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: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 49 insertions, 3 deletions

diff --git a/fs/aio.c b/fs/aio.c
index ac1c1587aa02..dbe699e9828c 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -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);
diff --git a/include/linux/aio.h b/include/linux/aio.h
index 43dc2ebfaa0e..b903fc02bdb7 100644
--- a/include/linux/aio.h
+++ b/include/linux/aio.h
@@ -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)
diff --git a/include/linux/aio_abi.h b/include/linux/aio_abi.h
index e3ca0a485cc6..9e0172931315 100644
--- a/include/linux/aio_abi.h
+++ b/include/linux/aio_abi.h
@@ -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