[musl] / src / aio / aio.c

#include <aio.h>
#include <pthread.h>
#include <semaphore.h>
#include <limits.h>
#include <errno.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/auxv.h>
#include "syscall.h"
#include "atomic.h"
#include "pthread_impl.h"

/* The following is a threads-based implementation of AIO with minimal
 * dependence on implementation details. Most synchronization is
 * performed with pthread primitives, but atomics and futex operations
 * are used for notification in a couple places where the pthread
 * primitives would be inefficient or impractical.
 *
 * For each fd with outstanding aio operations, an aio_queue structure
 * is maintained. These are reference-counted and destroyed by the last
 * aio worker thread to exit. Accessing any member of the aio_queue
 * structure requires a lock on the aio_queue. Adding and removing aio
 * queues themselves requires a write lock on the global map object,
 * a 4-level table mapping file descriptor numbers to aio queues. A
 * read lock on the map is used to obtain locks on existing queues by
 * excluding destruction of the queue by a different thread while it is
 * being locked.
 *
 * Each aio queue has a list of active threads/operations. Presently there
 * is a one to one relationship between threads and operations. The only
 * members of the aio_thread structure which are accessed by other threads
 * are the linked list pointers, op (which is immutable), running (which
 * is updated atomically), and err (which is synchronized via running),
 * so no locking is necessary. Most of the other other members are used
 * for sharing data between the main flow of execution and cancellation
 * cleanup handler.
 *
 * Taking any aio locks requires having all signals blocked. This is
 * necessary because aio_cancel is needed by close, and close is required
 * to be async-signal safe. All aio worker threads run with all signals
 * blocked permanently.
 */

struct aio_args {
        struct aiocb *cb;
        int op;
        int err;
        sem_t sem;
};

struct aio_thread {
        pthread_t td;
        struct aiocb *cb;
        struct aio_thread *next, *prev;
        struct aio_queue *q;
        volatile int running;
        int err, op;
        ssize_t ret;
};

struct aio_queue {
        int fd, seekable, append, ref, init;
        pthread_mutex_t lock;
        pthread_cond_t cond;
        struct aio_thread *head;
};

static pthread_rwlock_t maplock = PTHREAD_RWLOCK_INITIALIZER;
static struct aio_queue *****map;
static volatile int aio_fd_cnt;
volatile int __aio_fut;

static struct aio_queue *__aio_get_queue(int fd, int need)
{
        if (fd < 0) return 0;
        int a=fd>>24;
        unsigned char b=fd>>16, c=fd>>8, d=fd;
        struct aio_queue *q = 0;
        pthread_rwlock_rdlock(&maplock);
        if ((!map || !map[a] || !map[a][b] || !map[a][b][c] || !(q=map[a][b][c][d])) && need) {
                pthread_rwlock_unlock(&maplock);
                pthread_rwlock_wrlock(&maplock);
                if (!map) map = calloc(sizeof *map, (-1U/2+1)>>24);
                if (!map) goto out;
                if (!map[a]) map[a] = calloc(sizeof **map, 256);
                if (!map[a]) goto out;
                if (!map[a][b]) map[a][b] = calloc(sizeof ***map, 256);
                if (!map[a][b]) goto out;
                if (!map[a][b][c]) map[a][b][c] = calloc(sizeof ****map, 256);
                if (!map[a][b][c]) goto out;
                if (!(q = map[a][b][c][d])) {
                        map[a][b][c][d] = q = calloc(sizeof *****map, 1);
                        if (q) {
                                q->fd = fd;
                                pthread_mutex_init(&q->lock, 0);
                                pthread_cond_init(&q->cond, 0);
                                a_inc(&aio_fd_cnt);
                        }
                }
        }
        if (q) pthread_mutex_lock(&q->lock);
out:
        pthread_rwlock_unlock(&maplock);
        return q;
}

static void __aio_unref_queue(struct aio_queue *q)
{
        if (q->ref > 1) {
                q->ref--;
                pthread_mutex_unlock(&q->lock);
                return;
        }

        /* This is potentially the last reference, but a new reference
         * may arrive since we cannot free the queue object without first
         * taking the maplock, which requires releasing the queue lock. */
        pthread_mutex_unlock(&q->lock);
        pthread_rwlock_wrlock(&maplock);
        pthread_mutex_lock(&q->lock);
        if (q->ref == 1) {
                int fd=q->fd;
                int a=fd>>24;
                unsigned char b=fd>>16, c=fd>>8, d=fd;
                map[a][b][c][d] = 0;
                a_dec(&aio_fd_cnt);
                pthread_rwlock_unlock(&maplock);
                pthread_mutex_unlock(&q->lock);
                free(q);
        } else {
                q->ref--;
                pthread_rwlock_unlock(&maplock);
                pthread_mutex_unlock(&q->lock);
        }
}

static void cleanup(void *ctx)
{
        struct aio_thread *at = ctx;
        struct aio_queue *q = at->q;
        struct aiocb *cb = at->cb;
        struct sigevent sev = cb->aio_sigevent;

        /* There are four potential types of waiters we could need to wake:
         *   1. Callers of aio_cancel/close.
         *   2. Callers of aio_suspend with a single aiocb.
         *   3. Callers of aio_suspend with a list.
         *   4. AIO worker threads waiting for sequenced operations.
         * Types 1-3 are notified via atomics/futexes, mainly for AS-safety
         * considerations. Type 4 is notified later via a cond var. */

        cb->__ret = at->ret;
        if (a_swap(&at->running, 0) < 0)
                __wake(&at->running, -1, 1);
        if (a_swap(&cb->__err, at->err) != EINPROGRESS)
                __wake(&cb->__err, -1, 1);
        if (a_swap(&__aio_fut, 0))
                __wake(&__aio_fut, -1, 1);

        pthread_mutex_lock(&q->lock);

        if (at->next) at->next->prev = at->prev;
        if (at->prev) at->prev->next = at->next;
        else q->head = at->next;

        /* Signal aio worker threads waiting for sequenced operations. */
        pthread_cond_broadcast(&q->cond);

        __aio_unref_queue(q);

        if (sev.sigev_notify == SIGEV_SIGNAL) {
                siginfo_t si = {
                        .si_signo = sev.sigev_signo,
                        .si_value = sev.sigev_value,
                        .si_code = SI_ASYNCIO,
                        .si_pid = getpid(),
                        .si_uid = getuid()
                };
                __syscall(SYS_rt_sigqueueinfo, si.si_pid, si.si_signo, &si);
        }
        if (sev.sigev_notify == SIGEV_THREAD) {
                a_store(&__pthread_self()->cancel, 0);
                sev.sigev_notify_function(sev.sigev_value);
        }
}

static void *io_thread_func(void *ctx)
{
        struct aio_thread at, *p;

        struct aio_args *args = ctx;
        struct aiocb *cb = args->cb;
        int fd = cb->aio_fildes;
        int op = args->op;
        void *buf = (void *)cb->aio_buf;
        size_t len = cb->aio_nbytes;
        off_t off = cb->aio_offset;

        struct aio_queue *q = __aio_get_queue(fd, 1);
        ssize_t ret;

        args->err = q ? 0 : EAGAIN;
        sem_post(&args->sem);
        if (!q) return 0;

        at.op = op;
        at.running = 1;
        at.ret = -1;
        at.err = ECANCELED;
        at.q = q;
        at.td = __pthread_self();
        at.cb = cb;
        at.prev = 0;
        if ((at.next = q->head)) at.next->prev = &at;
        q->head = &at;
        q->ref++;

        if (!q->init) {
                int seekable = lseek(fd, 0, SEEK_CUR) >= 0;
                q->seekable = seekable;
                q->append = !seekable || (fcntl(fd, F_GETFL) & O_APPEND);
                q->init = 1;
        }

        pthread_cleanup_push(cleanup, &at);

        /* Wait for sequenced operations. */
        if (op!=LIO_READ && (op!=LIO_WRITE || q->append)) {
                for (;;) {
                        for (p=at.next; p && p->op!=LIO_WRITE; p=p->next);
                        if (!p) break;
                        pthread_cond_wait(&q->cond, &q->lock);
                }
        }

        pthread_mutex_unlock(&q->lock);

        switch (op) {
        case LIO_WRITE:
                ret = q->append ? write(fd, buf, len) : pwrite(fd, buf, len, off);
                break;
        case LIO_READ:
                ret = !q->seekable ? read(fd, buf, len) : pread(fd, buf, len, off);
                break;
        case O_SYNC:
                ret = fsync(fd);
                break;
        case O_DSYNC:
                ret = fdatasync(fd);
                break;
        }
        at.ret = ret;
        at.err = ret<0 ? errno : 0;
        
        pthread_cleanup_pop(1);

        return 0;
}

static size_t io_thread_stack_size = MINSIGSTKSZ+2048;
static pthread_once_t init_stack_size_once;

static void init_stack_size()
{
        unsigned long val = __getauxval(AT_MINSIGSTKSZ);
        if (val > MINSIGSTKSZ) io_thread_stack_size = val + 512;
}

static int submit(struct aiocb *cb, int op)
{
        int ret = 0;
        pthread_attr_t a;
        sigset_t allmask, origmask;
        pthread_t td;
        struct aio_args args = { .cb = cb, .op = op };
        sem_init(&args.sem, 0, 0);

        if (cb->aio_sigevent.sigev_notify == SIGEV_THREAD) {
                if (cb->aio_sigevent.sigev_notify_attributes)
                        a = *cb->aio_sigevent.sigev_notify_attributes;
                else
                        pthread_attr_init(&a);
        } else {
                pthread_once(&init_stack_size_once, init_stack_size);
                pthread_attr_init(&a);
                pthread_attr_setstacksize(&a, io_thread_stack_size);
                pthread_attr_setguardsize(&a, 0);
        }
        pthread_attr_setdetachstate(&a, PTHREAD_CREATE_DETACHED);
        sigfillset(&allmask);
        pthread_sigmask(SIG_BLOCK, &allmask, &origmask);
        cb->__err = EINPROGRESS;
        if (pthread_create(&td, &a, io_thread_func, &args)) {
                errno = EAGAIN;
                ret = -1;
        }
        pthread_sigmask(SIG_SETMASK, &origmask, 0);

        if (!ret) {
                while (sem_wait(&args.sem));
                if (args.err) {
                        errno = args.err;
                        ret = -1;
                }
        }

        return ret;
}

int aio_read(struct aiocb *cb)
{
        return submit(cb, LIO_READ);
}

int aio_write(struct aiocb *cb)
{
        return submit(cb, LIO_WRITE);
}

int aio_fsync(int op, struct aiocb *cb)
{
        if (op != O_SYNC && op != O_DSYNC) {
                errno = EINVAL;
                return -1;
        }
        return submit(cb, op);
}

ssize_t aio_return(struct aiocb *cb)
{
        return cb->__ret;
}

int aio_error(const struct aiocb *cb)
{
        a_barrier();
        return cb->__err & 0x7fffffff;
}

int aio_cancel(int fd, struct aiocb *cb)
{
        sigset_t allmask, origmask;
        int ret = AIO_ALLDONE;
        struct aio_thread *p;
        struct aio_queue *q;

        /* Unspecified behavior case. Report an error. */
        if (cb && fd != cb->aio_fildes) {
                errno = EINVAL;
                return -1;
        }

        sigfillset(&allmask);
        pthread_sigmask(SIG_BLOCK, &allmask, &origmask);

        if (!(q = __aio_get_queue(fd, 0))) {
                if (fcntl(fd, F_GETFD) < 0) ret = -1;
                goto done;
        }

        for (p = q->head; p; p = p->next) {
                if (cb && cb != p->cb) continue;
                /* Transition target from running to running-with-waiters */
                if (a_cas(&p->running, 1, -1)) {
                        pthread_cancel(p->td);
                        __wait(&p->running, 0, -1, 1);
                        if (p->err == ECANCELED) ret = AIO_CANCELED;
                }
        }

        pthread_mutex_unlock(&q->lock);
done:
        pthread_sigmask(SIG_SETMASK, &origmask, 0);
        return ret;
}

int __aio_close(int fd)
{
        a_barrier();
        if (aio_fd_cnt) aio_cancel(fd, 0);
        return fd;
}

weak_alias(aio_cancel, aio_cancel64);
weak_alias(aio_error, aio_error64);
weak_alias(aio_fsync, aio_fsync64);
weak_alias(aio_read, aio_read64);
weak_alias(aio_write, aio_write64);
weak_alias(aio_return, aio_return64);