From 4ad1d0cfbf96c3dbcf0ba104eb9f81bae6bdc6da Mon Sep 17 00:00:00 2001 From: Ulrich Drepper Date: Tue, 4 Jul 2000 00:22:32 +0000 Subject: (charmap_read): Prepend the condition filename == NULL. --- linuxthreads/spinlock.c | 134 +++++++++++++++++++++++++++++++++++++----------- 1 file changed, 103 insertions(+), 31 deletions(-) (limited to 'linuxthreads/spinlock.c') diff --git a/linuxthreads/spinlock.c b/linuxthreads/spinlock.c index c91a7cfa84..02ab9a9613 100644 --- a/linuxthreads/spinlock.c +++ b/linuxthreads/spinlock.c @@ -24,12 +24,20 @@ #include "spinlock.h" #include "restart.h" -/* The status field of a spinlock has the following meaning: - 0: spinlock is free - 1: spinlock is taken, no thread is waiting on it - ADDR: psinlock is taken, ADDR is address of thread descriptor for - first waiting thread, other waiting threads are linked via - their p_nextlock field. +/* The status field of a spinlock is a pointer whose least significant + bit is a locked flag. + + Thus the field values have the following meanings: + + status == 0: spinlock is free + status == 1: spinlock is taken; no thread is waiting on it + + (status & 1) == 1: spinlock is taken and (status & ~1L) is a + pointer to the first waiting thread; other + waiting threads are linked via the p_nextlock + field. + (status & 1) == 0: same as above, but spinlock is not taken. + The waiting list is not sorted by priority order. Actually, we always insert at top of list (sole insertion mode that can be performed without locking). @@ -38,29 +46,70 @@ This is safe because there are no concurrent __pthread_unlock operations -- only the thread that locked the mutex can unlock it. */ + void internal_function __pthread_lock(struct _pthread_fastlock * lock, pthread_descr self) { +#if defined HAS_COMPARE_AND_SWAP long oldstatus, newstatus; - int spurious_wakeup_count = 0; + int successful_seizure, spurious_wakeup_count = 0; + int spin_count = 0; +#endif + +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP + { + __pthread_acquire(&lock->__spinlock); + return 0; + } +#endif + +#if defined HAS_COMPARE_AND_SWAP +again: + + /* On SMP, try spinning to get the lock. */ + + if (__pthread_smp_kernel) { + int max_count = lock->__spinlock * 2 + 10; + + for (spin_count = 0; spin_count < max_count; spin_count++) { + if (((oldstatus = lock->__status) & 1) == 0) { + if(__compare_and_swap(&lock->__status, oldstatus, oldstatus | 1)) + { + if (spin_count) + lock->__spinlock += (spin_count - lock->__spinlock) / 8; + return; + } + } + } + + lock->__spinlock += (spin_count - lock->__spinlock) / 8; + } + + /* No luck, try once more or suspend. */ do { oldstatus = lock->__status; - if (oldstatus == 0) { - newstatus = 1; + successful_seizure = 0; + + if ((oldstatus & 1) == 0) { + newstatus = oldstatus | 1; + successful_seizure = 1; } else { if (self == NULL) self = thread_self(); - newstatus = (long) self; + newstatus = (long) self | 1; } + if (self != NULL) { - THREAD_SETMEM(self, p_nextlock, (pthread_descr) oldstatus); + THREAD_SETMEM(self, p_nextlock, (pthread_descr) (oldstatus & ~1L)); /* Make sure the store in p_nextlock completes before performing the compare-and-swap */ MEMORY_BARRIER(); } - } while(! compare_and_swap(&lock->__status, oldstatus, newstatus, - &lock->__spinlock)); + } while(! __compare_and_swap(&lock->__status, oldstatus, newstatus)); /* Suspend with guard against spurious wakeup. This can happen in pthread_cond_timedwait_relative, when the thread @@ -68,7 +117,7 @@ void internal_function __pthread_lock(struct _pthread_fastlock * lock, locks the queue to remove itself. At that point it may still be on the queue, and may be resumed by a condition signal. */ - if (oldstatus != 0) { + if (!successful_seizure) { for (;;) { suspend(self); if (self->p_nextlock != NULL) { @@ -78,37 +127,50 @@ void internal_function __pthread_lock(struct _pthread_fastlock * lock, } break; } + goto again; } /* Put back any resumes we caught that don't belong to us. */ while (spurious_wakeup_count--) restart(self); +#endif } int __pthread_unlock(struct _pthread_fastlock * lock) { +#if defined HAS_COMPARE_AND_SWAP long oldstatus; pthread_descr thr, * ptr, * maxptr; int maxprio; +#endif +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP + { + lock->__spinlock = 0; + WRITE_MEMORY_BARRIER(); + return 0; + } +#endif + +#if defined HAS_COMPARE_AND_SWAP again: oldstatus = lock->__status; - if (oldstatus == 0 || oldstatus == 1) { - /* No threads are waiting for this lock. Please note that we also - enter this case if the lock is not taken at all. If this wouldn't - be done here we would crash further down. */ - if (! compare_and_swap_with_release_semantics (&lock->__status, - oldstatus, 0, - &lock->__spinlock)) - goto again; - return 0; + + while ((oldstatus = lock->__status) == 1) { + if (__compare_and_swap_with_release_semantics(&lock->__status, + oldstatus, 0)) + return 0; } + /* Find thread in waiting queue with maximal priority */ ptr = (pthread_descr *) &lock->__status; - thr = (pthread_descr) oldstatus; + thr = (pthread_descr) (oldstatus & ~1L); maxprio = 0; maxptr = ptr; - while (thr != (pthread_descr) 1) { + while (thr != 0) { if (thr->p_priority >= maxprio) { maxptr = ptr; maxprio = thr->p_priority; @@ -128,16 +190,25 @@ again: /* Remove max prio thread from waiting list. */ if (maxptr == (pthread_descr *) &lock->__status) { /* If max prio thread is at head, remove it with compare-and-swap - to guard against concurrent lock operation */ - thr = (pthread_descr) oldstatus; - if (! compare_and_swap_with_release_semantics - (&lock->__status, oldstatus, (long)(thr->p_nextlock), - &lock->__spinlock)) + to guard against concurrent lock operation. This removal + also has the side effect of marking the lock as released + because the new status comes from thr->p_nextlock whose + least significant bit is clear. */ + thr = (pthread_descr) (oldstatus & ~1L); + if (! __compare_and_swap_with_release_semantics + (&lock->__status, oldstatus, (long)(thr->p_nextlock))) goto again; } else { - /* No risk of concurrent access, remove max prio thread normally */ + /* No risk of concurrent access, remove max prio thread normally. + But in this case we must also flip the least significant bit + of the status to mark the lock as released. */ thr = *maxptr; *maxptr = thr->p_nextlock; + + do { + oldstatus = lock->__status; + } while (!__compare_and_swap_with_release_semantics(&lock->__status, + oldstatus, oldstatus & ~1L)); } /* Prevent reordering of store to *maxptr above and store to thr->p_nextlock below */ @@ -147,6 +218,7 @@ again: restart(thr); return 0; +#endif } /* -- cgit 1.4.1