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Add comments for the generic lowlevellock implementation.

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Patch by Bernard Ogden <bernie.ogden@linaro.org>.
This commit is contained in:
Torvald Riegel 2014-12-15 22:49:29 +01:00
parent 045a6bcdd2
commit d52c62df3d
4 changed files with 124 additions and 18 deletions
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21
ChangeLog
View File

@ -1,3 +1,24 @@
2014-12-15 Bernard Ogden <bernie.ogden@linaro.org>
* nptl/lowlevellock.c (__lll_lock_wait_private): Add comments.
(__lll_lock_wait): Likewise.
(__lll_timedlock_wait): Likewise.
(__lll_timedwait_tid): Likewise.
* nptl/lowlevelrobustlock.c (__lll_robust_lock_wait): Likewise.
(__lll_robust_timedlock_wait): Likewise.
* sysdeps/nptl/lowlevellock.h (lll_trylock): Likewise.
(lll_cond_trylock): Likewise.
(__lll_lock): Likewise.
(__lll_robust_lock): Likewise.
(__lll_cond_lock): Likewise.
(lll_robust_cond_lock): Likewise.
(__lll_timedlock): Likewise.
(__lll_robust_timedlock): Likewise.
(__lll_unlock): Likewise.
(__lll_robust_unlock): Likewise.
(lll_wait_tid): Likewise.
(lll_timedwait_tid): Likewise.
2014-12-15 Torvald Riegel <triegel@redhat.com>
* nptl/tst-sem4.c (do_test): Start with a fresh semaphore state.

21
nptl/lowlevellock.c
View File

@ -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 up 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;
}

9
nptl/lowlevelrobustlock.c
View File

@ -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;
/* Try to put the 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:
@ -67,7 +70,7 @@ __lll_robust_timedlock_wait (int *futex, const struct timespec *abstime,
int tid = THREAD_GETMEM (THREAD_SELF, tid);
int oldval = *futex;
/* If the futex changed meanwhile try locking again. */
/* If the futex changed meanwhile, try locking again. */
if (oldval == 0)
goto 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;
/* Try to put the 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);

91
sysdeps/nptl/lowlevellock.h
View File

@ -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 with no waiters) 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, ensure that it is >1 (acquired, possibly with waiters)
and then block until we acquire the lock, at which point FUTEX will still be
>1. The lock is always 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, ensure that it is set to FUTEX | FUTEX_WAITERS
(acquired, possibly with waiters) and block until we acquire the lock.
FUTEX will now be ID | FUTEX_WAITERS and we return 0.
If the previous owner of the lock dies before we acquire the lock then FUTEX
will be the value of id as set by the previous owner, with FUTEX_OWNER_DIED
set (FUTEX_WAITERS may or may not be set). We return this value to indicate
that the lock is not acquired. */
#define __lll_robust_lock(futex, id, private) \
({ \
int *__futex = (futex); \
@ -69,6 +128,10 @@ 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). 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 +142,8 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
#define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private)
/* As __lll_robust_lock, but set to ID | FUTEX_WAITERS (acquired, possibly with
waiters) if FUTEX is 0. */
#define lll_robust_cond_lock(futex, id, private) \
__lll_robust_lock (&(futex), (id) | FUTEX_WAITERS, private)
@ -88,8 +153,9 @@ 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 +170,8 @@ 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 +189,9 @@ 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
that acquires the lock will set FUTEX to >1. */
#define __lll_unlock(futex, private) \
((void) \
({ \
@ -136,6 +207,9 @@ 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 that acquires the
lock will set FUTEX_WAITERS. */
#define __lll_robust_unlock(futex, private) \
((void) \
({ \
@ -159,15 +233,12 @@ 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 +249,8 @@ 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; \