Re: [PATCH v3] lowlevellock comments

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

Ping^2

On 10 November 2014 09:25, Bernie Ogden <bernie.ogden@linaro.org> wrote:
> 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;                                     \