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.

diff --git a/src/internal/pthread_impl.h b/src/internal/pthread_impl.h
index bbb4502..82d6242 100644 (file)
--- 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 883acb2..fd2bb07 100644 (file)
--- 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 ab7da9e..91aefd4 100644 (file)
--- 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 2898c41..f82664d 100644 (file)
--- 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 736d101..ccaab4e 100644 (file)
--- 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 aed1adc..db432ba 100644 (file)
--- 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 d6c7f9a..1edc95d 100644 (file)
--- 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;
 }