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SubjectRe: [PATCH 0/2] eventfd: new EFD_STATE flag
On Wed, 26 Aug 2009, Davide Libenzi wrote:

> > I see no kernel equivalent to read(), but that's easily done.
>
> Adding an in-kernel read based on "ctx", that is no problem at all.

Something like the untested below.
I had thought you said the eventfd readers where in userspace, but I might
have misunderstood you.



- Davide



---
fs/eventfd.c | 51 +++++++++++++++++++++++++++++++++---------------
include/linux/eventfd.h | 7 ++++++
2 files changed, 43 insertions(+), 15 deletions(-)
Index: linux-2.6.mod/fs/eventfd.c
===================================================================
--- linux-2.6.mod.orig/fs/eventfd.c 2009-08-26 15:58:03.000000000 -0700
+++ linux-2.6.mod/fs/eventfd.c 2009-08-26 16:20:03.000000000 -0700
@@ -130,26 +130,33 @@ static unsigned int eventfd_poll(struct
return events;
}

-static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
- loff_t *ppos)
+/**
+ * eventfd_ctx_read - Reads the eventfd counter or wait if it is zero.
+ * @ctx: [in] Pointer to eventfd context.
+ * @no_wait: [in] Different from zero if the operation should not block.
+ * @cnt: [out] Pointer to the 64bit conter value.
+ *
+ * Returns zero if successful, or the following error codes:
+ *
+ * -EAGAIN : The operation would have blocked but @no_wait was nonzero.
+ * -ERESTARTSYS : A signal interrupted the wait operation.
+ */
+ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt)
{
- struct eventfd_ctx *ctx = file->private_data;
ssize_t res;
- __u64 ucnt = 0;
DECLARE_WAITQUEUE(wait, current);

- if (count < sizeof(ucnt))
- return -EINVAL;
spin_lock_irq(&ctx->wqh.lock);
+ *cnt = 0;
res = -EAGAIN;
if (ctx->count > 0)
- res = sizeof(ucnt);
- else if (!(file->f_flags & O_NONBLOCK)) {
+ res = 0;
+ else if (!no_wait) {
__add_wait_queue(&ctx->wqh, &wait);
- for (res = 0;;) {
+ for (;;) {
set_current_state(TASK_INTERRUPTIBLE);
if (ctx->count > 0) {
- res = sizeof(ucnt);
+ res = 0;
break;
}
if (signal_pending(current)) {
@@ -163,18 +170,32 @@ static ssize_t eventfd_read(struct file
__remove_wait_queue(&ctx->wqh, &wait);
__set_current_state(TASK_RUNNING);
}
- if (likely(res > 0)) {
- ucnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
- ctx->count -= ucnt;
+ if (likely(res == 0)) {
+ *cnt = (ctx->flags & EFD_SEMAPHORE) ? 1 : ctx->count;
+ ctx->count -= *cnt;
if (waitqueue_active(&ctx->wqh))
wake_up_locked_poll(&ctx->wqh, POLLOUT);
}
spin_unlock_irq(&ctx->wqh.lock);
- if (res > 0 && put_user(ucnt, (__u64 __user *) buf))
- return -EFAULT;

return res;
}
+EXPORT_SYMBOL_GPL(eventfd_ctx_read);
+
+static ssize_t eventfd_read(struct file *file, char __user *buf, size_t count,
+ loff_t *ppos)
+{
+ struct eventfd_ctx *ctx = file->private_data;
+ ssize_t res;
+ __u64 cnt;
+
+ if (count < sizeof(cnt))
+ return -EINVAL;
+ if ((res = eventfd_ctx_read(ctx, file->f_flags & O_NONBLOCK, &cnt)) < 0)
+ return res;
+
+ return put_user(cnt, (__u64 __user *) buf) ? -EFAULT: sizeof(cnt);
+}

static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t count,
loff_t *ppos)
Index: linux-2.6.mod/include/linux/eventfd.h
===================================================================
--- linux-2.6.mod.orig/include/linux/eventfd.h 2009-08-26 15:58:03.000000000 -0700
+++ linux-2.6.mod/include/linux/eventfd.h 2009-08-26 16:17:01.000000000 -0700
@@ -33,6 +33,7 @@ struct file *eventfd_fget(int fd);
struct eventfd_ctx *eventfd_ctx_fdget(int fd);
struct eventfd_ctx *eventfd_ctx_fileget(struct file *file);
int eventfd_signal(struct eventfd_ctx *ctx, int n);
+ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait, __u64 *cnt);

#else /* CONFIG_EVENTFD */

@@ -55,6 +56,12 @@ static inline void eventfd_ctx_put(struc

}

+static inline ssize_t eventfd_ctx_read(struct eventfd_ctx *ctx, int no_wait,
+ __u64 *cnt)
+{
+ return -ENOSYS;
+}
+
#endif

#endif /* _LINUX_EVENTFD_H */

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