Commit 9c3060be authored by Davide Libenzi's avatar Davide Libenzi Committed by Linus Torvalds
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signal/timer/event: KAIO eventfd support example

This is an example about how to add eventfd support to the current KAIO code,
in order to enable KAIO to post readiness events to a pollable fd (hence
compatible with POSIX select/poll).  The KAIO code simply signals the eventfd
fd when events are ready, and this triggers a POLLIN in the fd.  This patch
uses a reserved for future use member of the struct iocb to pass an eventfd
file descriptor, that KAIO will use to post events every time a request
completes.  At that point, an aio_getevents() will return the completed result
to a struct io_event.  I made a quick test program to verify the patch, and it
runs fine here:

The test program uses poll(2), but it'd, of course, work with select and epoll

This can allow to schedule both block I/O and other poll-able devices
requests, and wait for results using select/poll/epoll.  In a typical
scenario, an application would submit KAIO request using aio_submit(), and
will also use epoll_ctl() on the whole other class of devices (that with the
addition of signals, timers and user events, now it's pretty much complete),
and then would:

	for_each_event {
		if (curr_event_is_kaiofd) {
		} else {
Signed-off-by: default avatarDavide Libenzi <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent fdb902b1
......@@ -30,6 +30,7 @@
#include <linux/highmem.h>
#include <linux/workqueue.h>
#include <linux/security.h>
#include <linux/eventfd.h>
#include <asm/kmap_types.h>
#include <asm/uaccess.h>
......@@ -417,6 +418,7 @@ static struct kiocb fastcall *__aio_get_req(struct kioctx *ctx)
req->private = NULL;
req->ki_iovec = NULL;
req->ki_eventfd = ERR_PTR(-EINVAL);
/* Check if the completion queue has enough free space to
* accept an event from this io.
......@@ -458,6 +460,8 @@ static inline void really_put_req(struct kioctx *ctx, struct kiocb *req)
if (!IS_ERR(req->ki_eventfd))
if (req->ki_dtor)
if (req->ki_iovec != &req->ki_inline_vec)
......@@ -942,6 +946,14 @@ int fastcall aio_complete(struct kiocb *iocb, long res, long res2)
return 1;
* Check if the user asked us to deliver the result through an
* eventfd. The eventfd_signal() function is safe to be called
* from IRQ context.
if (!IS_ERR(iocb->ki_eventfd))
eventfd_signal(iocb->ki_eventfd, 1);
info = &ctx->ring_info;
/* add a completion event to the ring buffer.
......@@ -1526,8 +1538,7 @@ int fastcall io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
ssize_t ret;
/* enforce forwards compatibility on users */
if (unlikely(iocb->aio_reserved1 || iocb->aio_reserved2 ||
iocb->aio_reserved3)) {
if (unlikely(iocb->aio_reserved1 || iocb->aio_reserved2)) {
pr_debug("EINVAL: io_submit: reserve field set\n");
return -EINVAL;
......@@ -1551,6 +1562,19 @@ int fastcall io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
return -EAGAIN;
if (iocb->aio_flags & IOCB_FLAG_RESFD) {
* If the IOCB_FLAG_RESFD flag of aio_flags is set, get an
* instance of the file* now. The file descriptor must be
* an eventfd() fd, and will be signaled for each completed
* event using the eventfd_signal() function.
req->ki_eventfd = eventfd_fget((int) iocb->aio_resfd);
if (unlikely(IS_ERR(req->ki_eventfd))) {
ret = PTR_ERR(req->ki_eventfd);
goto out_put_req;
req->ki_filp = file;
ret = put_user(req->ki_key, &user_iocb->aio_key);
......@@ -119,6 +119,12 @@ struct kiocb {
struct list_head ki_list; /* the aio core uses this
* for cancellation */
* If the aio_resfd field of the userspace iocb is not zero,
* this is the underlying file* to deliver event to.
struct file *ki_eventfd;
#define is_sync_kiocb(iocb) ((iocb)->ki_key == KIOCB_SYNC_KEY)
......@@ -45,6 +45,14 @@ enum {
* Valid flags for the "aio_flags" member of the "struct iocb".
* IOCB_FLAG_RESFD - Set if the "aio_resfd" member of the "struct iocb"
* is valid.
#define IOCB_FLAG_RESFD (1 << 0)
/* read() from /dev/aio returns these structures. */
struct io_event {
__u64 data; /* the data field from the iocb */
......@@ -84,7 +92,15 @@ struct iocb {
/* extra parameters */
__u64 aio_reserved2; /* TODO: use this for a (struct sigevent *) */
__u64 aio_reserved3;
/* flags for the "struct iocb" */
__u32 aio_flags;
* if the IOCB_FLAG_RESFD flag of "aio_flags" is set, this is an
* eventfd to signal AIO readiness to
__u32 aio_resfd;
}; /* 64 bytes */
#undef IFBIG
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