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aio-posix: fix concurrent access to poll_disable_cnt 

It is valid for an aio_set_fd_handler to happen concurrently with
aio_poll.  In that case, poll_disable_cnt can change under the heels
of aio_poll, and the assertion on poll_disable_cnt can fail in
run_poll_handlers.

Therefore, this patch simply checks the counter on every polling
iteration.  There are no particular needs for ordering, since the
polling loop is terminated anyway by aio_notify at the end of
aio_set_fd_handler.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20180912171040.1732-2-pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
This commit is contained in:
Paolo Bonzini 2018-09-12 19:10:38 +02:00 committed by Fam Zheng
parent c5e4e49258
commit d7be5dd19c
1 changed files with 15 additions and 11 deletions
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26
util/aio-posix.c
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@ -211,6 +211,7 @@ void aio_set_fd_handler(AioContext *ctx,
AioHandler *node;
bool is_new = false;
bool deleted = false;
int poll_disable_change;
qemu_lockcnt_lock(&ctx->list_lock);
@ -244,11 +245,9 @@ void aio_set_fd_handler(AioContext *ctx,
QLIST_REMOVE(node, node);
deleted = true;
}
if (!node->io_poll) {
ctx->poll_disable_cnt--;
}
poll_disable_change = -!node->io_poll;
} else {
poll_disable_change = !io_poll - (node && !node->io_poll);
if (node == NULL) {
/* Alloc and insert if it's not already there */
node = g_new0(AioHandler, 1);
@ -257,10 +256,6 @@ void aio_set_fd_handler(AioContext *ctx,
g_source_add_poll(&ctx->source, &node->pfd);
is_new = true;
ctx->poll_disable_cnt += !io_poll;
} else {
ctx->poll_disable_cnt += !io_poll - !node->io_poll;
}
/* Update handler with latest information */
@ -274,6 +269,15 @@ void aio_set_fd_handler(AioContext *ctx,
node->pfd.events |= (io_write ? G_IO_OUT | G_IO_ERR : 0);
}
/* No need to order poll_disable_cnt writes against other updates;
* the counter is only used to avoid wasting time and latency on
* iterated polling when the system call will be ultimately necessary.
* Changing handlers is a rare event, and a little wasted polling until
* the aio_notify below is not an issue.
*/
atomic_set(&ctx->poll_disable_cnt,
atomic_read(&ctx->poll_disable_cnt) + poll_disable_change);
aio_epoll_update(ctx, node, is_new);
qemu_lockcnt_unlock(&ctx->list_lock);
aio_notify(ctx);
@ -525,7 +529,6 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns)
assert(ctx->notify_me);
assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
assert(ctx->poll_disable_cnt == 0);
trace_run_poll_handlers_begin(ctx, max_ns);
@ -533,7 +536,8 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns)
do {
progress = run_poll_handlers_once(ctx);
} while (!progress && qemu_clock_get_ns(QEMU_CLOCK_REALTIME) < end_time);
} while (!progress && qemu_clock_get_ns(QEMU_CLOCK_REALTIME) < end_time
&& !atomic_read(&ctx->poll_disable_cnt));
trace_run_poll_handlers_end(ctx, progress);
@ -552,7 +556,7 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns)
*/
static bool try_poll_mode(AioContext *ctx, bool blocking)
{
if (blocking && ctx->poll_max_ns && ctx->poll_disable_cnt == 0) {
if (blocking && ctx->poll_max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
/* See qemu_soonest_timeout() uint64_t hack */
int64_t max_ns = MIN((uint64_t)aio_compute_timeout(ctx),
(uint64_t)ctx->poll_ns);