mirror of https://github.com/xemu-project/xemu.git
AioContext: optimize clearing the EventNotifier
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>
This commit is contained in:
Paolo Bonzini
Stefan Hajnoczi
parent
21a03d17f2
commit
05e514b1d4
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@ -276,7 +276,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
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aio_context_acquire(ctx);
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}
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event_notifier_test_and_clear(&ctx->notifier);
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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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@ -338,7 +338,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
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}
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if (first) {
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event_notifier_test_and_clear(&ctx->notifier);
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aio_notify_accept(ctx);
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progress |= aio_bh_poll(ctx);
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first = false;
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}
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10
async.c
10
async.c
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@ -203,7 +203,7 @@ aio_ctx_check(GSource *source)
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QEMUBH *bh;
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atomic_and(&ctx->notify_me, ~1);
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event_notifier_test_and_clear(&ctx->notifier);
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aio_notify_accept(ctx);
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for (bh = ctx->first_bh; bh; bh = bh->next) {
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if (!bh->deleted && bh->scheduled) {
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@ -267,6 +267,14 @@ void aio_notify(AioContext *ctx)
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smp_mb();
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if (ctx->notify_me) {
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event_notifier_set(&ctx->notifier);
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atomic_mb_set(&ctx->notified, true);
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}
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}
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void aio_notify_accept(AioContext *ctx)
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{
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if (atomic_xchg(&ctx->notified, false)) {
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event_notifier_test_and_clear(&ctx->notifier);
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}
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}
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@ -0,0 +1,152 @@
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/*
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* This model describes the interaction between ctx->notified
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* and ctx->notifier.
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*
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* Author: Paolo Bonzini <pbonzini@redhat.com>
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*
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* This file is in the public domain. If you really want a license,
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* the WTFPL will do.
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*
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* To verify the buggy version:
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* spin -a -DBUG1 docs/aio_notify_bug.promela
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* gcc -O2 pan.c
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* ./a.out -a -f
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* (or -DBUG2)
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*
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* To verify the fixed version:
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* spin -a docs/aio_notify_bug.promela
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* gcc -O2 pan.c
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* ./a.out -a -f
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*
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* Add -DCHECK_REQ to test an alternative invariant and the
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* "notify_me" optimization.
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*/
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int notify_me;
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bool notified;
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bool event;
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bool req;
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bool notifier_done;
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#ifdef CHECK_REQ
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#define USE_NOTIFY_ME 1
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#else
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#define USE_NOTIFY_ME 0
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#endif
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#ifdef BUG
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#error Please define BUG1 or BUG2 instead.
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#endif
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active proctype notifier()
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{
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do
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:: true -> {
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req = 1;
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if
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:: !USE_NOTIFY_ME || notify_me ->
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#if defined BUG1
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/* CHECK_REQ does not detect this bug! */
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notified = 1;
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event = 1;
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#elif defined BUG2
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if
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:: !notified -> event = 1;
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:: else -> skip;
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fi;
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notified = 1;
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#else
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event = 1;
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notified = 1;
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#endif
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:: else -> skip;
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fi
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}
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:: true -> break;
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od;
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notifier_done = 1;
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}
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#define AIO_POLL \
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notify_me++; \
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if \
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:: !req -> { \
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if \
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:: event -> skip; \
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fi; \
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} \
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:: else -> skip; \
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fi; \
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notify_me--; \
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\
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atomic { old = notified; notified = 0; } \
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if \
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:: old -> event = 0; \
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:: else -> skip; \
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fi; \
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\
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req = 0;
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active proctype waiter()
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{
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bool old;
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do
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:: true -> AIO_POLL;
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od;
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}
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/* Same as waiter(), but disappears after a while. */
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active proctype temporary_waiter()
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{
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bool old;
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do
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:: true -> AIO_POLL;
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:: true -> break;
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od;
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}
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#ifdef CHECK_REQ
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never {
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do
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:: req -> goto accept_if_req_not_eventually_false;
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:: true -> skip;
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od;
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accept_if_req_not_eventually_false:
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if
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:: req -> goto accept_if_req_not_eventually_false;
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fi;
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assert(0);
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}
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#else
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/* There must be infinitely many transitions of event as long
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* as the notifier does not exit.
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*
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* If event stayed always true, the waiters would be busy looping.
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* If event stayed always false, the waiters would be sleeping
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* forever.
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*/
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never {
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do
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:: !event -> goto accept_if_event_not_eventually_true;
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:: event -> goto accept_if_event_not_eventually_false;
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:: true -> skip;
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od;
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accept_if_event_not_eventually_true:
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if
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:: !event && notifier_done -> do :: true -> skip; od;
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:: !event && !notifier_done -> goto accept_if_event_not_eventually_true;
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fi;
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assert(0);
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accept_if_event_not_eventually_false:
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if
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:: event -> goto accept_if_event_not_eventually_false;
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fi;
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assert(0);
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}
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#endif
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@ -99,7 +99,19 @@ struct AioContext {
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*/
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int walking_bh;
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/* Used for aio_notify. */
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/* Used by aio_notify.
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*
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* "notified" is used to avoid expensive event_notifier_test_and_clear
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* calls. When it is clear, the EventNotifier is clear, or one thread
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* is going to clear "notified" before processing more events. False
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* positives are possible, i.e. "notified" could be set even though the
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* EventNotifier is clear.
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*
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* Note that event_notifier_set *cannot* be optimized the same way. For
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* more information on the problem that would result, see "#ifdef BUG2"
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* in the docs/aio_notify_accept.promela formal model.
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*/
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bool notified;
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EventNotifier notifier;
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/* Thread pool for performing work and receiving completion callbacks */
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@ -173,6 +185,24 @@ QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
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*/
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void aio_notify(AioContext *ctx);
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/**
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* aio_notify_accept: Acknowledge receiving an aio_notify.
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*
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* aio_notify() uses an EventNotifier in order to wake up a sleeping
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* aio_poll() or g_main_context_iteration(). Calls to aio_notify() are
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* usually rare, but the AioContext has to clear the EventNotifier on
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* every aio_poll() or g_main_context_iteration() in order to avoid
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* busy waiting. This event_notifier_test_and_clear() cannot be done
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* using the usual aio_context_set_event_notifier(), because it must
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* be done before processing all events (file descriptors, bottom halves,
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* timers).
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*
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* aio_notify_accept() is an optimized event_notifier_test_and_clear()
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* that is specific to an AioContext's notifier; it is used internally
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* to clear the EventNotifier only if aio_notify() had been called.
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*/
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void aio_notify_accept(AioContext *ctx);
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/**
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* aio_bh_poll: Poll bottom halves for an AioContext.
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*
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