From 60164570111873175111cf8a5b973375e492eee9 Mon Sep 17 00:00:00 2001 From: Rich Felker Date: Tue, 27 Sep 2011 13:50:29 -0400 Subject: [PATCH] process-shared barrier support, based on discussion with bdonlan this implementation is rather heavy-weight, but it's the first solution i've found that's actually correct. all waiters actually wait twice at the barrier so that they can synchronize exit, and they hold a "vm lock" that prevents changes to virtual memory mappings (and blocks pthread_barrier_destroy) until all waiters are finished inspecting the barrier. thus, it is safe for any thread to destroy and/or unmap the barrier's memory as soon as pthread_barrier_wait returns, without further synchronization. --- src/internal/pthread_impl.h | 8 ++- src/mman/mmap.c | 13 +++- src/mman/munmap.c | 11 ++- src/thread/pthread_barrier_destroy.c | 6 ++ src/thread/pthread_barrier_init.c | 4 +- src/thread/pthread_barrier_wait.c | 74 +++++++++++++++++++-- src/thread/pthread_barrierattr_setpshared.c | 2 +- 7 files changed, 102 insertions(+), 16 deletions(-) diff --git a/src/internal/pthread_impl.h b/src/internal/pthread_impl.h index bbb4502f..82d62426 100644 --- a/src/internal/pthread_impl.h +++ b/src/internal/pthread_impl.h @@ -73,10 +73,12 @@ struct __timer { #define _c_waiters2 __u.__i[7] #define _rw_lock __u.__i[0] #define _rw_waiters __u.__i[1] -#define _b_inst __u.__p[0] +#define _b_lock __u.__i[0] +#define _b_waiters __u.__i[1] #define _b_limit __u.__i[2] -#define _b_lock __u.__i[3] -#define _b_waiters __u.__i[4] +#define _b_count __u.__i[3] +#define _b_seq __u.__i[4] +#define _b_inst __u.__p[4] #include "pthread_arch.h" diff --git a/src/mman/mmap.c b/src/mman/mmap.c index 883acb28..fd2bb07e 100644 --- a/src/mman/mmap.c +++ b/src/mman/mmap.c @@ -5,16 +5,25 @@ #include "syscall.h" #include "libc.h" +static void dummy1(int x) { } +static void dummy0(void) { } +weak_alias(dummy1, __vm_lock); +weak_alias(dummy0, __vm_unlock); + void *__mmap(void *start, size_t len, int prot, int flags, int fd, off_t off) { + void *ret; if (sizeof(off_t) > sizeof(long)) if (((long)off & 0xfff) | ((long)((unsigned long long)off>>(12 + 8*(sizeof(off_t)-sizeof(long)))))) start = (void *)-1; + if (flags & MAP_FIXED) __vm_lock(-1); #ifdef SYS_mmap2 - return (void *)syscall(SYS_mmap2, start, len, prot, flags, fd, off>>12); + ret = (void *)syscall(SYS_mmap2, start, len, prot, flags, fd, off>>12); #else - return (void *)syscall(SYS_mmap, start, len, prot, flags, fd, off); + ret = (void *)syscall(SYS_mmap, start, len, prot, flags, fd, off); #endif + if (flags & MAP_FIXED) __vm_unlock(); + return ret; } weak_alias(__mmap, mmap); diff --git a/src/mman/munmap.c b/src/mman/munmap.c index ab7da9e2..91aefd40 100644 --- a/src/mman/munmap.c +++ b/src/mman/munmap.c @@ -3,9 +3,18 @@ #include "syscall.h" #include "libc.h" +static void dummy1(int x) { } +static void dummy0(void) { } +weak_alias(dummy1, __vm_lock); +weak_alias(dummy0, __vm_unlock); + int __munmap(void *start, size_t len) { - return syscall(SYS_munmap, start, len); + int ret; + __vm_lock(-1); + ret = syscall(SYS_munmap, start, len); + __vm_unlock(); + return ret; } weak_alias(__munmap, munmap); diff --git a/src/thread/pthread_barrier_destroy.c b/src/thread/pthread_barrier_destroy.c index 2898c41a..f82664d5 100644 --- a/src/thread/pthread_barrier_destroy.c +++ b/src/thread/pthread_barrier_destroy.c @@ -1,6 +1,12 @@ #include "pthread_impl.h" +void __vm_lock(int), __vm_unlock(void); + int pthread_barrier_destroy(pthread_barrier_t *b) { + if (b->_b_limit < 0) { + __vm_lock(-1); + __vm_unlock(); + } return 0; } diff --git a/src/thread/pthread_barrier_init.c b/src/thread/pthread_barrier_init.c index 736d1014..ccaab4eb 100644 --- a/src/thread/pthread_barrier_init.c +++ b/src/thread/pthread_barrier_init.c @@ -2,7 +2,7 @@ int pthread_barrier_init(pthread_barrier_t *b, const pthread_barrierattr_t *a, unsigned count) { - if (!count) return EINVAL; - *b = (pthread_barrier_t){ ._b_limit = count-1 }; + if (count-1 > INT_MAX-1) return EINVAL; + *b = (pthread_barrier_t){ ._b_limit = count-1 | (a?*a:0) }; return 0; } diff --git a/src/thread/pthread_barrier_wait.c b/src/thread/pthread_barrier_wait.c index aed1adc8..db432ba3 100644 --- a/src/thread/pthread_barrier_wait.c +++ b/src/thread/pthread_barrier_wait.c @@ -1,5 +1,62 @@ #include "pthread_impl.h" +static int vmlock[2]; + +void __vm_lock(int inc) +{ + for (;;) { + int v = vmlock[0]; + if (inc*v < 0) __wait(vmlock, vmlock+1, v, 1); + else if (a_cas(vmlock, v, v+inc)==v) break; + } +} + +void __vm_unlock(void) +{ + if (vmlock[0]>0) a_dec(vmlock); + else a_inc(vmlock); + if (vmlock[1]) __wake(vmlock, 1, 1); +} + +static int pshared_barrier_wait(pthread_barrier_t *b) +{ + int limit = (b->_b_limit & INT_MAX) + 1; + int seq; + int ret = 0; + + if (!limit) return PTHREAD_BARRIER_SERIAL_THREAD; + + while (a_swap(&b->_b_lock, 1)) + __wait(&b->_b_lock, &b->_b_waiters, 1, 0); + + seq = b->_b_seq; + + if (++b->_b_count == limit) { + ret = PTHREAD_BARRIER_SERIAL_THREAD; + b->_b_seq++; + __wake(&b->_b_seq, -1, 0); + } else { + a_store(&b->_b_lock, 0); + if (b->_b_waiters) __wake(&b->_b_lock, 1, 0); + __wait(&b->_b_seq, 0, seq, 0); + } + + __vm_lock(+1); + + if (a_fetch_add(&b->_b_count, -1)==1) { + b->_b_seq++; + __wake(&b->_b_seq, -1, 0); + a_store(&b->_b_lock, 0); + if (b->_b_waiters) __wake(&b->_b_lock, 1, 0); + } else { + __wait(&b->_b_seq, 0, seq+1, 0); + } + + __vm_unlock(); + + return 0; +} + struct instance { int count; @@ -16,9 +73,12 @@ int pthread_barrier_wait(pthread_barrier_t *b) /* Trivial case: count was set at 1 */ if (!limit) return PTHREAD_BARRIER_SERIAL_THREAD; + /* Process-shared barriers require a separate, inefficient wait */ + if (limit < 0) return pshared_barrier_wait(b); + /* Otherwise we need a lock on the barrier object */ while (a_swap(&b->_b_lock, 1)) - __wait(&b->_b_lock, &b->_b_waiters, 1, 0); + __wait(&b->_b_lock, &b->_b_waiters, 1, 1); inst = b->_b_inst; /* First thread to enter the barrier becomes the "instance owner" */ @@ -27,7 +87,7 @@ int pthread_barrier_wait(pthread_barrier_t *b) int spins = 10000; b->_b_inst = inst = &new_inst; a_store(&b->_b_lock, 0); - if (b->_b_waiters) __wake(&b->_b_lock, 1, 0); + if (b->_b_waiters) __wake(&b->_b_lock, 1, 1); while (spins-- && !inst->finished) a_spin(); a_inc(&inst->finished); @@ -40,19 +100,19 @@ int pthread_barrier_wait(pthread_barrier_t *b) if (++inst->count == limit) { b->_b_inst = 0; a_store(&b->_b_lock, 0); - if (b->_b_waiters) __wake(&b->_b_lock, 1, 0); + if (b->_b_waiters) __wake(&b->_b_lock, 1, 1); a_store(&inst->last, 1); if (inst->waiters) - __wake(&inst->last, -1, 0); + __wake(&inst->last, -1, 1); } else { a_store(&b->_b_lock, 0); - if (b->_b_waiters) __wake(&b->_b_lock, 1, 0); - __wait(&inst->last, &inst->waiters, 0, 0); + if (b->_b_waiters) __wake(&b->_b_lock, 1, 1); + __wait(&inst->last, &inst->waiters, 0, 1); } /* Last thread to exit the barrier wakes the instance owner */ if (a_fetch_add(&inst->count,-1)==1 && a_fetch_add(&inst->finished,1)) - __wake(&inst->finished, 1, 0); + __wake(&inst->finished, 1, 1); return 0; } diff --git a/src/thread/pthread_barrierattr_setpshared.c b/src/thread/pthread_barrierattr_setpshared.c index d6c7f9a3..1edc95dc 100644 --- a/src/thread/pthread_barrierattr_setpshared.c +++ b/src/thread/pthread_barrierattr_setpshared.c @@ -2,6 +2,6 @@ int pthread_barrierattr_setpshared(pthread_barrierattr_t *a, int pshared) { - *a = !!pshared; + *a = pshared ? INT_MIN : 0; return 0; } -- 2.25.1