_private:
 
 out_unlock:
 	double_unlock_hb(hb1, hb2);
+	hb_waiters_dec(hb2);
 
 	/*
 	 * drop_futex_key_refs() must be called outside the spinlocks. During
@@ -2875,9 +2896,28 @@ SYSCALL_DEFINE6(futex, u32 __user *, uaddr, int, op, u32, val,
 	return do_futex(uaddr, op, val, tp, uaddr2, val2, val3);
 }
 
-static int __init futex_init(void)
+static void __init futex_detect_cmpxchg(void)
 {
+#ifndef CONFIG_HAVE_FUTEX_CMPXCHG
 	u32 curval;
+
+	/*
+	 * This will fail and we want it. Some arch implementations do
+	 * runtime detection of the futex_atomic_cmpxchg_inatomic()
+	 * functionality. We want to know that before we call in any
+	 * of the complex code paths. Also we want to prevent
+	 * registration of robust lists in that case. NULL is
+	 * guaranteed to fault and we get -EFAULT on functional
+	 * implementation, the non-functional ones will return
+	 * -ENOSYS.
+	 */
+	if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
+		futex_cmpxchg_enabled = 1;
+#endif
+}
+
+static int __init futex_init(void)
+{
 	unsigned int futex_shift;
 	unsigned long i;
 
@@ -2893,18 +2933,8 @@ static int __init futex_init(void)
 					       &futex_shift, NULL,
 					       futex_hashsize, futex_hashsize);
 	futex_hashsize = 1UL << futex_shift;
-	/*
-	 * This will fail and we want it. Some arch implementations do
-	 * runtime detection of the futex_atomic_cmpxchg_inatomic()
-	 * functionality. We want to know that before we call in any
-	 * of the complex code paths. Also we want to prevent
-	 * registration of robust lists in that case. NULL is
-	 * guaranteed to fault and we get -EFAULT on functional
-	 * implementation, the non-functional ones will return
-	 * -ENOSYS.
-	 */
-	if (cmpxchg_futex_value_locked(&curval, NULL, 0, 0) == -EFAULT)
-		futex_cmpxchg_enabled = 1;
+
+	futex_detect_cmpxchg();
 
 	for (i = 0; i < futex_hashsize; i++) {
 		atomic_set(&futex_queues[i].waiters, 0);