diff options
Diffstat (limited to 'libpthread/linuxthreads/spinlock.c')
| -rw-r--r-- | libpthread/linuxthreads/spinlock.c | 616 |
1 files changed, 572 insertions, 44 deletions
diff --git a/libpthread/linuxthreads/spinlock.c b/libpthread/linuxthreads/spinlock.c index b1a99d975..5d4bd20e4 100644 --- a/libpthread/linuxthreads/spinlock.c +++ b/libpthread/linuxthreads/spinlock.c @@ -14,20 +14,42 @@ /* Internal locks */ +#define __FORCE_GLIBC +#include <features.h> #include <errno.h> #include <sched.h> #include <time.h> +#include <stdlib.h> +#include <limits.h> #include "pthread.h" #include "internals.h" #include "spinlock.h" #include "restart.h" -/* The status field of a fastlock has the following meaning: - 0: fastlock is free - 1: fastlock is taken, no thread is waiting on it - ADDR: fastlock is taken, ADDR is address of thread descriptor for - first waiting thread, other waiting threads are linked via - their p_nextlock field. +static void __pthread_acquire(int * spinlock); + +static inline void __pthread_release(int * spinlock) +{ + WRITE_MEMORY_BARRIER(); + *spinlock = __LT_SPINLOCK_INIT; + __asm __volatile ("" : "=m" (*spinlock) : "0" (*spinlock)); +} + + +/* 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). @@ -36,35 +58,95 @@ 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; + int spin_count; +#endif + +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + { + __pthread_acquire(&lock->__spinlock); + return; + } +#endif + +#if defined HAS_COMPARE_AND_SWAP + /* First try it without preparation. Maybe it's a completely + uncontested lock. */ + if (lock->__status == 0 && __compare_and_swap (&lock->__status, 0, 1)) + return; + + spurious_wakeup_count = 0; + spin_count = 0; + + /* On SMP, try spinning to get the lock. */ +#if 0 + if (__pthread_smp_kernel) { + int max_count = lock->__spinlock * 2 + 10; + + if (max_count > MAX_ADAPTIVE_SPIN_COUNT) + max_count = MAX_ADAPTIVE_SPIN_COUNT; + + 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; + READ_MEMORY_BARRIER(); + return; + } + } +#ifdef BUSY_WAIT_NOP + BUSY_WAIT_NOP; +#endif + __asm __volatile ("" : "=m" (lock->__status) : "0" (lock->__status)); + } + + lock->__spinlock += (spin_count - lock->__spinlock) / 8; + } +#endif + +again: + + /* 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) { - ASSERT(self->p_nextlock == NULL); - THREAD_SETMEM(self, p_nextlock, (pthread_descr) oldstatus); + THREAD_SETMEM(self, p_nextlock, (pthread_descr) (oldstatus)); + /* 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. + /* Suspend with guard against spurious wakeup. This can happen in pthread_cond_timedwait_relative, when the thread wakes up due to timeout and is still on the condvar queue, and then 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) { @@ -74,61 +156,506 @@ 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); + + READ_MEMORY_BARRIER(); +#endif } -void internal_function __pthread_unlock(struct _pthread_fastlock * lock) +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 || defined TEST_FOR_COMPARE_AND_SWAP + { + __pthread_release(&lock->__spinlock); + return 0; + } +#endif + +#if defined HAS_COMPARE_AND_SWAP + WRITE_MEMORY_BARRIER(); 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(&lock->__status, oldstatus, 0, &lock->__spinlock)) - goto again; - return; + 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) { + + /* Before we iterate over the wait queue, we need to execute + a read barrier, otherwise we may read stale contents of nodes that may + just have been inserted by other processors. One read barrier is enough to + ensure we have a stable list; we don't need one for each pointer chase + through the list, because we are the owner of the lock; other threads + can only add nodes at the front; if a front node is consistent, + the ones behind it must also be. */ + + READ_MEMORY_BARRIER(); + + while (thr != 0) { if (thr->p_priority >= maxprio) { maxptr = ptr; maxprio = thr->p_priority; } ptr = &(thr->p_nextlock); - thr = *ptr; + thr = (pthread_descr)((long)(thr->p_nextlock) & ~1L); } + /* 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(&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) & ~1L)) goto again; } else { - /* No risk of concurrent access, remove max prio thread normally */ - thr = *maxptr; + /* 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 = (pthread_descr)((long)*maxptr & ~1L); *maxptr = thr->p_nextlock; + + /* Ensure deletion from linked list completes before we + release the lock. */ + WRITE_MEMORY_BARRIER(); + + do { + oldstatus = lock->__status; + } while (!__compare_and_swap_with_release_semantics(&lock->__status, + oldstatus, oldstatus & ~1L)); } - /* Wake up the selected waiting thread */ + + /* Wake up the selected waiting thread. Woken thread can check + its own p_nextlock field for NULL to detect that it has been removed. No + barrier is needed here, since restart() and suspend() take + care of memory synchronization. */ + thr->p_nextlock = NULL; restart(thr); + + return 0; +#endif +} + +/* + * Alternate fastlocks do not queue threads directly. Instead, they queue + * these wait queue node structures. When a timed wait wakes up due to + * a timeout, it can leave its wait node in the queue (because there + * is no safe way to remove from the quue). Some other thread will + * deallocate the abandoned node. + */ + + +struct wait_node { + struct wait_node *next; /* Next node in null terminated linked list */ + pthread_descr thr; /* The thread waiting with this node */ + int abandoned; /* Atomic flag */ +}; + +static long wait_node_free_list; +static int wait_node_free_list_spinlock; + +/* Allocate a new node from the head of the free list using an atomic + operation, or else using malloc if that list is empty. A fundamental + assumption here is that we can safely access wait_node_free_list->next. + That's because we never free nodes once we allocate them, so a pointer to a + node remains valid indefinitely. */ + +static struct wait_node *wait_node_alloc(void) +{ + struct wait_node *new_node = 0; + + __pthread_acquire(&wait_node_free_list_spinlock); + if (wait_node_free_list != 0) { + new_node = (struct wait_node *) wait_node_free_list; + wait_node_free_list = (long) new_node->next; + } + WRITE_MEMORY_BARRIER(); + __pthread_release(&wait_node_free_list_spinlock); + + if (new_node == 0) + return malloc(sizeof *wait_node_alloc()); + + return new_node; +} + +/* Return a node to the head of the free list using an atomic + operation. */ + +static void wait_node_free(struct wait_node *wn) +{ + __pthread_acquire(&wait_node_free_list_spinlock); + wn->next = (struct wait_node *) wait_node_free_list; + wait_node_free_list = (long) wn; + WRITE_MEMORY_BARRIER(); + __pthread_release(&wait_node_free_list_spinlock); + return; +} + +#if defined HAS_COMPARE_AND_SWAP + +/* Remove a wait node from the specified queue. It is assumed + that the removal takes place concurrently with only atomic insertions at the + head of the queue. */ + +static void wait_node_dequeue(struct wait_node **pp_head, + struct wait_node **pp_node, + struct wait_node *p_node) +{ + /* If the node is being deleted from the head of the + list, it must be deleted using atomic compare-and-swap. + Otherwise it can be deleted in the straightforward way. */ + + if (pp_node == pp_head) { + /* We don't need a read barrier between these next two loads, + because it is assumed that the caller has already ensured + the stability of *p_node with respect to p_node. */ + + long oldvalue = (long) p_node; + long newvalue = (long) p_node->next; + + if (__compare_and_swap((long *) pp_node, oldvalue, newvalue)) + return; + + /* Oops! Compare and swap failed, which means the node is + no longer first. We delete it using the ordinary method. But we don't + know the identity of the node which now holds the pointer to the node + being deleted, so we must search from the beginning. */ + + for (pp_node = pp_head; p_node != *pp_node; ) { + pp_node = &(*pp_node)->next; + READ_MEMORY_BARRIER(); /* Stabilize *pp_node for next iteration. */ + } + } + + *pp_node = p_node->next; + return; +} + +#endif + +void __pthread_alt_lock(struct _pthread_fastlock * lock, + pthread_descr self) +{ +#if defined HAS_COMPARE_AND_SWAP + long oldstatus, newstatus; +#endif + struct wait_node wait_node; + +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + { + int suspend_needed = 0; + __pthread_acquire(&lock->__spinlock); + + if (lock->__status == 0) + lock->__status = 1; + else { + if (self == NULL) + self = thread_self(); + + wait_node.abandoned = 0; + wait_node.next = (struct wait_node *) lock->__status; + wait_node.thr = self; + lock->__status = (long) &wait_node; + suspend_needed = 1; + } + + __pthread_release(&lock->__spinlock); + + if (suspend_needed) + suspend (self); + return; + } +#endif + +#if defined HAS_COMPARE_AND_SWAP + do { + oldstatus = lock->__status; + if (oldstatus == 0) { + newstatus = 1; + } else { + if (self == NULL) + self = thread_self(); + wait_node.thr = self; + newstatus = (long) &wait_node; + } + wait_node.abandoned = 0; + wait_node.next = (struct wait_node *) oldstatus; + /* Make sure the store in wait_node.next completes before performing + the compare-and-swap */ + MEMORY_BARRIER(); + } while(! __compare_and_swap(&lock->__status, oldstatus, newstatus)); + + /* Suspend. Note that unlike in __pthread_lock, we don't worry + here about spurious wakeup. That's because this lock is not + used in situations where that can happen; the restart can + only come from the previous lock owner. */ + + if (oldstatus != 0) + suspend(self); + + READ_MEMORY_BARRIER(); +#endif +} + +/* Timed-out lock operation; returns 0 to indicate timeout. */ + +int __pthread_alt_timedlock(struct _pthread_fastlock * lock, + pthread_descr self, const struct timespec *abstime) +{ + long oldstatus = 0; +#if defined HAS_COMPARE_AND_SWAP + long newstatus; +#endif + struct wait_node *p_wait_node = wait_node_alloc(); + + /* Out of memory, just give up and do ordinary lock. */ + if (p_wait_node == 0) { + __pthread_alt_lock(lock, self); + return 1; + } + +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + { + __pthread_acquire(&lock->__spinlock); + + if (lock->__status == 0) + lock->__status = 1; + else { + if (self == NULL) + self = thread_self(); + + p_wait_node->abandoned = 0; + p_wait_node->next = (struct wait_node *) lock->__status; + p_wait_node->thr = self; + lock->__status = (long) p_wait_node; + oldstatus = 1; /* force suspend */ + } + + __pthread_release(&lock->__spinlock); + goto suspend; + } +#endif + +#if defined HAS_COMPARE_AND_SWAP + do { + oldstatus = lock->__status; + if (oldstatus == 0) { + newstatus = 1; + } else { + if (self == NULL) + self = thread_self(); + p_wait_node->thr = self; + newstatus = (long) p_wait_node; + } + p_wait_node->abandoned = 0; + p_wait_node->next = (struct wait_node *) oldstatus; + /* Make sure the store in wait_node.next completes before performing + the compare-and-swap */ + MEMORY_BARRIER(); + } while(! __compare_and_swap(&lock->__status, oldstatus, newstatus)); +#endif + +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + suspend: +#endif + + /* If we did not get the lock, do a timed suspend. If we wake up due + to a timeout, then there is a race; the old lock owner may try + to remove us from the queue. This race is resolved by us and the owner + doing an atomic testandset() to change the state of the wait node from 0 + to 1. If we succeed, then it's a timeout and we abandon the node in the + queue. If we fail, it means the owner gave us the lock. */ + + if (oldstatus != 0) { + if (timedsuspend(self, abstime) == 0) { + if (!testandset(&p_wait_node->abandoned)) + return 0; /* Timeout! */ + + /* Eat oustanding resume from owner, otherwise wait_node_free() below + will race with owner's wait_node_dequeue(). */ + suspend(self); + } + } + + wait_node_free(p_wait_node); + + READ_MEMORY_BARRIER(); + + return 1; /* Got the lock! */ +} + +void __pthread_alt_unlock(struct _pthread_fastlock *lock) +{ + struct wait_node *p_node, **pp_node, *p_max_prio, **pp_max_prio; + struct wait_node ** const pp_head = (struct wait_node **) &lock->__status; + int maxprio; + + WRITE_MEMORY_BARRIER(); + +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + { + __pthread_acquire(&lock->__spinlock); + } +#endif + + while (1) { + + /* If no threads are waiting for this lock, try to just + atomically release it. */ +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + { + if (lock->__status == 0 || lock->__status == 1) { + lock->__status = 0; + break; + } + } +#endif + +#if defined TEST_FOR_COMPARE_AND_SWAP + else +#endif + +#if defined HAS_COMPARE_AND_SWAP + { + long oldstatus = lock->__status; + if (oldstatus == 0 || oldstatus == 1) { + if (__compare_and_swap_with_release_semantics (&lock->__status, oldstatus, 0)) + break; + else + continue; + } + } +#endif + + /* Process the entire queue of wait nodes. Remove all abandoned + wait nodes and put them into the global free queue, and + remember the one unabandoned node which refers to the thread + having the highest priority. */ + + pp_max_prio = pp_node = pp_head; + p_max_prio = p_node = *pp_head; + maxprio = INT_MIN; + + READ_MEMORY_BARRIER(); /* Prevent access to stale data through p_node */ + + while (p_node != (struct wait_node *) 1) { + int prio; + + if (p_node->abandoned) { + /* Remove abandoned node. */ +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + *pp_node = p_node->next; +#endif +#if defined TEST_FOR_COMPARE_AND_SWAP + else +#endif +#if defined HAS_COMPARE_AND_SWAP + wait_node_dequeue(pp_head, pp_node, p_node); +#endif + wait_node_free(p_node); + /* Note that the next assignment may take us to the beginning + of the queue, to newly inserted nodes, if pp_node == pp_head. + In that case we need a memory barrier to stabilize the first of + these new nodes. */ + p_node = *pp_node; + if (pp_node == pp_head) + READ_MEMORY_BARRIER(); /* No stale reads through p_node */ + continue; + } else if ((prio = p_node->thr->p_priority) >= maxprio) { + /* Otherwise remember it if its thread has a higher or equal priority + compared to that of any node seen thus far. */ + maxprio = prio; + pp_max_prio = pp_node; + p_max_prio = p_node; + } + + /* This canno6 jump backward in the list, so no further read + barrier is needed. */ + pp_node = &p_node->next; + p_node = *pp_node; + } + + /* If all threads abandoned, go back to top */ + if (maxprio == INT_MIN) + continue; + + ASSERT (p_max_prio != (struct wait_node *) 1); + + /* Now we want to to remove the max priority thread's wait node from + the list. Before we can do this, we must atomically try to change the + node's abandon state from zero to nonzero. If we succeed, that means we + have the node that we will wake up. If we failed, then it means the + thread timed out and abandoned the node in which case we repeat the + whole unlock operation. */ + + if (!testandset(&p_max_prio->abandoned)) { +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + *pp_max_prio = p_max_prio->next; +#endif +#if defined TEST_FOR_COMPARE_AND_SWAP + else +#endif +#if defined HAS_COMPARE_AND_SWAP + wait_node_dequeue(pp_head, pp_max_prio, p_max_prio); +#endif + restart(p_max_prio->thr); + break; + } + } + +#if defined TEST_FOR_COMPARE_AND_SWAP + if (!__pthread_has_cas) +#endif +#if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP + { + __pthread_release(&lock->__spinlock); + } +#endif } + /* Compare-and-swap emulation with a spinlock */ #ifdef TEST_FOR_COMPARE_AND_SWAP @@ -137,24 +664,25 @@ int __pthread_has_cas = 0; #if !defined HAS_COMPARE_AND_SWAP || defined TEST_FOR_COMPARE_AND_SWAP -static void __pthread_acquire(int * spinlock); - int __pthread_compare_and_swap(long * ptr, long oldval, long newval, int * spinlock) { int res; - if (testandset(spinlock)) __pthread_acquire(spinlock); + + __pthread_acquire(spinlock); + if (*ptr == oldval) { *ptr = newval; res = 1; } else { res = 0; } - *spinlock = 0; + + __pthread_release(spinlock); + return res; } -/* This function is called if the inlined test-and-set - in __pthread_compare_and_swap() failed */ +#endif /* The retry strategy is as follows: - We test and set the spinlock MAX_SPIN_COUNT times, calling @@ -179,6 +707,8 @@ static void __pthread_acquire(int * spinlock) int cnt = 0; struct timespec tm; + READ_MEMORY_BARRIER(); + while (testandset(spinlock)) { if (cnt < MAX_SPIN_COUNT) { sched_yield(); @@ -191,5 +721,3 @@ static void __pthread_acquire(int * spinlock) } } } - -#endif |