[PATCH v3] lowlevellock comments

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

Here's a v3. All comments addressed - replies to comments I have something to
say about inline below.

On 6 October 2014 13:26, Torvald Riegel <triegel@redhat.com> wrote:
>> To make the >1 state easier to refer to in comments I've gone for:
>> >1 - locked-with-waiters, there _may_ be waiters
>>
>> If I'm right then strictly >1 means locked-possibly-with-waiters, but that's
>> really getting too much of a mouthful (as 'potentially contended' would have
>> been in the earlier terminology).
>
> But "locked possibly with waiters" is the right description, so I'd
> rather use this.

OK, changed throughout to 'acquired, possibly with waiters' (based on the
suggested text below).

>> > Please use locked, or unlocked terms everywhere.
>>
>> I've gone for unlocked, locked and locked with waiters.
>
> My vote would be for "acquired" / "not acquired", as in "the lock has
> been acquired".

Switched to acquired/not acquired, as no-one defended locked/unlocked.

>> +   thread ID while the clone is running and is reset to zero
>> +   afterwards.       */
>
> Could you add who resets it?

Done (it's the kernel).

>>
>> +/* Uses futexes to avoid unnecessary calls into the kernel. Likely paths are
>
> I'd say it uses futexes to block using the kernel.  That's the purpose
> of the futex call, so that we don't need to just spin-wait.
<snip>
>> +   the cases with no waiters and hence do not involve syscalls. Unlikely paths
>> +   usually will involve syscalls to wait or to wake.
>
> The "likely" path is not necessarily likely, but we try to make
> uncontended mutexes fast, so that's why we avoid the syscalls in these
> cases. I would suggest:
>
> Low-level locks use a combination of atomic operations (to acquire and
> release lock ownership) and futex operations (to block until the state
> of a lock changes).  A lock can be in one of three states:
> 0:  not acquired,
> 1:  acquired; no other threads are blocked or about to block for changes
> to the lock state,
>>1:  acquired, other threads are potentially blocked or about to block
> for changes to the lock state.
>
> We expect that the common case is an uncontended lock, so we just need
> to transition the lock between states 0 and 1; releasing the lock does
> not need to wake any other blocked threads.  If the lock is contended
> and a thread decides to block using a futex operation, then this thread
> needs to first change the state to >1; if this state is observed during
> lock release, the releasing thread will wake one of the potentially
> blocked threads.

Used your text, tweaked slightly to make the difference between 1 and >1 even
more explicit. Also reworded the >1 description to a form that makes a bit
more sense to the pedant in me.

>> +   cond_locks are never in the locked state, they are either unlocked or
>> +   locked with waiters.
>
> Change that to the following?:
>
> Condition variables contain an optimization for broadcasts that requeues
> waiting threads on a lock's futex.  Therefore, there is a special
> variant of the locks (whose name contains "cond") that makes sure to
> always set the lock state to >1 and not just 1.

And just used this as is.

>> +/* If LOCK is 0 (unlocked), set to 1 (locked) and return 0 to indicate the
>> +   lock was acquired. Otherwise leave the lock unchanged and return non-zero
>> +   to indicate the lock was not acquired.  */
>
> If you use acquired / not acquired instead of locked / unlocked, this
> gets simpler.

In that I can be a bit less verbose about the meaning of returns? Done.

>> +   2) The previous owner of the lock dies. FUTEX will be
>> +      ID | FUTEX_OWNER_DIED. FUTEX_WAITERS may also be set. Return FUTEX to
>> +      indicate that the lock is not acquired.  */
>
> I'd say "this value" instead of the second occurence of "FUTEX" to
> highlight that we do NOT read FUTEX twice.

Fixed - also put the FUTEX_WAITERS part in brackets and cleared up some
ambiguity.


2014-10-23  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..99cca8e 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);
     }
 
@@ -83,6 +83,11 @@ __lll_timedlock_wait (int *futex, const struct timespec *abstime, int private)
 }
 
 
+/* The kernel notifies a process which uses CLONE_CHILD_CLEARTID via futex
+   wake-up when the clone terminates.  The memory location contains the
+   thread ID while the clone is running and is reset to zero by the kernel
+   afterwards.  The kernel up to version 3.16.3 does not use the private futex
+   operations for futex wake-up when the clone terminates.  */
 int
 __lll_timedwait_tid (int *tidp, const struct timespec *abstime)
 {
@@ -113,8 +118,10 @@ __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 or the wait times out.
+         The kernel to version 3.16.3 does not use the private futex
+         operations for futex wake-up when the clone terminates.
+      */
       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..2f4f314 100644
--- a/nptl/lowlevelrobustlock.c
+++ b/nptl/lowlevelrobustlock.c
@@ -36,14 +36,17 @@ __lll_robust_lock_wait (int *futex, int private)
 
   do
     {
+      /* If the owner died, return the present value of the futex.  */
       if (__glibc_unlikely (oldval & FUTEX_OWNER_DIED))
 	return oldval;
 
+      /* Attempt to put lock into state 'acquired, possibly with waiters'.  */
       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 the owner died, return the present value of the futex.  */
       if (__glibc_unlikely (oldval & FUTEX_OWNER_DIED))
 	return oldval;
 
+      /* Attempt to put lock into state 'acquired, possibly with waiters'.  */
       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..8f6e270 100644
--- a/sysdeps/nptl/lowlevellock.h
+++ b/sysdeps/nptl/lowlevellock.h
@@ -22,9 +22,53 @@
 #include <atomic.h>
 #include <lowlevellock-futex.h>
 
+/* Low-level locks use a combination of atomic operations (to acquire and
+   release lock ownership) and futex operations (to block until the state
+   of a lock changes).  A lock can be in one of three states:
+   0:  not acquired,
+   1:  acquired with no waiters; no other threads are blocked or about to block
+       for changes to the lock state,
+   >1: acquired, possibly with waiters; there may be other threads blocked or
+       about to block for changes to the lock state.
+
+   We expect that the common case is an uncontended lock, so we just need
+   to transition the lock between states 0 and 1; releasing the lock does
+   not need to wake any other blocked threads.  If the lock is contended
+   and a thread decides to block using a futex operation, then this thread
+   needs to first change the state to >1; if this state is observed during
+   lock release, the releasing thread will wake one of the potentially
+   blocked threads.
+
+   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.
+
+   Condition variables contain an optimization for broadcasts that requeues
+   waiting threads on a lock's futex.  Therefore, there is a special
+   variant of the locks (whose name contains "cond") that makes sure to
+   always set the lock state to >1 and not just 1.
+
+   Robust locks set the lock to the id of the owner.  This allows detection
+   of the case where the owner exits without releasing the lock.  Flags are
+   OR'd with the owner id to record additional information about lock state.
+   Therefore the states of robust locks are:
+    0: not acquired
+   id: acquired (by user identified by id & FUTEX_TID_MASK)
+
+   The following flags may be set in the robust lock value:
+   FUTEX_WAITERS     - possibly has waiters
+   FUTEX_OWNER_DIED  - owning user has exited without releasing the futex.  */
+
+
+/* If LOCK is 0 (not acquired), set to 1 (acquired) and return 0.  Otherwise
+   leave lock unchanged and return non-zero to indicate that the lock was not
+   acquired.  */
 #define lll_trylock(lock)	\
   atomic_compare_and_exchange_bool_acq (&(lock), 1, 0)
 
+/* If LOCK is 0 (not acquired), set to 2 (acquired, possibly with waiters) and
+   return 0.  Otherwise leave lock unchanged and return non-zero to indicate
+   that the lock was not acquired.  */
 #define lll_cond_trylock(lock)	\
   atomic_compare_and_exchange_bool_acq (&(lock), 2, 0)
 
@@ -35,6 +79,13 @@ 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 (not acquired), set to 1 (acquired with no waiters) and
+   return.  Otherwise block until we acquire the lock, at which point FUTEX is
+   2 (acquired, possibly with waiters), then return.  The lock is aways
+   acquired on return.  */
 #define __lll_lock(futex, private)                                      \
   ((void)                                                               \
    ({                                                                   \
@@ -52,6 +103,14 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
   __lll_lock (&(futex), private)
 
 
+/* If FUTEX is 0 (not acquired), set to ID (acquired with no waiters) and
+   return 0.  Otherwise, block until either:
+   1) We acquire the lock, at which point FUTEX will be ID | FUTEX_WAITERS
+      (acquired, possibly with waiters), then return 0.
+   2) The previous owner of the lock dies.  FUTEX will be the value of id as
+      set by the previous owner, with FUTEX_OWNER_DIED set (FUTEX_WAITERS may
+      also be set).  Return this value to indicate that the lock is not
+      acquired.  */
 #define __lll_robust_lock(futex, id, private)                           \
   ({                                                                    \
     int *__futex = (futex);                                             \
@@ -69,6 +128,12 @@ 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 (acquired, possibly with waiters).  condvar
+   locks only have 'not acquired' and 'acquired, possibly with waiters' states,
+   so there is no need to check FUTEX before setting.
+   If FUTEX was 0 (not acquired) then return.  Otherwise, block until the lock
+   is acquired, at which point FUTEX is 2 (acquired, possibly with waiters).
+   The lock is always acquired on return.  */
 #define __lll_cond_lock(futex, private)                                 \
   ((void)                                                               \
    ({                                                                   \
@@ -79,6 +144,14 @@ 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 (not acquired), set to ID | FUTEX_WAITERS (acquired, possibly
+   with waiters) and return 0.  Otherwise, block until either:
+   1) We acquire the lock, at which point FUTEX will be ID | FUTEX_WAITERS
+      (acquired, possibly with waiters), then return 0.
+   2) The previous owner of the lock dies.  FUTEX will be the value of id as
+      set by the previous owner, with FUTEX_OWNER_DIED set (FUTEX_WAITERS may
+      also be set).  Return this value to indicate that the lock is not
+      acquired.  */
 #define lll_robust_cond_lock(futex, id, private)	\
   __lll_robust_lock (&(futex), (id) | FUTEX_WAITERS, private)
 
@@ -88,8 +161,9 @@ 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.  If the timeout occurs then return
+   ETIMEDOUT.  If ABSTIME is invalid, return EINVAL.  */
 #define __lll_timedlock(futex, abstime, private)                \
   ({                                                            \
     int *__futex = (futex);                                     \
@@ -104,6 +178,8 @@ extern int __lll_robust_timedlock_wait (int *futex, const struct timespec *,
   __lll_timedlock (&(futex), abstime, private)
 
 
+/* As __lll_robust_lock, but with a timeout.  If the timeout occurs then return
+   ETIMEDOUT.  If ABSTIME is invalid, return EINVAL.  */
 #define __lll_robust_timedlock(futex, abstime, id, private)             \
   ({                                                                    \
     int *__futex = (futex);                                             \
@@ -121,6 +197,9 @@ 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 (not acquired), releasing the lock.  If FUTEX
+   was >1 (acquired, possibly with waiters), then wake any waiters.  The waiter
+   who acquires the lock will set FUTEX to >1.  */
 #define __lll_unlock(futex, private)                    \
   ((void)                                               \
    ({                                                   \
@@ -136,6 +215,9 @@ 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 (not acquired), releasing the lock.  If FUTEX
+   had FUTEX_WAITERS set then wake any waiters.  The waiter who acquires the
+   lock will set FUTEX_WAITERS.  */
 #define __lll_robust_unlock(futex, private)             \
   ((void)                                               \
    ({                                                   \
@@ -159,15 +241,12 @@ 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
-   afterwards.	*/
+   wake-up when the clone terminates.  The memory location contains the
+   thread ID while the clone is running and is reset to zero by the kernel
+   afterwards.  The kernel up to version 3.16.3 does not use the private futex
+   operations for futex wake-up when the clone terminates.  */
 #define lll_wait_tid(tid) \
   do {					\
     __typeof (tid) __tid;		\
@@ -178,6 +257,8 @@ 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.  If the timeout occurs then return
+   ETIMEDOUT.  If ABSTIME is invalid, return EINVAL.  */
 #define lll_timedwait_tid(tid, abstime) \
   ({							\
     int __res = 0;					\