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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: 4fe2c96315 ("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>
This commit is contained in:
Pavel Begunkov 2019-11-21 11:54:28 +03:00 committed by Jens Axboe
parent b76da70fc3
commit 1b4a51b6d0
1 changed files with 18 additions and 68 deletions
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86
fs/io_uring.c
View File

@ -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 @@ err_req:
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 @@ out:
* 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);