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![Paolo Bonzini](/assets/img/avatar_default.png)
It is pretty rare for aio_notify to actually set the EventNotifier. It can happen with worker threads such as thread-pool.c's, but otherwise it should never be set thanks to the ctx->notify_me optimization. The previous patch, unfortunately, added an unconditional call to event_notifier_test_and_clear; now add a userspace fast path that avoids the call. Note that it is not possible to do the same with event_notifier_set; it would break, as proved (again) by the included formal model. This patch survived over 3000 reboots on aarch64 KVM. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-7-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
300 lines
7.8 KiB
C
300 lines
7.8 KiB
C
/*
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* QEMU aio implementation
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*
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* Copyright IBM, Corp. 2008
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*
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* Authors:
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* Anthony Liguori <aliguori@us.ibm.com>
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*
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* This work is licensed under the terms of the GNU GPL, version 2. See
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* the COPYING file in the top-level directory.
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*
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* Contributions after 2012-01-13 are licensed under the terms of the
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* GNU GPL, version 2 or (at your option) any later version.
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*/
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#include "qemu-common.h"
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#include "block/block.h"
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#include "qemu/queue.h"
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#include "qemu/sockets.h"
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struct AioHandler
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{
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GPollFD pfd;
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IOHandler *io_read;
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IOHandler *io_write;
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int deleted;
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void *opaque;
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QLIST_ENTRY(AioHandler) node;
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};
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static AioHandler *find_aio_handler(AioContext *ctx, int fd)
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{
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AioHandler *node;
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QLIST_FOREACH(node, &ctx->aio_handlers, node) {
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if (node->pfd.fd == fd)
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if (!node->deleted)
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return node;
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}
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return NULL;
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}
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void aio_set_fd_handler(AioContext *ctx,
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int fd,
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IOHandler *io_read,
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IOHandler *io_write,
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void *opaque)
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{
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AioHandler *node;
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node = find_aio_handler(ctx, fd);
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/* Are we deleting the fd handler? */
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if (!io_read && !io_write) {
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if (node) {
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g_source_remove_poll(&ctx->source, &node->pfd);
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/* If the lock is held, just mark the node as deleted */
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if (ctx->walking_handlers) {
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node->deleted = 1;
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node->pfd.revents = 0;
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} else {
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/* Otherwise, delete it for real. We can't just mark it as
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* deleted because deleted nodes are only cleaned up after
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* releasing the walking_handlers lock.
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*/
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QLIST_REMOVE(node, node);
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g_free(node);
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}
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}
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} else {
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if (node == NULL) {
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/* Alloc and insert if it's not already there */
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node = g_new0(AioHandler, 1);
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node->pfd.fd = fd;
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QLIST_INSERT_HEAD(&ctx->aio_handlers, node, node);
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g_source_add_poll(&ctx->source, &node->pfd);
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}
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/* Update handler with latest information */
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node->io_read = io_read;
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node->io_write = io_write;
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node->opaque = opaque;
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node->pfd.events = (io_read ? G_IO_IN | G_IO_HUP | G_IO_ERR : 0);
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node->pfd.events |= (io_write ? G_IO_OUT | G_IO_ERR : 0);
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}
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aio_notify(ctx);
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}
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void aio_set_event_notifier(AioContext *ctx,
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EventNotifier *notifier,
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EventNotifierHandler *io_read)
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{
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aio_set_fd_handler(ctx, event_notifier_get_fd(notifier),
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(IOHandler *)io_read, NULL, notifier);
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}
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bool aio_prepare(AioContext *ctx)
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{
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return false;
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}
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bool aio_pending(AioContext *ctx)
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{
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AioHandler *node;
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QLIST_FOREACH(node, &ctx->aio_handlers, node) {
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int revents;
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revents = node->pfd.revents & node->pfd.events;
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if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read) {
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return true;
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}
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if (revents & (G_IO_OUT | G_IO_ERR) && node->io_write) {
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return true;
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}
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}
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return false;
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}
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bool aio_dispatch(AioContext *ctx)
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{
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AioHandler *node;
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bool progress = false;
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/*
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* If there are callbacks left that have been queued, we need to call them.
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* Do not call select in this case, because it is possible that the caller
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* does not need a complete flush (as is the case for aio_poll loops).
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*/
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if (aio_bh_poll(ctx)) {
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progress = true;
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}
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/*
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* We have to walk very carefully in case aio_set_fd_handler is
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* called while we're walking.
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*/
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node = QLIST_FIRST(&ctx->aio_handlers);
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while (node) {
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AioHandler *tmp;
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int revents;
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ctx->walking_handlers++;
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revents = node->pfd.revents & node->pfd.events;
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node->pfd.revents = 0;
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if (!node->deleted &&
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(revents & (G_IO_IN | G_IO_HUP | G_IO_ERR)) &&
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node->io_read) {
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node->io_read(node->opaque);
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/* aio_notify() does not count as progress */
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if (node->opaque != &ctx->notifier) {
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progress = true;
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}
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}
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if (!node->deleted &&
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(revents & (G_IO_OUT | G_IO_ERR)) &&
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node->io_write) {
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node->io_write(node->opaque);
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progress = true;
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}
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tmp = node;
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node = QLIST_NEXT(node, node);
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ctx->walking_handlers--;
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if (!ctx->walking_handlers && tmp->deleted) {
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QLIST_REMOVE(tmp, node);
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g_free(tmp);
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}
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}
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/* Run our timers */
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progress |= timerlistgroup_run_timers(&ctx->tlg);
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return progress;
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}
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/* These thread-local variables are used only in a small part of aio_poll
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* around the call to the poll() system call. In particular they are not
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* used while aio_poll is performing callbacks, which makes it much easier
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* to think about reentrancy!
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*
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* Stack-allocated arrays would be perfect but they have size limitations;
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* heap allocation is expensive enough that we want to reuse arrays across
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* calls to aio_poll(). And because poll() has to be called without holding
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* any lock, the arrays cannot be stored in AioContext. Thread-local data
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* has none of the disadvantages of these three options.
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*/
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static __thread GPollFD *pollfds;
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static __thread AioHandler **nodes;
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static __thread unsigned npfd, nalloc;
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static __thread Notifier pollfds_cleanup_notifier;
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static void pollfds_cleanup(Notifier *n, void *unused)
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{
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g_assert(npfd == 0);
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g_free(pollfds);
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g_free(nodes);
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nalloc = 0;
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}
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static void add_pollfd(AioHandler *node)
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{
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if (npfd == nalloc) {
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if (nalloc == 0) {
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pollfds_cleanup_notifier.notify = pollfds_cleanup;
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qemu_thread_atexit_add(&pollfds_cleanup_notifier);
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nalloc = 8;
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} else {
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g_assert(nalloc <= INT_MAX);
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nalloc *= 2;
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}
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pollfds = g_renew(GPollFD, pollfds, nalloc);
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nodes = g_renew(AioHandler *, nodes, nalloc);
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}
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nodes[npfd] = node;
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pollfds[npfd] = (GPollFD) {
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.fd = node->pfd.fd,
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.events = node->pfd.events,
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};
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npfd++;
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}
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bool aio_poll(AioContext *ctx, bool blocking)
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{
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AioHandler *node;
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int i, ret;
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bool progress;
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int64_t timeout;
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aio_context_acquire(ctx);
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progress = false;
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/* aio_notify can avoid the expensive event_notifier_set if
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* everything (file descriptors, bottom halves, timers) will
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* be re-evaluated before the next blocking poll(). This is
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* already true when aio_poll is called with blocking == false;
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* if blocking == true, it is only true after poll() returns,
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* so disable the optimization now.
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*/
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if (blocking) {
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atomic_add(&ctx->notify_me, 2);
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}
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ctx->walking_handlers++;
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assert(npfd == 0);
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/* fill pollfds */
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QLIST_FOREACH(node, &ctx->aio_handlers, node) {
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if (!node->deleted && node->pfd.events) {
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add_pollfd(node);
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}
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}
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timeout = blocking ? aio_compute_timeout(ctx) : 0;
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/* wait until next event */
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if (timeout) {
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aio_context_release(ctx);
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}
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ret = qemu_poll_ns((GPollFD *)pollfds, npfd, timeout);
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if (blocking) {
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atomic_sub(&ctx->notify_me, 2);
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}
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if (timeout) {
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aio_context_acquire(ctx);
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}
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aio_notify_accept(ctx);
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/* if we have any readable fds, dispatch event */
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if (ret > 0) {
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for (i = 0; i < npfd; i++) {
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nodes[i]->pfd.revents = pollfds[i].revents;
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}
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}
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npfd = 0;
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ctx->walking_handlers--;
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/* Run dispatch even if there were no readable fds to run timers */
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if (aio_dispatch(ctx)) {
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progress = true;
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}
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aio_context_release(ctx);
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return progress;
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}
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