Re: [PATCH -mm 4/5][AIO] - AIO completion signal notification

From: Sébastien Dugué
Date: Thu Nov 30 2006 - 10:51:58 EST


AIO completion signal notification

The current 2.6 kernel does not support notification of user space via
an RT signal upon an asynchronous IO completion. The POSIX specification
states that when an AIO request completes, a signal can be delivered to
the application as notification.

This patch adds a struct sigevent *aio_sigeventp to the iocb.
The relevant fields (pid, signal number and value) are stored in the kiocb
for use when the request completes.

That sigevent structure is filled by the application as part of the AIO
request preparation. Upon request completion, the kernel notifies the
application using those sigevent parameters. If SIGEV_NONE has been specified,
then the old behaviour is retained and the application must rely on polling
the completion queue using io_getevents().


A struct sigevent *aio_sigeventp field is added to struct iocb in
include/linux/aio_abi.h

A struct aio_notify containing the sigevent parameters is defined in aio.h:

struct aio_notify {
struct task_struct *target;
__u16 signo;
__u16 notify;
sigval_t value;
};

A struct aio_notify ki_notify is added to struct kiocb in include/linux/aio.h

In io_submit_one(), if the application provided a sigevent then
setup_sigevent() is called which does the following:

- check access to the user sigevent and make a local copy

- if the requested notification is SIGEV_NONE, then nothing to do

- fill in the kiocb->ki_notify fields (notify, signo, value)

- check sigevent consistency, get the signal target task and
save it in kiocb->ki_notify.target

- preallocate a sigqueue for this event using sigqueue_alloc()

Upon request completion, in aio_complete(), if notification is needed for
this request (iocb->ki_notify.notify != SIGEV_NONE), then aio_send_signal()
is called to signal the target task as follows:

- fill in the siginfo struct to be sent to the task

- if notify is SIGEV_THREAD_ID then send signal to specific task
using send_sigqueue()

- else send signal to task group using send_5group_sigqueue()



Notes concerning sigqueue preallocation:

To ensure reliable delivery of completion notification, the sigqueue is
preallocated in the submission path so that there is no chance it can fail
in the completion path.

Unlike the posix-timers case (currently the single other user of sigqueue
preallocation), where the sigqueue is allocated for the lifetime of the timer
and freed at timer destruction time, the aio case is a bit more tricky due to
the async nature of the whole thing.

In the aio case, the sigqueue exists for the lifetime of the request, therefore
it must be freed only once the signal for the request completion has been
delivered. This involves changing __sigqueue_free() to free the sigqueue when the
signal is collected if si_code is SI_ASYNCIO even if it was preallocated as well
as explicitly calling sigqueue_free() in submission and completion error paths.


fs/aio.c | 115 ++++++++++++++++++++++++++++++++++++++++++++++--
fs/compat.c | 18 +++++++
include/linux/aio.h | 12 +++++
include/linux/aio_abi.h | 3 -
kernel/signal.c | 2
5 files changed, 144 insertions(+), 6 deletions(-)

Signed-off-by: Sébastien Dugué <sebastien.dugue@xxxxxxxx>
Signed-off-by: Laurent Vivier <laurent.vivier@xxxxxxxx>

Index: linux-2.6.19-rc6-mm2/fs/aio.c
===================================================================
--- linux-2.6.19-rc6-mm2.orig/fs/aio.c 2006-11-30 10:54:16.000000000 +0100
+++ linux-2.6.19-rc6-mm2/fs/aio.c 2006-11-30 15:18:52.000000000 +0100
@@ -416,6 +416,7 @@ static struct kiocb fastcall *__aio_get_
req->ki_dtor = NULL;
req->private = NULL;
req->ki_iovec = NULL;
+ req->ki_notify.sigq = NULL;
INIT_LIST_HEAD(&req->ki_run_list);

/* Check if the completion queue has enough free space to
@@ -463,6 +464,12 @@ static inline void really_put_req(struct
req->ki_dtor(req);
if (req->ki_iovec != &req->ki_inline_vec)
kfree(req->ki_iovec);
+
+ /* Release task ref */
+ if (req->ki_notify.notify == SIGEV_THREAD_ID ||
+ req->ki_notify.notify == SIGEV_SIGNAL)
+ put_task_struct(req->ki_notify.target);
+
kmem_cache_free(kiocb_cachep, req);
ctx->reqs_active--;

@@ -929,6 +936,79 @@ void fastcall kick_iocb(struct kiocb *io
}
EXPORT_SYMBOL(kick_iocb);

+static int aio_send_signal(struct aio_notify *notify)
+{
+ struct sigqueue *sigq = notify->sigq;
+ struct siginfo *info = &sigq->info;
+ int ret;
+
+ memset(info, 0, sizeof(struct siginfo));
+
+ info->si_signo = notify->signo;
+ info->si_errno = 0;
+ info->si_code = SI_ASYNCIO;
+ info->si_pid = 0;
+ info->si_uid = 0;
+ info->si_value = notify->value;
+
+ if (notify->notify & SIGEV_THREAD_ID)
+ ret = send_sigqueue(notify->signo, sigq, notify->target);
+ else
+ ret = send_group_sigqueue(notify->signo, sigq, notify->target);
+
+ return ret;
+}
+
+static long aio_setup_sigevent(struct aio_notify *notify,
+ struct sigevent __user *user_event)
+{
+ sigevent_t event;
+ struct task_struct *target;
+
+ if (copy_from_user(&event, user_event, sizeof (event)))
+ return -EFAULT;
+
+ if (event.sigev_notify == SIGEV_NONE)
+ return 0;
+
+ notify->notify = event.sigev_notify;
+ notify->signo = event.sigev_signo;
+ notify->value = event.sigev_value;
+
+ read_lock(&tasklist_lock);
+ target = good_sigevent(&event);
+
+ if (unlikely(!target || (target->flags & PF_EXITING)))
+ goto out_unlock;
+
+ /*
+ * At this point, we know that notify is either SIGEV_SIGNAL or
+ * SIGEV_THREAD_ID and the target task is valid. So get a reference
+ * on the task, it will be dropped in really_put_req() when
+ * we're done with the request.
+ */
+ get_task_struct(target);
+ notify->target = target;
+ read_unlock(&tasklist_lock);
+
+ /*
+ * NOTE: we cannot free the sigqueue in the completion path as
+ * the signal may not have been delivered to the target task.
+ * Therefore it has to be freed in __sigqueue_free() when the
+ * signal is collected if si_code is SI_ASYNCIO.
+ */
+ notify->sigq = sigqueue_alloc();
+
+ if (unlikely(!notify->sigq))
+ return -EAGAIN;
+
+ return 0;
+
+out_unlock:
+ read_unlock(&tasklist_lock);
+ return -EINVAL;
+}
+
/* aio_complete
* Called when the io request on the given iocb is complete.
* Returns true if this is the last user of the request. The
@@ -976,8 +1056,11 @@ int fastcall aio_complete(struct kiocb *
* cancelled requests don't get events, userland was given one
* when the event got cancelled.
*/
- if (kiocbIsCancelled(iocb))
+ if (kiocbIsCancelled(iocb)) {
+ if (iocb->ki_notify.sigq)
+ sigqueue_free(iocb->ki_notify.sigq);
goto put_rq;
+ }

ring = kmap_atomic(info->ring_pages[0], KM_IRQ1);

@@ -1008,6 +1091,14 @@ int fastcall aio_complete(struct kiocb *

pr_debug("added to ring %p at [%lu]\n", iocb, tail);

+ if (iocb->ki_notify.notify != SIGEV_NONE) {
+ ret = aio_send_signal(&iocb->ki_notify);
+
+ /* If signal generation failed, release the sigqueue */
+ if (ret)
+ sigqueue_free(iocb->ki_notify.sigq);
+ }
+
pr_debug("%ld retries: %zd of %zd\n", iocb->ki_retried,
iocb->ki_nbytes - iocb->ki_left, iocb->ki_nbytes);
put_rq:
@@ -1549,8 +1640,7 @@ int fastcall io_submit_one(struct kioctx
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_reserved3)) {
pr_debug("EINVAL: io_submit: reserve field set\n");
return -EINVAL;
}
@@ -1559,6 +1649,7 @@ int fastcall io_submit_one(struct kioctx
if (unlikely(
(iocb->aio_buf != (unsigned long)iocb->aio_buf) ||
(iocb->aio_nbytes != (size_t)iocb->aio_nbytes) ||
+ (iocb->aio_sigeventp != (unsigned long)iocb->aio_sigeventp) ||
((ssize_t)iocb->aio_nbytes < 0)
)) {
pr_debug("EINVAL: io_submit: overflow check\n");
@@ -1593,10 +1684,21 @@ int fastcall io_submit_one(struct kioctx
INIT_LIST_HEAD(&req->ki_wait.task_list);
req->ki_retried = 0;

+ /* handle setting up the sigevent for POSIX AIO signals */
+ req->ki_notify.notify = SIGEV_NONE;
+
+ if (iocb->aio_sigeventp) {
+ ret = aio_setup_sigevent(&req->ki_notify,
+ (struct sigevent __user *)(unsigned long)
+ iocb->aio_sigeventp);
+ if (ret)
+ goto out_put_req;
+ }
+
ret = aio_setup_iocb(req);

if (ret)
- goto out_put_req;
+ goto out_sigqfree;

spin_lock_irq(&ctx->ctx_lock);
aio_run_iocb(req);
@@ -1609,6 +1711,11 @@ int fastcall io_submit_one(struct kioctx
aio_put_req(req); /* drop extra ref to req */
return 0;

+out_sigqfree:
+ /* Undo the sigqueue alloc if someting went bad */
+ if (req->ki_notify.sigq)
+ sigqueue_free(req->ki_notify.sigq);
+
out_put_req:
aio_put_req(req); /* drop extra ref to req */
aio_put_req(req); /* drop i/o ref to req */
Index: linux-2.6.19-rc6-mm2/include/linux/aio_abi.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/linux/aio_abi.h 2006-11-30 10:54:16.000000000 +0100
+++ linux-2.6.19-rc6-mm2/include/linux/aio_abi.h 2006-11-30 15:06:05.000000000 +0100
@@ -82,8 +82,9 @@ struct iocb {
__u64 aio_nbytes;
__s64 aio_offset;

+ __u64 aio_sigeventp; /* pointer to struct sigevent */
+
/* extra parameters */
- __u64 aio_reserved2; /* TODO: use this for a (struct sigevent *) */
__u64 aio_reserved3;
}; /* 64 bytes */

Index: linux-2.6.19-rc6-mm2/include/linux/aio.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/linux/aio.h 2006-11-30 13:18:15.000000000 +0100
+++ linux-2.6.19-rc6-mm2/include/linux/aio.h 2006-11-30 15:06:05.000000000 +0100
@@ -7,6 +7,7 @@
#include <linux/uio.h>

#include <asm/atomic.h>
+#include <asm/siginfo.h>

#define AIO_MAXSEGS 4
#define AIO_KIOGRP_NR_ATOMIC 8
@@ -49,6 +50,14 @@ struct kioctx;
#define kiocbIsKicked(iocb) test_bit(KIF_KICKED, &(iocb)->ki_flags)
#define kiocbIsCancelled(iocb) test_bit(KIF_CANCELLED, &(iocb)->ki_flags)

+struct aio_notify {
+ struct task_struct *target;
+ __u16 signo;
+ __u16 notify;
+ sigval_t value;
+ struct sigqueue *sigq;
+};
+
/* is there a better place to document function pointer methods? */
/**
* ki_retry - iocb forward progress callback
@@ -118,6 +127,9 @@ struct kiocb {

struct list_head ki_list; /* the aio core uses this
* for cancellation */
+
+ /* to notify a process on I/O event */
+ struct aio_notify ki_notify;
};

#define is_sync_kiocb(iocb) ((iocb)->ki_key == KIOCB_SYNC_KEY)
Index: linux-2.6.19-rc6-mm2/kernel/signal.c
===================================================================
--- linux-2.6.19-rc6-mm2.orig/kernel/signal.c 2006-11-30 13:33:10.000000000 +0100
+++ linux-2.6.19-rc6-mm2/kernel/signal.c 2006-11-30 15:06:05.000000000 +0100
@@ -297,7 +297,7 @@ static struct sigqueue *__sigqueue_alloc

static void __sigqueue_free(struct sigqueue *q)
{
- if (q->flags & SIGQUEUE_PREALLOC)
+ if (q->flags & SIGQUEUE_PREALLOC && q->info.si_code != SI_ASYNCIO)
return;
atomic_dec(&q->user->sigpending);
free_uid(q->user);
Index: linux-2.6.19-rc6-mm2/fs/compat.c
===================================================================
--- linux-2.6.19-rc6-mm2.orig/fs/compat.c 2006-11-30 13:12:32.000000000 +0100
+++ linux-2.6.19-rc6-mm2/fs/compat.c 2006-11-30 15:06:05.000000000 +0100
@@ -663,6 +663,7 @@ compat_sys_io_submit(aio_context_t ctx_i
compat_uptr_t uptr;
struct iocb __user *user_iocb;
struct iocb tmp;
+ struct compat_sigevent __user *uevent;

if (unlikely(get_user(uptr, iocb + i))) {
ret = -EFAULT;
@@ -676,6 +677,23 @@ compat_sys_io_submit(aio_context_t ctx_i
break;
}

+ uevent = (struct compat_sigevent __user *)tmp.aio_sigeventp;
+
+ if (uevent) {
+ struct sigevent __user *event = NULL;
+ struct sigevent kevent;
+
+ event = compat_alloc_user_space(sizeof(*event));
+
+ if (get_compat_sigevent(&kevent, uevent) ||
+ copy_to_user(event, &kevent, sizeof(*event))) {
+ ret = -EFAULT;
+ break;
+ }
+
+ tmp.aio_sigeventp = (__u64)event;
+ }
+
ret = io_submit_one(ctx, user_iocb, &tmp);
if (ret)
break;
-
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