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:

http://www.xmailserver.org/eventfd-aio-test.c



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

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:

	epoll_wait(...);
	for_each_event {
		if (curr_event_is_kaiofd) {
			aio_getevents();
			dispatch_aio_events();
		} else {
			dispatch_epoll_event();
		}
	}

Signed-off-by: default avatarDavide Libenzi <davidel@xmailserver.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent fdb902b1

fs/aio.c

+26 −2
Original line number Diff line number Diff line
@@ -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;

	INIT_LIST_HEAD(&req->ki_run_list);

	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) 
{

	assert_spin_locked(&ctx->ctx_lock);



	if (!IS_ERR(req->ki_eventfd))

		fput(req->ki_eventfd);

	if (req->ki_dtor)

		req->ki_dtor(req);

	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, 
		fput(file);

		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);

include/linux/aio.h

+6 −0
Original line number Diff line number Diff line
@@ -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)

include/linux/aio_abi.h

+17 −1
Original line number Diff line number Diff line
@@ -45,6 +45,14 @@ enum { 
	IOCB_CMD_PWRITEV = 8,

};



/*

 * 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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