Re: [PATCH v3] lowlevellock comments

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

Ping

On 23 October 2014 13:12, Bernard Ogden <bernie.ogden@linaro.org> wrote:
> 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;                                     \