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Subject[patch 13/13] signal/timer/event fds v9 - 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>



- Davide



Index: linux-2.6.21-rc5.fds/fs/aio.c
===================================================================
--- linux-2.6.21-rc5.fds.orig/fs/aio.c 2007-03-31 12:45:30.000000000 -0700
+++ linux-2.6.21-rc5.fds/fs/aio.c 2007-03-31 12:46:31.000000000 -0700
@@ -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>
@@ -421,6 +422,7 @@
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.
@@ -462,6 +464,8 @@
{
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)
@@ -946,6 +950,14 @@
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.
@@ -1555,6 +1567,19 @@
fput(file);
return -EAGAIN;
}
+ if (iocb->aio_resfd != 0) {
+ /*
+ * If the aio_resfd field of the iocb is not zero, 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 (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);
Index: linux-2.6.21-rc5.fds/include/linux/aio.h
===================================================================
--- linux-2.6.21-rc5.fds.orig/include/linux/aio.h 2007-03-31 12:45:30.000000000 -0700
+++ linux-2.6.21-rc5.fds/include/linux/aio.h 2007-03-31 12:46:31.000000000 -0700
@@ -119,6 +119,12 @@

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)
Index: linux-2.6.21-rc5.fds/include/linux/aio_abi.h
===================================================================
--- linux-2.6.21-rc5.fds.orig/include/linux/aio_abi.h 2007-03-31 12:45:30.000000000 -0700
+++ linux-2.6.21-rc5.fds/include/linux/aio_abi.h 2007-03-31 12:46:31.000000000 -0700
@@ -84,7 +84,11 @@

/* extra parameters */
__u64 aio_reserved2; /* TODO: use this for a (struct sigevent *) */
- __u64 aio_reserved3;
+ __u32 aio_reserved3;
+ /*
+ * If different from 0, this is an eventfd to deliver AIO results to
+ */
+ __u32 aio_resfd;
}; /* 64 bytes */

#undef IFBIG
-
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