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0ceb849bd3
In many cases, the call to event_notifier_set in aio_notify is unnecessary. In particular, if we are executing aio_dispatch, or if aio_poll is not blocking, we know that we will soon get to the next loop iteration (if necessary); the thread that hosts the AioContext's event loop does not need any nudging. The patch includes a Promela formal model that shows that this really works and does not need any further complication such as generation counts. It needs a memory barrier though. The generation counts are not needed because any change to ctx->dispatching after the memory barrier is okay for aio_notify. If it changes from zero to one, it is the right thing to skip event_notifier_set. If it changes from one to zero, the event_notifier_set is unnecessary but harmless. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
275 lines
7.6 KiB
C
275 lines
7.6 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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int pollfds_idx;
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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_malloc0(sizeof(AioHandler));
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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->pollfds_idx = -1;
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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_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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static 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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* 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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bool aio_poll(AioContext *ctx, bool blocking)
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{
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AioHandler *node;
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bool was_dispatching;
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int ret;
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bool progress;
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was_dispatching = ctx->dispatching;
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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 happens
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* in two cases:
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*
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* 1) when aio_poll is called with blocking == false
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*
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* 2) when we are called after poll(). If we are called before
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* poll(), bottom halves will not be re-evaluated and we need
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* aio_notify() if blocking == true.
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*
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* The first aio_dispatch() only does something when AioContext is
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* running as a GSource, and in that case aio_poll is used only
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* with blocking == false, so this optimization is already quite
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* effective. However, the code is ugly and should be restructured
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* to have a single aio_dispatch() call. To do this, we need to
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* reorganize aio_poll into a prepare/poll/dispatch model like
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* glib's.
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*
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* If we're in a nested event loop, ctx->dispatching might be true.
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* In that case we can restore it just before returning, but we
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* have to clear it now.
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*/
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aio_set_dispatching(ctx, !blocking);
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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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blocking = false;
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progress = true;
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}
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/* Re-evaluate condition (1) above. */
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aio_set_dispatching(ctx, !blocking);
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if (aio_dispatch(ctx)) {
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progress = true;
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}
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if (progress && !blocking) {
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goto out;
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}
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ctx->walking_handlers++;
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g_array_set_size(ctx->pollfds, 0);
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/* fill pollfds */
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QLIST_FOREACH(node, &ctx->aio_handlers, node) {
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node->pollfds_idx = -1;
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if (!node->deleted && node->pfd.events) {
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GPollFD pfd = {
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.fd = node->pfd.fd,
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.events = node->pfd.events,
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};
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node->pollfds_idx = ctx->pollfds->len;
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g_array_append_val(ctx->pollfds, pfd);
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}
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}
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ctx->walking_handlers--;
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/* wait until next event */
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ret = qemu_poll_ns((GPollFD *)ctx->pollfds->data,
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ctx->pollfds->len,
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blocking ? timerlistgroup_deadline_ns(&ctx->tlg) : 0);
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/* if we have any readable fds, dispatch event */
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if (ret > 0) {
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QLIST_FOREACH(node, &ctx->aio_handlers, node) {
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if (node->pollfds_idx != -1) {
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GPollFD *pfd = &g_array_index(ctx->pollfds, GPollFD,
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node->pollfds_idx);
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node->pfd.revents = pfd->revents;
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}
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}
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}
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/* Run dispatch even if there were no readable fds to run timers */
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aio_set_dispatching(ctx, true);
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if (aio_dispatch(ctx)) {
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progress = true;
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}
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out:
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aio_set_dispatching(ctx, was_dispatching);
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return progress;
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}
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