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umtx operation UMTX_OP_MUTEX_WAKE has a side-effect that it accesses

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a mutex after a thread has unlocked it, it event writes data to the mutex
memory to clear contention bit, there is a race that other threads
can lock it and unlock it, then destroy it, so it should not write
data to the mutex memory if there isn't any waiter.
The new operation UMTX_OP_MUTEX_WAKE2 try to fix the problem. It
requires thread library to clear the lock word entirely, then
call the WAKE2 operation to check if there is any waiter in kernel,
and try to wake up a thread, if necessary, the contention bit is set again
by the operation. This also mitgates the chance that other threads find
the contention bit and try to enter kernel to compete with each other
to wake up sleeping thread, this is unnecessary. With this change, the
mutex owner is no longer holding the mutex until it reaches a point
where kernel umtx queue is locked, it releases the mutex as soon as
possible.
Performance is improved when the mutex is contensted heavily.  On Intel
i3-2310M, the runtime of a benchmark program is reduced from 26.87 seconds
to 2.39 seconds, it even is better than UMTX_OP_MUTEX_WAKE which is
deprecated now. http://people.freebsd.org/~davidxu/bench/mutex_perf.c
This commit is contained in:
David Xu 2012-04-05 02:24:08 +00:00
parent 6188bf47c8
commit 17ce606321
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=233912
4 changed files with 103 additions and 9 deletions
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2
lib/libthr/thread/thr_private.h
View File

@ -834,8 +834,6 @@ ssize_t __sys_write(int, const void *, size_t);
void __sys_exit(int);
#endif
int _umtx_op_err(void *, int op, u_long, void *, void *) __hidden;
static inline int
_thr_isthreaded(void)
{

21
lib/libthr/thread/thr_umtx.h
View File

@ -35,6 +35,7 @@
#define DEFAULT_UMUTEX {0,0,{0,0},{0,0,0,0}}
#define DEFAULT_URWLOCK {0,0,0,0,{0,0,0,0}}
int _umtx_op_err(void *, int op, u_long, void *, void *) __hidden;
int __thr_umutex_lock(struct umutex *mtx, uint32_t id) __hidden;
int __thr_umutex_lock_spin(struct umutex *mtx, uint32_t id) __hidden;
int __thr_umutex_timedlock(struct umutex *mtx, uint32_t id,
@ -121,9 +122,23 @@ _thr_umutex_timedlock(struct umutex *mtx, uint32_t id,
static inline int
_thr_umutex_unlock(struct umutex *mtx, uint32_t id)
{
if (atomic_cmpset_rel_32(&mtx->m_owner, id, UMUTEX_UNOWNED))
return (0);
return (__thr_umutex_unlock(mtx, id));
uint32_t flags = mtx->m_flags;
if ((flags & (UMUTEX_PRIO_PROTECT | UMUTEX_PRIO_INHERIT)) == 0) {
uint32_t owner;
do {
owner = mtx->m_owner;
if (__predict_false((owner & ~UMUTEX_CONTESTED) != id))
return (EPERM);
} while (__predict_false(!atomic_cmpset_rel_32(&mtx->m_owner,
owner, UMUTEX_UNOWNED)));
if ((owner & UMUTEX_CONTESTED))
(void)_umtx_op_err(mtx, UMTX_OP_MUTEX_WAKE2, flags, 0, 0);
return (0);
}
if (atomic_cmpset_rel_32(&mtx->m_owner, id, UMUTEX_UNOWNED))
return (0);
return (__thr_umutex_unlock(mtx, id));
}
static inline int

84
sys/kern/kern_umtx.c
View File

@ -1319,6 +1319,78 @@ do_wake_umutex(struct thread *td, struct umutex *m)
return (0);
}
/*
* Check if the mutex has waiters and tries to fix contention bit.
*/
static int
do_wake2_umutex(struct thread *td, struct umutex *m, uint32_t flags)
{
struct umtx_key key;
uint32_t owner, old;
int type;
int error;
int count;
switch(flags & (UMUTEX_PRIO_INHERIT | UMUTEX_PRIO_PROTECT)) {
case 0:
type = TYPE_NORMAL_UMUTEX;
break;
case UMUTEX_PRIO_INHERIT:
type = TYPE_PI_UMUTEX;
break;
case UMUTEX_PRIO_PROTECT:
type = TYPE_PP_UMUTEX;
break;
default:
return (EINVAL);
}
if ((error = umtx_key_get(m, type, GET_SHARE(flags),
&key)) != 0)
return (error);
owner = 0;
umtxq_lock(&key);
umtxq_busy(&key);
count = umtxq_count(&key);
umtxq_unlock(&key);
/*
* Only repair contention bit if there is a waiter, this means the mutex
* is still being referenced by userland code, otherwise don't update
* any memory.
*/
if (count > 1) {
owner = fuword32(__DEVOLATILE(uint32_t *, &m->m_owner));
while ((owner & UMUTEX_CONTESTED) ==0) {
old = casuword32(&m->m_owner, owner,
owner|UMUTEX_CONTESTED);
if (old == owner)
break;
owner = old;
}
} else if (count == 1) {
owner = fuword32(__DEVOLATILE(uint32_t *, &m->m_owner));
while ((owner & ~UMUTEX_CONTESTED) != 0 &&
(owner & UMUTEX_CONTESTED) == 0) {
old = casuword32(&m->m_owner, owner,
owner|UMUTEX_CONTESTED);
if (old == owner)
break;
owner = old;
}
}
umtxq_lock(&key);
if (owner == -1) {
error = EFAULT;
umtxq_signal(&key, INT_MAX);
}
else if (count != 0 && (owner & ~UMUTEX_CONTESTED) == 0)
umtxq_signal(&key, 1);
umtxq_unbusy(&key);
umtxq_unlock(&key);
umtx_key_release(&key);
return (error);
}
static inline struct umtx_pi *
umtx_pi_alloc(int flags)
{
@ -3152,6 +3224,12 @@ __umtx_op_sem_wake(struct thread *td, struct _umtx_op_args *uap)
return do_sem_wake(td, uap->obj);
}
static int
__umtx_op_wake2_umutex(struct thread *td, struct _umtx_op_args *uap)
{
return do_wake2_umutex(td, uap->obj, uap->val);
}
typedef int (*_umtx_op_func)(struct thread *td, struct _umtx_op_args *uap);
static _umtx_op_func op_table[] = {
@ -3176,7 +3254,8 @@ static _umtx_op_func op_table[] = {
__umtx_op_wake_umutex, /* UMTX_OP_UMUTEX_WAKE */
__umtx_op_sem_wait, /* UMTX_OP_SEM_WAIT */
__umtx_op_sem_wake, /* UMTX_OP_SEM_WAKE */
__umtx_op_nwake_private /* UMTX_OP_NWAKE_PRIVATE */
__umtx_op_nwake_private, /* UMTX_OP_NWAKE_PRIVATE */
__umtx_op_wake2_umutex /* UMTX_OP_UMUTEX_WAKE2 */
};
int
@ -3478,7 +3557,8 @@ static _umtx_op_func op_table_compat32[] = {
__umtx_op_wake_umutex, /* UMTX_OP_UMUTEX_WAKE */
__umtx_op_sem_wait_compat32, /* UMTX_OP_SEM_WAIT */
__umtx_op_sem_wake, /* UMTX_OP_SEM_WAKE */
__umtx_op_nwake_private32 /* UMTX_OP_NWAKE_PRIVATE */
__umtx_op_nwake_private32, /* UMTX_OP_NWAKE_PRIVATE */
__umtx_op_wake2_umutex /* UMTX_OP_UMUTEX_WAKE2 */
};
int

5
sys/sys/umtx.h
View File

@ -76,11 +76,12 @@
#define UMTX_OP_WAIT_UINT_PRIVATE 15
#define UMTX_OP_WAKE_PRIVATE 16
#define UMTX_OP_MUTEX_WAIT 17
#define UMTX_OP_MUTEX_WAKE 18
#define UMTX_OP_MUTEX_WAKE 18 /* deprecated */
#define UMTX_OP_SEM_WAIT 19
#define UMTX_OP_SEM_WAKE 20
#define UMTX_OP_NWAKE_PRIVATE 21
#define UMTX_OP_MAX 22
#define UMTX_OP_MUTEX_WAKE2 22
#define UMTX_OP_MAX 23
/* Flags for UMTX_OP_CV_WAIT */
#define CVWAIT_CHECK_UNPARKING 0x01