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io_uring: signal registered eventfd to process deferred task work

Browse source 
Some workloads rely on a registered eventfd (via
io_uring_register_eventfd(3)) in order to wake up and process the
io_uring.

In the case of a ring setup with IORING_SETUP_DEFER_TASKRUN, that eventfd
also needs to be signalled when there are tasks to run.

This changes an old behaviour which assumed 1 eventfd signal implied at
least 1 CQE, however only when this new flag is set (and so old users will
not notice). This should be expected with the IORING_SETUP_DEFER_TASKRUN
flag as it is not guaranteed that every task will result in a CQE.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220830125013.570060-7-dylany@fb.com
[axboe: fold in call_rcu() serialization fix]
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Dylan Yudaken 2022-08-30 05:50:12 -07:00 committed by Jens Axboe
parent d8e9214f11
commit 21a091b970
2 changed files with 63 additions and 23 deletions
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2
include/linux/io_uring_types.h
View file

@ -184,6 +184,8 @@ struct io_ev_fd {
struct eventfd_ctx *cq_ev_fd;
unsigned int eventfd_async: 1;
struct rcu_head rcu;
atomic_t refs;
atomic_t ops;
};
struct io_alloc_cache {

84
io_uring/io_uring.c
View file

@ -125,6 +125,11 @@ enum {
IO_CHECK_CQ_DROPPED_BIT,
};
enum {
IO_EVENTFD_OP_SIGNAL_BIT,
IO_EVENTFD_OP_FREE_BIT,
};
struct io_defer_entry {
struct list_head list;
struct io_kiocb *req;
@ -478,33 +483,28 @@ static __cold void io_queue_deferred(struct io_ring_ctx *ctx)
}
}
static void io_eventfd_put(struct rcu_head *rcu)
static void io_eventfd_ops(struct rcu_head *rcu)
{
struct io_ev_fd *ev_fd = container_of(rcu, struct io_ev_fd, rcu);
int ops = atomic_xchg(&ev_fd->ops, 0);
eventfd_ctx_put(ev_fd->cq_ev_fd);
kfree(ev_fd);
if (ops & BIT(IO_EVENTFD_OP_SIGNAL_BIT))
eventfd_signal(ev_fd->cq_ev_fd, 1);
/* IO_EVENTFD_OP_FREE_BIT may not be set here depending on callback
* ordering in a race but if references are 0 we know we have to free
* it regardless.
*/
if (atomic_dec_and_test(&ev_fd->refs)) {
eventfd_ctx_put(ev_fd->cq_ev_fd);
kfree(ev_fd);
}
}
static void io_eventfd_signal(struct io_ring_ctx *ctx)
{
struct io_ev_fd *ev_fd;
bool skip;
spin_lock(&ctx->completion_lock);
/*
* Eventfd should only get triggered when at least one event has been
* posted. Some applications rely on the eventfd notification count only
* changing IFF a new CQE has been added to the CQ ring. There's no
* depedency on 1:1 relationship between how many times this function is
* called (and hence the eventfd count) and number of CQEs posted to the
* CQ ring.
*/
skip = ctx->cached_cq_tail == ctx->evfd_last_cq_tail;
ctx->evfd_last_cq_tail = ctx->cached_cq_tail;
spin_unlock(&ctx->completion_lock);
if (skip)
return;
struct io_ev_fd *ev_fd = NULL;
rcu_read_lock();
/*
@ -522,13 +522,46 @@ static void io_eventfd_signal(struct io_ring_ctx *ctx)
goto out;
if (READ_ONCE(ctx->rings->cq_flags) & IORING_CQ_EVENTFD_DISABLED)
goto out;
if (ev_fd->eventfd_async && !io_wq_current_is_worker())
goto out;
if (!ev_fd->eventfd_async || io_wq_current_is_worker())
if (likely(eventfd_signal_allowed())) {
eventfd_signal(ev_fd->cq_ev_fd, 1);
} else {
atomic_inc(&ev_fd->refs);
if (!atomic_fetch_or(BIT(IO_EVENTFD_OP_SIGNAL_BIT), &ev_fd->ops))
call_rcu(&ev_fd->rcu, io_eventfd_ops);
else
atomic_dec(&ev_fd->refs);
}
out:
rcu_read_unlock();
}
static void io_eventfd_flush_signal(struct io_ring_ctx *ctx)
{
bool skip;
spin_lock(&ctx->completion_lock);
/*
* Eventfd should only get triggered when at least one event has been
* posted. Some applications rely on the eventfd notification count
* only changing IFF a new CQE has been added to the CQ ring. There's
* no depedency on 1:1 relationship between how many times this
* function is called (and hence the eventfd count) and number of CQEs
* posted to the CQ ring.
*/
skip = ctx->cached_cq_tail == ctx->evfd_last_cq_tail;
ctx->evfd_last_cq_tail = ctx->cached_cq_tail;
spin_unlock(&ctx->completion_lock);
if (skip)
return;
io_eventfd_signal(ctx);
}
void __io_commit_cqring_flush(struct io_ring_ctx *ctx)
{
if (ctx->off_timeout_used || ctx->drain_active) {
@ -540,7 +573,7 @@ void __io_commit_cqring_flush(struct io_ring_ctx *ctx)
spin_unlock(&ctx->completion_lock);
}
if (ctx->has_evfd)
io_eventfd_signal(ctx);
io_eventfd_flush_signal(ctx);
}
static inline void io_cqring_ev_posted(struct io_ring_ctx *ctx)
@ -1071,6 +1104,8 @@ static void io_req_local_work_add(struct io_kiocb *req)
if (ctx->flags & IORING_SETUP_TASKRUN_FLAG)
atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);
if (ctx->has_evfd)
io_eventfd_signal(ctx);
io_cqring_wake(ctx);
}
@ -2474,6 +2509,8 @@ static int io_eventfd_register(struct io_ring_ctx *ctx, void __user *arg,
ev_fd->eventfd_async = eventfd_async;
ctx->has_evfd = true;
rcu_assign_pointer(ctx->io_ev_fd, ev_fd);
atomic_set(&ev_fd->refs, 1);
atomic_set(&ev_fd->ops, 0);
return 0;
}
@ -2486,7 +2523,8 @@ static int io_eventfd_unregister(struct io_ring_ctx *ctx)
if (ev_fd) {
ctx->has_evfd = false;
rcu_assign_pointer(ctx->io_ev_fd, NULL);
call_rcu(&ev_fd->rcu, io_eventfd_put);
if (!atomic_fetch_or(BIT(IO_EVENTFD_OP_FREE_BIT), &ev_fd->ops))
call_rcu(&ev_fd->rcu, io_eventfd_ops);
return 0;
}