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Re: [Qemu-block] [PATCH 1/3] aio-posix: fix concurrent access to poll_di
From: |
Fam Zheng |
Subject: |
Re: [Qemu-block] [PATCH 1/3] aio-posix: fix concurrent access to poll_disable_cnt |
Date: |
Thu, 13 Sep 2018 14:56:39 +0800 |
User-agent: |
Mutt/1.10.1 (2018-07-13) |
On Wed, 09/12 19:10, Paolo Bonzini wrote:
> 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 <address@hidden>
> ---
> util/aio-posix.c | 26 +++++++++++++++-----------
> 1 file changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/util/aio-posix.c b/util/aio-posix.c
> index 131ba6b4a8..5c29380575 100644
> --- a/util/aio-posix.c
> +++ b/util/aio-posix.c
> @@ -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);
Why not atomic_add?
> +
> 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);
> --
> 2.17.1
>
>
Reviewed-by: Fam Zheng <address@hidden>