From: Jens Axboe axboe@kernel.dk
[ Upstream commit 592b4805432af075468876771c0f7d41273ccb3c ]
A previous commit added looping around handling traditional task_work as an optimization, and while that may seem like a good idea, it's also possible to run into application starvation doing so. If the task_work generation is bursty, we can get very deep task_work queues, and we can end up looping in here for a very long time.
One immediately observable problem with that is handling network traffic using provided buffers, where flooding incoming traffic and looping task_work handling will very quickly lead to buffer starvation as we keep running task_work rather than returning to the application so it can handle the associated CQEs and also provide buffers back.
Fixes: 3a0c037b0e16 ("io_uring: batch task_work") Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- io_uring/io_uring.c | 45 +++++++-------------------------------------- 1 file changed, 7 insertions(+), 38 deletions(-)
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c index cd9a137ad6cef..5233a20d01b54 100644 --- a/io_uring/io_uring.c +++ b/io_uring/io_uring.c @@ -1176,12 +1176,11 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx, struct io_tw_state *ts)
static unsigned int handle_tw_list(struct llist_node *node, struct io_ring_ctx **ctx, - struct io_tw_state *ts, - struct llist_node *last) + struct io_tw_state *ts) { unsigned int count = 0;
- while (node && node != last) { + do { struct llist_node *next = node->next; struct io_kiocb *req = container_of(node, struct io_kiocb, io_task_work.node); @@ -1205,7 +1204,7 @@ static unsigned int handle_tw_list(struct llist_node *node, *ctx = NULL; cond_resched(); } - } + } while (node);
return count; } @@ -1224,22 +1223,6 @@ static inline struct llist_node *io_llist_xchg(struct llist_head *head, return xchg(&head->first, new); }
-/** - * io_llist_cmpxchg - possibly swap all entries in a lock-less list - * @head: the head of lock-less list to delete all entries - * @old: expected old value of the first entry of the list - * @new: new entry as the head of the list - * - * perform a cmpxchg on the first entry of the list. - */ - -static inline struct llist_node *io_llist_cmpxchg(struct llist_head *head, - struct llist_node *old, - struct llist_node *new) -{ - return cmpxchg(&head->first, old, new); -} - static __cold void io_fallback_tw(struct io_uring_task *tctx, bool sync) { struct llist_node *node = llist_del_all(&tctx->task_list); @@ -1274,9 +1257,7 @@ void tctx_task_work(struct callback_head *cb) struct io_ring_ctx *ctx = NULL; struct io_uring_task *tctx = container_of(cb, struct io_uring_task, task_work); - struct llist_node fake = {}; struct llist_node *node; - unsigned int loops = 0; unsigned int count = 0;
if (unlikely(current->flags & PF_EXITING)) { @@ -1284,21 +1265,9 @@ void tctx_task_work(struct callback_head *cb) return; }
- do { - loops++; - node = io_llist_xchg(&tctx->task_list, &fake); - count += handle_tw_list(node, &ctx, &ts, &fake); - - /* skip expensive cmpxchg if there are items in the list */ - if (READ_ONCE(tctx->task_list.first) != &fake) - continue; - if (ts.locked && !wq_list_empty(&ctx->submit_state.compl_reqs)) { - io_submit_flush_completions(ctx); - if (READ_ONCE(tctx->task_list.first) != &fake) - continue; - } - node = io_llist_cmpxchg(&tctx->task_list, &fake, NULL); - } while (node != &fake); + node = llist_del_all(&tctx->task_list); + if (node) + count = handle_tw_list(node, &ctx, &ts);
ctx_flush_and_put(ctx, &ts);
@@ -1306,7 +1275,7 @@ void tctx_task_work(struct callback_head *cb) if (unlikely(atomic_read(&tctx->in_cancel))) io_uring_drop_tctx_refs(current);
- trace_io_uring_task_work_run(tctx, count, loops); + trace_io_uring_task_work_run(tctx, count, 1); }
static inline void io_req_local_work_add(struct io_kiocb *req, unsigned flags)