io_uring: drain next sqe instead of shadowing
There's an issue with the shadow drain logic in that we drop the
completion lock after deciding to defer a request, then re-grab it later
and assume that the state is still the same. In the mean time, someone
else completing a request could have found and issued it. This can cause
a stall in the queue, by having a shadow request inserted that nobody is
going to drain.
Additionally, if we fail allocating the shadow request, we simply ignore
the drain.
Instead of using a shadow request, defer the next request/link instead.
This also has the following advantages:
- removes semi-duplicated code
- doesn't allocate memory for shadows
- works better if only the head marked for drain
- doesn't need complex synchronisation
On the flip side, it removes the shadow->seq ==
last_drain_in_in_link->seq optimization. That shouldn't be a common
case, and can always be added back, if needed.
Fixes: 4fe2c963154c ("io_uring: add support for link with drain")
Cc: Jackie Liu <liuyun01@kylinos.cn>
Reported-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 27fefb5..ca980c5 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -186,6 +186,7 @@ struct io_ring_ctx {
bool compat;
bool account_mem;
bool cq_overflow_flushed;
+ bool drain_next;
/*
* Ring buffer of indices into array of io_uring_sqe, which is
@@ -346,7 +347,7 @@ struct io_kiocb {
#define REQ_F_LINK 64 /* linked sqes */
#define REQ_F_LINK_TIMEOUT 128 /* has linked timeout */
#define REQ_F_FAIL_LINK 256 /* fail rest of links */
-#define REQ_F_SHADOW_DRAIN 512 /* link-drain shadow req */
+#define REQ_F_DRAIN_LINK 512 /* link should be fully drained */
#define REQ_F_TIMEOUT 1024 /* timeout request */
#define REQ_F_ISREG 2048 /* regular file */
#define REQ_F_MUST_PUNT 4096 /* must be punted even for NONBLOCK */
@@ -620,11 +621,6 @@ static void io_commit_cqring(struct io_ring_ctx *ctx)
__io_commit_cqring(ctx);
while ((req = io_get_deferred_req(ctx)) != NULL) {
- if (req->flags & REQ_F_SHADOW_DRAIN) {
- /* Just for drain, free it. */
- __io_free_req(req);
- continue;
- }
req->flags |= REQ_F_IO_DRAINED;
io_queue_async_work(req);
}
@@ -2973,6 +2969,12 @@ static void io_queue_sqe(struct io_kiocb *req)
{
int ret;
+ if (unlikely(req->ctx->drain_next)) {
+ req->flags |= REQ_F_IO_DRAIN;
+ req->ctx->drain_next = false;
+ }
+ req->ctx->drain_next = (req->flags & REQ_F_DRAIN_LINK);
+
ret = io_req_defer(req);
if (ret) {
if (ret != -EIOCBQUEUED) {
@@ -2985,57 +2987,16 @@ static void io_queue_sqe(struct io_kiocb *req)
__io_queue_sqe(req);
}
-static void io_queue_link_head(struct io_kiocb *req, struct io_kiocb *shadow)
+static inline void io_queue_link_head(struct io_kiocb *req)
{
- int ret;
- int need_submit = false;
- struct io_ring_ctx *ctx = req->ctx;
-
if (unlikely(req->flags & REQ_F_FAIL_LINK)) {
- ret = -ECANCELED;
- goto err;
- }
- if (!shadow) {
+ io_cqring_add_event(req, -ECANCELED);
+ io_double_put_req(req);
+ } else
io_queue_sqe(req);
- return;
- }
-
- /*
- * Mark the first IO in link list as DRAIN, let all the following
- * IOs enter the defer list. all IO needs to be completed before link
- * list.
- */
- req->flags |= REQ_F_IO_DRAIN;
- ret = io_req_defer(req);
- if (ret) {
- if (ret != -EIOCBQUEUED) {
-err:
- io_cqring_add_event(req, ret);
- if (req->flags & REQ_F_LINK)
- req->flags |= REQ_F_FAIL_LINK;
- io_double_put_req(req);
- if (shadow)
- __io_free_req(shadow);
- return;
- }
- } else {
- /*
- * If ret == 0 means that all IOs in front of link io are
- * running done. let's queue link head.
- */
- need_submit = true;
- }
-
- /* Insert shadow req to defer_list, blocking next IOs */
- spin_lock_irq(&ctx->completion_lock);
- trace_io_uring_defer(ctx, shadow, true);
- list_add_tail(&shadow->list, &ctx->defer_list);
- spin_unlock_irq(&ctx->completion_lock);
-
- if (need_submit)
- __io_queue_sqe(req);
}
+
#define SQE_VALID_FLAGS (IOSQE_FIXED_FILE|IOSQE_IO_DRAIN|IOSQE_IO_LINK)
static void io_submit_sqe(struct io_kiocb *req, struct io_submit_state *state,
@@ -3072,6 +3033,9 @@ static void io_submit_sqe(struct io_kiocb *req, struct io_submit_state *state,
struct io_kiocb *prev = *link;
struct io_uring_sqe *sqe_copy;
+ if (s->sqe->flags & IOSQE_IO_DRAIN)
+ (*link)->flags |= REQ_F_DRAIN_LINK | REQ_F_IO_DRAIN;
+
if (READ_ONCE(s->sqe->opcode) == IORING_OP_LINK_TIMEOUT) {
ret = io_timeout_setup(req);
/* common setup allows offset being set, we don't */
@@ -3190,7 +3154,6 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
{
struct io_submit_state state, *statep = NULL;
struct io_kiocb *link = NULL;
- struct io_kiocb *shadow_req = NULL;
int i, submitted = 0;
bool mm_fault = false;
@@ -3229,18 +3192,6 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
sqe_flags = req->submit.sqe->flags;
- if (link && (sqe_flags & IOSQE_IO_DRAIN)) {
- if (!shadow_req) {
- shadow_req = io_get_req(ctx, NULL);
- if (unlikely(!shadow_req))
- goto out;
- shadow_req->flags |= (REQ_F_IO_DRAIN | REQ_F_SHADOW_DRAIN);
- refcount_dec(&shadow_req->refs);
- }
- shadow_req->sequence = req->submit.sequence;
- }
-
-out:
req->submit.ring_file = ring_file;
req->submit.ring_fd = ring_fd;
req->submit.has_user = *mm != NULL;
@@ -3256,14 +3207,13 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,
* that's the end of the chain. Submit the previous link.
*/
if (!(sqe_flags & IOSQE_IO_LINK) && link) {
- io_queue_link_head(link, shadow_req);
+ io_queue_link_head(link);
link = NULL;
- shadow_req = NULL;
}
}
if (link)
- io_queue_link_head(link, shadow_req);
+ io_queue_link_head(link);
if (statep)
io_submit_state_end(&state);