Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions

Davide Libenzi
2009-06-22T00:00:48Z

On Sun, 21 Jun 2009, Gregory Haskins wrote:

> This looks great, Davide. I am fairly certain I can now solve the races
> and even implement Michael's DEASSIGN feature with this patch in place.
> I will actually fire it up tomorrow when I am back in the office and
> give it a spin, but I do not spy any more races via visual inspection.
>
> Kind Regards,
> -Greg
>
> PS: I was wrong with my previous statement about requiring an embeddable
> object (eventfd_notifier for me, eventfd_pollcb for you). I think you
> can technically solve this issue minimally by merely locking the POLLHUP
> and exposing the kref. However, I think that leads to an more awkward
> interface (e.g. we already have eventfd_fget() plus we add a new one
> like eventfd_refget(), which might confuse users), so I prefer what you
> did here. Just thought I would throw that out there in case you would
> prefer to change even fewer lines.

I actually ended up exposing the eventfd context anyway, since IMO is a
better option instead of holding references to the eventfd file (that
makes VFS people uneasy).
So eventfd_signal() now accepts the eventfd context instead of the file
pointer.
Inside KAIO we had it holding a reference to the underlying file*, same
thing LGUEST (Rusty Cc-ed), that has been changed to fit the new API.
And since eventfd_ctx_put() doesn't sleep, KAIO code got simpler a little.
The other change have been to make KAIO select EVENTFD, instead of making
the ugly empty stubs inside eventfd.h to accomodate (KAIO && !EVENTFD).
The eventfd_pollcb_register() remained to be accepting a file*, first
because it needs it ;) , second because it can unracely detect POLLHUP due
to the caller holding a reference to the file*.

- Davide

---
drivers/lguest/lg.h | 4 -
drivers/lguest/lguest_user.c | 4 -
fs/aio.c | 24 +-----
fs/eventfd.c | 170 ++++++++++++++++++++++++++++++++++++++++---
include/linux/aio.h | 5 -
include/linux/eventfd.h | 28 ++++---
init/Kconfig | 1
7 files changed, 192 insertions(+), 44 deletions(-)

Index: linux-2.6.mod/fs/eventfd.c
===================================================================
--- linux-2.6.mod.orig/fs/eventfd.c 2009-06-21 09:52:10.000000000 -0700
+++ linux-2.6.mod/fs/eventfd.c 2009-06-21 16:51:27.000000000 -0700
@@ -17,32 +17,38 @@
#include <linux/eventfd.h>
#include <linux/syscalls.h>
#include <linux/module.h>
+#include <linux/kref.h>

struct eventfd_ctx {
+ struct kref kref;
wait_queue_head_t wqh;
/*
* Every time that a write(2) is performed on an eventfd, the
* value of the __u64 being written is added to "count" and a
* wakeup is performed on "wqh". A read(2) will return the "count"
* value to userspace, and will reset "count" to zero. The kernel
- * size eventfd_signal() also, adds to the "count" counter and
+ * side eventfd_signal() also, adds to the "count" counter and
* issue a wakeup.
*/
__u64 count;
unsigned int flags;
};

-/*
- * Adds "n" to the eventfd counter "count". Returns "n" in case of
- * success, or a value lower then "n" in case of coutner overflow.
- * This function is supposed to be called by the kernel in paths
- * that do not allow sleeping. In this function we allow the counter
- * to reach the ULLONG_MAX value, and we signal this as overflow
- * condition by returining a POLLERR to poll(2).
+/**
+ * eventfd_signal - Adds @n to the eventfd counter. This function is
+ * supposed to be called by the kernel in paths that do not
+ * allow sleeping. In this function we allow the counter
+ * to reach the ULLONG_MAX value, and we signal this as
+ * overflow condition by returining a POLLERR to poll(2).
+ *
+ * @ctx: [in] Pointer to the eventfd context.
+ * @n: [in] Value of the counter to be added to the eventfd internal counter.
+ *
+ * Returns: In case of success, it returns @n, otherwise (in case of overflow
+ * of the eventfd 64bit internal counter) a value lower than @n.
*/
-int eventfd_signal(struct file *file, int n)
+int eventfd_signal(struct eventfd_ctx *ctx, int n)
{
- struct eventfd_ctx *ctx = file->private_data;
unsigned long flags;

if (n < 0)
@@ -59,9 +65,30 @@ int eventfd_signal(struct file *file, in
}
EXPORT_SYMBOL_GPL(eventfd_signal);

+static void eventfd_free(struct kref *kref)
+{
+ struct eventfd_ctx *ctx = container_of(kref, struct eventfd_ctx, kref);
+
+ kfree(ctx);
+}
+
+static struct eventfd_ctx *eventfd_get(struct eventfd_ctx *ctx)
+{
+ kref_get(&ctx->kref);
+ return ctx;
+}
+
+static void eventfd_put(struct eventfd_ctx *ctx)
+{
+ kref_put(&ctx->kref, eventfd_free);
+}
+
static int eventfd_release(struct inode *inode, struct file *file)
{
- kfree(file->private_data);
+ struct eventfd_ctx *ctx = file->private_data;
+
+ wake_up_poll(&ctx->wqh, POLLHUP);
+ eventfd_put(ctx);
return 0;
}

@@ -185,6 +212,14 @@ static const struct file_operations even
.write = eventfd_write,
};

+/**
+ * eventfd_fget - Acquire a reference of an eventfd file descriptor.
+ *
+ * @fd: [in] Eventfd file descriptor.
+ *
+ * Returns: A pointer to the eventfd file structure in case of success, or a
+ * proper error pointer in case of failure.
+ */
struct file *eventfd_fget(int fd)
{
struct file *file;
@@ -201,6 +236,59 @@ struct file *eventfd_fget(int fd)
}
EXPORT_SYMBOL_GPL(eventfd_fget);

+/**
+ * eventfd_ctx_get - Acquires a reference to the internal eventfd context.
+ *
+ * @file: [in] Pointer to the eventfd file.
+ *
+ * Returns: In case of success, returns a pointer to the eventfd context,
+ * otherwise a proper error code.
+ */
+struct eventfd_ctx *eventfd_ctx_get(struct file *file)
+{
+ return file->f_op == &eventfd_fops ? eventfd_get(file->private_data) :
+ ERR_PTR(-EINVAL);
+}
+EXPORT_SYMBOL_GPL(eventfd_ctx_get);
+
+/**
+ * eventfd_ctx_put - Releases a reference to the internal eventfd context
+ * (previously acquired with eventfd_ctx_get()).
+ *
+ * @ctx: [in] Pointer to eventfd context.
+ *
+ * Returns: Nothing.
+ */
+void eventfd_ctx_put(struct eventfd_ctx *ctx)
+{
+ eventfd_put(ctx);
+}
+EXPORT_SYMBOL_GPL(eventfd_ctx_put);
+
+/**
+ * eventfd_ctx_fdget - Acquires a reference to the internal eventfd context
+ * given an eventfd file descriptor.
+ *
+ * @fd: [in] Eventfd file descriptor.
+ *
+ * Returns: In case of success, it returns a pointer to the internal eventfd
+ * context, otherwise a proper error code.
+ */
+struct eventfd_ctx *eventfd_ctx_fdget(int fd)
+{
+ struct file *file;
+ struct eventfd_ctx *ctx;
+
+ file = eventfd_fget(fd);
+ if (IS_ERR(file))
+ return (struct eventfd_ctx *) file;
+ ctx = eventfd_ctx_get(file);
+ fput(file);
+
+ return ctx;
+}
+EXPORT_SYMBOL_GPL(eventfd_ctx_fdget);
+
SYSCALL_DEFINE2(eventfd2, unsigned int, count, int, flags)
{
int fd;
@@ -217,6 +305,7 @@ SYSCALL_DEFINE2(eventfd2, unsigned int,
if (!ctx)
return -ENOMEM;

+ kref_init(&ctx->kref);
init_waitqueue_head(&ctx->wqh);
ctx->count = count;
ctx->flags = flags;
@@ -237,3 +326,62 @@ SYSCALL_DEFINE1(eventfd, unsigned int, c
return sys_eventfd2(count, 0);
}

+static void eventfd_pollcb_ptqueue(struct file *file, wait_queue_head_t *wqh,
+ poll_table *pt)
+{
+ struct eventfd_pollcb *ecb;
+
+ ecb = container_of(pt, struct eventfd_pollcb, pt);
+ add_wait_queue(wqh, &ecb->wait);
+}
+
+/**
+ * eventfd_pollcb_register - Registers a wakeup callback with the eventfd
+ * file. The wakeup callback will be called from
+ * atomic context, and should handle the POLLHUP
+ * event accordingly in order to notice the last
+ * instance of the eventfd descriptor going away.
+ *
+ * @file: [in] Pointer to the eventfd file.
+ * @ecb: [out] Pointer to the eventfd callback structure.
+ * @cbf: [in] Pointer to the wakeup callback function.
+ * @events: [out] Pointer to the events that were ready at the time of the
+ * callback registration.
+ *
+ * Returns: Zero in case of success, or a proper error code.
+ */
+int eventfd_pollcb_register(struct file *file, struct eventfd_pollcb *ecb,
+ wait_queue_func_t cbf, unsigned int *events)
+{
+ struct eventfd_ctx *ctx;
+
+ ctx = file->private_data;
+ if (file->f_op != &eventfd_fops)
+ return -EINVAL;
+
+ init_waitqueue_func_entry(&ecb->wait, cbf);
+ init_poll_funcptr(&ecb->pt, eventfd_pollcb_ptqueue);
+ ecb->ctx = eventfd_get(ctx);
+
+ *events = file->f_op->poll(file, &ecb->pt);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(eventfd_pollcb_register);
+
+/**
+ * eventfd_pollcb_unregister - Unregisters a wakeup callback previously registered
+ * with eventfd_pollcb_register().
+ *
+ * @ecb: [in] Pointer to the eventfd callback structure previously registered with
+ * eventfd_pollcb_register().
+ *
+ * Returns: Nothing.
+ */
+void eventfd_pollcb_unregister(struct eventfd_pollcb *ecb)
+{
+ remove_wait_queue(&ecb->ctx->wqh, &ecb->wait);
+ eventfd_put(ecb->ctx);
+}
+EXPORT_SYMBOL_GPL(eventfd_pollcb_unregister);
+
Index: linux-2.6.mod/include/linux/eventfd.h
===================================================================
--- linux-2.6.mod.orig/include/linux/eventfd.h 2009-06-21 09:52:10.000000000 -0700
+++ linux-2.6.mod/include/linux/eventfd.h 2009-06-21 16:49:19.000000000 -0700
@@ -8,10 +8,10 @@
#ifndef _LINUX_EVENTFD_H
#define _LINUX_EVENTFD_H

-#ifdef CONFIG_EVENTFD
-
-/* For O_CLOEXEC and O_NONBLOCK */
#include <linux/fcntl.h>
+#include <linux/wait.h>
+#include <linux/poll.h>
+#include <linux/file.h>

/*
* CAREFUL: Check include/asm-generic/fcntl.h when defining
@@ -27,16 +27,22 @@
#define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)

-struct file *eventfd_fget(int fd);
-int eventfd_signal(struct file *file, int n);
-
-#else /* CONFIG_EVENTFD */
+struct eventfd_ctx;

-#define eventfd_fget(fd) ERR_PTR(-ENOSYS)
-static inline int eventfd_signal(struct file *file, int n)
-{ return 0; }
+struct eventfd_pollcb {
+ poll_table pt;
+ struct eventfd_ctx *ctx;
+ wait_queue_t wait;
+};

-#endif /* CONFIG_EVENTFD */
+struct file *eventfd_fget(int fd);
+struct eventfd_ctx *eventfd_ctx_fdget(int fd);
+struct eventfd_ctx *eventfd_ctx_get(struct file *file);
+void eventfd_ctx_put(struct eventfd_ctx *ctx);
+int eventfd_signal(struct eventfd_ctx *ctx, int n);
+int eventfd_pollcb_register(struct file *file, struct eventfd_pollcb *ecb,
+ wait_queue_func_t cbf, unsigned int *events);
+void eventfd_pollcb_unregister(struct eventfd_pollcb *ecb);

#endif /* _LINUX_EVENTFD_H */

Index: linux-2.6.mod/fs/aio.c
===================================================================
--- linux-2.6.mod.orig/fs/aio.c 2009-06-21 14:36:52.000000000 -0700
+++ linux-2.6.mod/fs/aio.c 2009-06-21 15:25:44.000000000 -0700
@@ -485,6 +485,8 @@ static inline void really_put_req(struct
{
assert_spin_locked(&ctx->ctx_lock);

+ if (req->ki_eventfd != NULL)
+ eventfd_ctx_put(req->ki_eventfd);
if (req->ki_dtor)
req->ki_dtor(req);
if (req->ki_iovec != &req->ki_inline_vec)
@@ -509,8 +511,6 @@ static void aio_fput_routine(struct work
/* Complete the fput(s) */
if (req->ki_filp != NULL)
__fput(req->ki_filp);
- if (req->ki_eventfd != NULL)
- __fput(req->ki_eventfd);

/* Link the iocb into the context's free list */
spin_lock_irq(&ctx->ctx_lock);
@@ -528,8 +528,6 @@ static void aio_fput_routine(struct work
*/
static int __aio_put_req(struct kioctx *ctx, struct kiocb *req)
{
- int schedule_putreq = 0;
-
dprintk(KERN_DEBUG "aio_put(%p): f_count=%ld\n",
req, atomic_long_read(&req->ki_filp->f_count));

@@ -549,24 +547,16 @@ static int __aio_put_req(struct kioctx *
* we would not be holding the last reference to the file*, so
* this function will be executed w/out any aio kthread wakeup.
*/
- if (unlikely(atomic_long_dec_and_test(&req->ki_filp->f_count)))
- schedule_putreq++;
- else
- req->ki_filp = NULL;
- if (req->ki_eventfd != NULL) {
- if (unlikely(atomic_long_dec_and_test(&req->ki_eventfd->f_count)))
- schedule_putreq++;
- else
- req->ki_eventfd = NULL;
- }
- if (unlikely(schedule_putreq)) {
+ if (unlikely(atomic_long_dec_and_test(&req->ki_filp->f_count))) {
get_ioctx(ctx);
spin_lock(&fput_lock);
list_add(&req->ki_list, &fput_head);
spin_unlock(&fput_lock);
queue_work(aio_wq, &fput_work);
- } else
+ } else {
+ req->ki_filp = NULL;
really_put_req(ctx, req);
+ }
return 1;
}

@@ -1622,7 +1612,7 @@ static int io_submit_one(struct kioctx *
* 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);
+ req->ki_eventfd = eventfd_ctx_fdget((int) iocb->aio_resfd);
if (IS_ERR(req->ki_eventfd)) {
ret = PTR_ERR(req->ki_eventfd);
req->ki_eventfd = NULL;
Index: linux-2.6.mod/include/linux/aio.h
===================================================================
--- linux-2.6.mod.orig/include/linux/aio.h 2009-06-21 14:36:52.000000000 -0700
+++ linux-2.6.mod/include/linux/aio.h 2009-06-21 15:32:42.000000000 -0700
@@ -12,6 +12,7 @@
#define AIO_MAXSEGS 4
#define AIO_KIOGRP_NR_ATOMIC 8

+struct eventfd_ctx;
struct kioctx;

/* Notes on cancelling a kiocb:
@@ -121,9 +122,9 @@ struct kiocb {

/*
* If the aio_resfd field of the userspace iocb is not zero,
- * this is the underlying file* to deliver event to.
+ * this is the underlying eventfd context to deliver events to.
*/
- struct file *ki_eventfd;
+ struct eventfd_ctx *ki_eventfd;
};

#define is_sync_kiocb(iocb) ((iocb)->ki_key == KIOCB_SYNC_KEY)
Index: linux-2.6.mod/init/Kconfig
===================================================================
--- linux-2.6.mod.orig/init/Kconfig 2009-06-21 15:42:52.000000000 -0700
+++ linux-2.6.mod/init/Kconfig 2009-06-21 15:43:25.000000000 -0700
@@ -925,6 +925,7 @@ config SHMEM

config AIO
bool "Enable AIO support" if EMBEDDED
+ select EVENTFD
default y
help
This option enables POSIX asynchronous I/O which may by used
Index: linux-2.6.mod/drivers/lguest/lg.h
===================================================================
--- linux-2.6.mod.orig/drivers/lguest/lg.h 2009-06-21 15:55:17.000000000 -0700
+++ linux-2.6.mod/drivers/lguest/lg.h 2009-06-21 15:56:00.000000000 -0700
@@ -80,9 +80,11 @@ struct lg_cpu {
struct lg_cpu_arch arch;
};

+struct eventfd_ctx;
+
struct lg_eventfd {
unsigned long addr;
- struct file *event;
+ struct eventfd_ctx *event;
};

struct lg_eventfd_map {
Index: linux-2.6.mod/drivers/lguest/lguest_user.c
===================================================================
--- linux-2.6.mod.orig/drivers/lguest/lguest_user.c 2009-06-21 15:55:17.000000000 -0700
+++ linux-2.6.mod/drivers/lguest/lguest_user.c 2009-06-21 15:57:11.000000000 -0700
@@ -50,7 +50,7 @@ static int add_eventfd(struct lguest *lg

/* Now append new entry. */
new->map[new->num].addr = addr;
- new->map[new->num].event = eventfd_fget(fd);
+ new->map[new->num].event = eventfd_ctx_fdget(fd);
if (IS_ERR(new->map[new->num].event)) {
kfree(new);
return PTR_ERR(new->map[new->num].event);
@@ -357,7 +357,7 @@ static int close(struct inode *inode, st

/* Release any eventfds they registered. */
for (i = 0; i < lg->eventfds->num; i++)
- fput(lg->eventfds->map[i].event);
+ eventfd_ctx_put(lg->eventfds->map[i].event);
kfree(lg->eventfds);

/* If lg->dead doesn't contain an error code it will be NULL or a