On 9/27/21 1:36 PM, Hao Xu wrote:
> For multishot mode, there may be cases like:
> io_poll_task_func()
> -> add_wait_queue()
> async_wake()
> ->io_req_task_work_add()
> this one mess up the running task_work list
> since req->io_task_work.node is in use.
By the time req->io_task_work.func() is called, node->next is undefined
and free to use. io_req_task_work_add() will override it without looking
at a prior value and that's fine, as the calling code doesn't touch it
after the callback.
> similar situation for req->io_task_work.fallback_node.
> Fix it by set node->next = NULL before we run the tw, so that when we
> add req back to the wait queue in middle of tw running, we can safely
> re-add it to the tw list.
I might be missing what is the problem you're trying to fix. Does the
above makes sense? It doesn't sound like node->next=NULL can solve
anything.
> Fixes: 7cbf1722d5fc ("io_uring: provide FIFO ordering for task_work")
> Signed-off-by: Hao Xu <haoxu@xxxxxxxxxxxxxxxxx>
> ---
> fs/io_uring.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index d0b358b9b589..f667d6286438 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1250,13 +1250,17 @@ static void io_fallback_req_func(struct work_struct *work)
> struct io_ring_ctx *ctx = container_of(work, struct io_ring_ctx,
> fallback_work.work);
> struct llist_node *node = llist_del_all(&ctx->fallback_llist);
> - struct io_kiocb *req, *tmp;
> + struct io_kiocb *req;
> bool locked = false;
>
> percpu_ref_get(&ctx->refs);
> - llist_for_each_entry_safe(req, tmp, node, io_task_work.fallback_node)
> + req = llist_entry(node, struct io_kiocb, io_task_work.fallback_node);
> + while (member_address_is_nonnull(req, io_task_work.fallback_node)) {
> + node = req->io_task_work.fallback_node.next;
> + req->io_task_work.fallback_node.next = NULL;
> req->io_task_work.func(req, &locked);
> -
> + req = llist_entry(node, struct io_kiocb, io_task_work.fallback_node);
> + }
> if (locked) {
> io_submit_flush_completions(ctx);
> mutex_unlock(&ctx->uring_lock);
> @@ -2156,6 +2160,7 @@ static void tctx_task_work(struct callback_head *cb)
> locked = mutex_trylock(&ctx->uring_lock);
> percpu_ref_get(&ctx->refs);
> }
> + node->next = NULL;
> req->io_task_work.func(req, &locked);
> node = next;
> } while (node);
>
--
Pavel Begunkov
