[PATCH] lowlevellock comments

[Bernard Ogden <bernie dot ogden at linaro dot org>]

This patch adds many (too many/too remedial?) comments to
lowlevellock.h, and a few comments to a couple of other files.

Needs review for both correctness and usefulness.

Thanks in advance.

2014-09-26  Bernard Ogden  <bernie.ogden@linaro.org>

	* nptl/lowlevellock.c: Add comments.
	* nptl/lowlevelrobustlock.c: Likewise.
	* sysdeps/nptl/lowlevellock.h: Likewise.

---

diff --git a/nptl/lowlevellock.c b/nptl/lowlevellock.c
index e198af7..9177af3 100644
--- a/nptl/lowlevellock.c
+++ b/nptl/lowlevellock.c
@@ -27,10 +27,10 @@ void
 __lll_lock_wait_private (int *futex)
 {
   if (*futex == 2)
-    lll_futex_wait (futex, 2, LLL_PRIVATE);
+    lll_futex_wait (futex, 2, LLL_PRIVATE); /* Wait if *futex == 2.  */
 
   while (atomic_exchange_acq (futex, 2) != 0)
-    lll_futex_wait (futex, 2, LLL_PRIVATE);
+    lll_futex_wait (futex, 2, LLL_PRIVATE); /* Wait if *futex == 2.  */
 }
 
 
@@ -40,10 +40,10 @@ void
 __lll_lock_wait (int *futex, int private)
 {
   if (*futex == 2)
-    lll_futex_wait (futex, 2, private);
+    lll_futex_wait (futex, 2, private); /* Wait if *futex == 2.  */
 
   while (atomic_exchange_acq (futex, 2) != 0)
-    lll_futex_wait (futex, 2, private);
+    lll_futex_wait (futex, 2, private); /* Wait if *futex == 2.  */
 }
 
 
@@ -75,7 +75,7 @@ __lll_timedlock_wait (int *futex, const struct timespec *abstime, int private)
       if (rt.tv_sec < 0)
 	return ETIMEDOUT;
 
-      /* Wait.  */
+      /* If *futex == 2, wait until woken or timeout.  */
       lll_futex_timed_wait (futex, 2, &rt, private);
     }
 
@@ -113,8 +113,8 @@ __lll_timedwait_tid (int *tidp, const struct timespec *abstime)
       if (rt.tv_sec < 0)
 	return ETIMEDOUT;
 
-      /* Wait until thread terminates.  The kernel so far does not use
-	 the private futex operations for this.  */
+      /* If *tidp == tid, wait until thread terminates.  The kernel so far
+         does not use the private futex operations for this.  */
       if (lll_futex_timed_wait (tidp, tid, &rt, LLL_SHARED) == -ETIMEDOUT)
 	return ETIMEDOUT;
     }
diff --git a/nptl/lowlevelrobustlock.c b/nptl/lowlevelrobustlock.c
index 3525807..60e80d0 100644
--- a/nptl/lowlevelrobustlock.c
+++ b/nptl/lowlevelrobustlock.c
@@ -36,14 +36,17 @@ __lll_robust_lock_wait (int *futex, int private)
 
   do
     {
+      /* If owner died, return the present value of the futex.  */
       if (__glibc_unlikely (oldval & FUTEX_OWNER_DIED))
 	return oldval;
 
+      /* Put lock into contended state.  */
       int newval = oldval | FUTEX_WAITERS;
       if (oldval != newval
 	  && atomic_compare_and_exchange_bool_acq (futex, newval, oldval))
 	continue;
 
+      /* If *futex == 2, wait until woken.  */
       lll_futex_wait (futex, newval, private);
 
     try:
@@ -100,15 +103,17 @@ __lll_robust_timedlock_wait (int *futex, const struct timespec *abstime,
 	return ETIMEDOUT;
 #endif
 
-      /* Wait.  */
+      /* If owner died, return the present value of the futex.  */
       if (__glibc_unlikely (oldval & FUTEX_OWNER_DIED))
 	return oldval;
 
+      /* Put lock into contended state.  */
       int newval = oldval | FUTEX_WAITERS;
       if (oldval != newval
 	  && atomic_compare_and_exchange_bool_acq (futex, newval, oldval))
 	continue;
 
+      /* If *futex == 2, wait until woken or timeout.  */
 #if (!defined __ASSUME_FUTEX_CLOCK_REALTIME \
      || !defined lll_futex_timed_wait_bitset)
       lll_futex_timed_wait (futex, newval, &rt, private);
diff --git a/sysdeps/nptl/lowlevellock.h b/sysdeps/nptl/lowlevellock.h
index 28f4ba3..0731e5c 100644
--- a/sysdeps/nptl/lowlevellock.h
+++ b/sysdeps/nptl/lowlevellock.h
@@ -22,9 +22,38 @@
 #include <atomic.h>
 #include <lowlevellock-futex.h>
 
+/* Uses futexes to avoid unneccessary calls into the kernel. Likely paths are
+   the uncontended cases and hence do not involve syscalls. Unlikely
+   (contended case) paths usually will involve syscalls to wait or to wake.
+
+   Much of this code takes a 'private' parameter. This may be:
+   LLL_PRIVATE: lock only shared within a process
+   LLL_SHARED:  lock may be shared across processes.  */
+
+/* The states of locks are:
+    0  -  untaken
+    1  -  taken (by one user)
+   >1  -  contended (taken by 1 user, there may be other users waiting)
+
+   cond_locks are never in the taken state, they are either untaken or
+   contended.
+
+   The states of robust locks are:
+    0  - untaken
+   id  - taken (by user identified by id & FUTEX_TID_MASK)
+
+   The following flags may be set in the robust lock value:
+   FUTEX_WAITERS     - contended (taken by one user, there may be waiters)
+   FUTEX_OWNER_DIED  - owning user has exited without releasing the futex.  */
+
+
+/* If lock is 0 (untaken), set to 1 (taken). Return 0. Lock acquired.
+   Otherwise lock is unchanged. Return nonzero. Lock not acquired.  */
 #define lll_trylock(lock)	\
   atomic_compare_and_exchange_bool_acq (&(lock), 1, 0)
 
+/* If lock is 0 (untaken), set to 2 (contended). Return 0. Lock acquired.
+   Otherwise lock is unchanged. Return nonzero. Lock not acquired.  */
 #define lll_cond_trylock(lock)	\
   atomic_compare_and_exchange_bool_acq (&(lock), 2, 0)
 
@@ -35,6 +64,12 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
 /* This is an expression rather than a statement even though its value is
    void, so that it can be used in a comma expression or as an expression
    that's cast to void.  */
+/* The inner conditional compiles to a call to __lll_lock_wait_private if
+   private is known at compile time to be LLL_PRIVATE, and to a call to
+   __lll_lock_wait otherwise.  */
+/* If futex is 0 (untaken), set to 1 (taken). Lock acquired.
+   Otherwise, block until lock is acquired. After blocking, futex is 2
+   (contended).  */
 #define __lll_lock(futex, private)                                      \
   ((void)                                                               \
    ({                                                                   \
@@ -52,6 +87,11 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
   __lll_lock (&(futex), private)
 
 
+/* If futex is 0 (untaken), set to id (taken). Return 0. Lock acquired.
+   Otherwise, block until either:
+   1) Lock is acquired. Return 0. futex is id | FUTEX_WAITERS (contended).
+   2) Previous owner of futex dies. Return value of futex, which is id of
+      dead owner with FUTEX_OWNER_DIED set. Lock not acquired.  */
 #define __lll_robust_lock(futex, id, private)                           \
   ({                                                                    \
     int *__futex = (futex);                                             \
@@ -69,6 +109,12 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
 /* This is an expression rather than a statement even though its value is
    void, so that it can be used in a comma expression or as an expression
    that's cast to void.  */
+/* Unconditionally set futex to 2 (contended). cond locks only have
+   untaken and contended states, so there is no need to check the value of
+   futex before setting.
+   If previous value of futex was 0 (untaken), lock is acquired.
+   Otherwise, block until lock is acquired. futex will still be 2 (contended)
+   after acquisition.  */
 #define __lll_cond_lock(futex, private)                                 \
   ((void)                                                               \
    ({                                                                   \
@@ -79,6 +125,12 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
 #define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private)
 
 
+/* If futex is 0 (untaken), set to id | FUTEX_WAITERS (contended). Lock
+   acquired.
+   Otherwise, block until either:
+   1) Lock is acquired. Return 0. futex is id | FUTEX_WAITERS (contended).
+   2) Previous owner of futex dies. Return the value of futex, which is id of
+      dead owner with FUTEX_OWNER_DIED set. Lock not acquired.  */
 #define lll_robust_cond_lock(futex, id, private)	\
   __lll_robust_lock (&(futex), (id) | FUTEX_WAITERS, private)
 
@@ -88,8 +140,7 @@ extern int __lll_timedlock_wait (int *futex, const struct timespec *,
 extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
 					int private) attribute_hidden;
 
-/* Take futex if it is untaken.
-   Otherwise block until either we get the futex or abstime runs out.  */
+/* As __lll_lock, but with a timeout.  */
 #define __lll_timedlock(futex, abstime, private)                \
   ({                                                            \
     int *__futex = (futex);                                     \
@@ -104,6 +155,7 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
   __lll_timedlock (&(futex), abstime, private)
 
 
+/* As __lll_robust_lock, but with a timeout.  */
 #define __lll_robust_timedlock(futex, abstime, id, private)             \
   ({                                                                    \
     int *__futex = (futex);                                             \
@@ -121,6 +173,9 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
 /* This is an expression rather than a statement even though its value is
    void, so that it can be used in a comma expression or as an expression
    that's cast to void.  */
+/* Unconditionally set futex to 0 (untaken). Lock is released.
+   If previous value of futex was > 1 (contended), wake any waiters. (Waiter
+   that acquires the lock will set futex to >1 (contended).)  */
 #define __lll_unlock(futex, private)                    \
   ((void)                                               \
    ({                                                   \
@@ -132,10 +187,12 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
 #define lll_unlock(futex, private)	\
   __lll_unlock (&(futex), private)
 
-
 /* This is an expression rather than a statement even though its value is
    void, so that it can be used in a comma expression or as an expression
    that's cast to void.  */
+/* Unconditionally set futex to 0 (untaken). Lock is released.
+   If previous value of futex had FUTEX_WAITERS set, wake any waiters. (Waiter
+   that acquires the lock will set FUTEX_WAITERS.)  */
 #define __lll_robust_unlock(futex, private)             \
   ((void)                                               \
    ({                                                   \
@@ -159,11 +216,6 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
 #define LLL_LOCK_INITIALIZER		(0)
 #define LLL_LOCK_INITIALIZER_LOCKED	(1)
 
-/* The states of a lock are:
-    0  -  untaken
-    1  -  taken by one user
-   >1  -  taken by more users */
-
 /* The kernel notifies a process which uses CLONE_CHILD_CLEARTID via futex
    wakeup when the clone terminates.  The memory location contains the
    thread ID while the clone is running and is reset to zero
@@ -178,6 +230,7 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
 extern int __lll_timedwait_tid (int *, const struct timespec *)
      attribute_hidden;
 
+/* As lll_wait_tid, but with a timeout.  */
 #define lll_timedwait_tid(tid, abstime) \
   ({							\
     int __res = 0;					\