Re: [PATCH 0/2] eventfd: new EFD_STATE flag

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Jan 10, 2010 at 11:04:23AM -0800, Davide Libenzi wrote:
> On Sun, 10 Jan 2010, Michael S. Tsirkin wrote:
> 
> > On Sun, Jan 10, 2010 at 09:27:34AM -0800, Davide Libenzi wrote:
> > > On Sun, 10 Jan 2010, Michael S. Tsirkin wrote:
> > > 
> > > > Not sure what you mean by irqfd locking.  IMO we must take waitqueue
> > > > lock, otherwise we can not prevent wait queue callback from being
> > > > invoked, right? Note that if we take our own lock under wait queue
> > > > callback, we won't be able to use this lock around remove_wait_queue.
> > > > Also note how fs/evetfd.c uses __remove_wait_queue after taking wait
> > > > queue lock - we'd like to do the same with our wait queue.
> > > > 
> > > > Could you clarify pls?
> > > 
> > > Wait a second. Looking at the current code in Linus tree, when you 
> > > deassign an irqfd, you are actually destroying it, so the idea of saving 
> > > the counter on deassign is not going to work.
> > > 
> > > 
> > > 
> > > - Davide
> > 
> > Yes, the only context passed between deassign and assign is the eventfd
> > itself. So I think the following code for deassign would work provided
> > eventfd_ctx_read_locked works with wait queue lock taken:
> > 
> >         spin_lock_irqsave(&irqfd->wqh.lock, flags);
> >         eventfd_ctx_read_locked(ctx->eventfd, 1, &ucnt);
> >         __remove_wait_queue(irqfd->wqh, &irqfd->wait);
> >         spin_lock_irqrestore(&irqfd->wqh.lock, flags);
> 
> As I said, you cannot do that from KVM, since write-witers will nedd to be 
> woke up.
> Use the eventfd_ctx_remove_wait_queue() below (NOTE: compiled, not tested).
> 
> 
> 
> - Davide
> 

Yes, I think this will work. Will test and report. Thanks!

> ---
>  fs/eventfd.c            |   88 +++++++++++++++++++++++++++++++++++++++---------
>  include/linux/eventfd.h |   15 ++++++++
>  2 files changed, 88 insertions(+), 15 deletions(-)
> 
> Index: linux-2.6.mod/fs/eventfd.c
> ===================================================================
> --- linux-2.6.mod.orig/fs/eventfd.c	2010-01-06 18:33:27.000000000 -0800
> +++ linux-2.6.mod/fs/eventfd.c	2010-01-10 11:02:00.000000000 -0800
> @@ -135,26 +135,71 @@ 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)
> +static void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt)
> +{
> +	*cnt = (ctx->flags & EFD_SEMAPHORE) ? 1: ctx->count;
> +	ctx->count -= *cnt;
> +}
> +
> +/**
> + * eventfd_ctx_remove_wait_queue - Read the current counter and removes wait queue.
> + * @ctx: [in] Pointer to eventfd context.
> + * @wait: [in] Wait queue to be removed.
> + * @cnt: [out] Pointer to the 64bit conter value.
> + *
> + * Returns zero if successful, or the following error codes:
> + *
> + * -EAGAIN      : The operation would have blocked.
> + *
> + * This is used to atomically remove a wait queue entry from the eventfd wait
> + * queue head, and read/reset the counter value.
> + */
> +int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
> +				  __u64 *cnt)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&ctx->wqh.lock, flags);
> +	eventfd_ctx_do_read(ctx, cnt);
> +	__remove_wait_queue(&ctx->wqh, wait);
> +	if (*cnt != 0 && waitqueue_active(&ctx->wqh))
> +		wake_up_locked_poll(&ctx->wqh, POLLOUT);
> +	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
> +
> +	return *cnt != 0 ? 0: -EAGAIN;
> +}
> +EXPORT_SYMBOL_GPL(eventfd_ctx_remove_wait_queue);
> +
> +/**
> + * 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.
> + *
> + * If @no_wait is zero, the function might sleep until the eventfd internal
> + * counter becomes greater than zero.
> + */
> +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)) {
> @@ -168,18 +213,31 @@ 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)) {
> +		eventfd_ctx_do_read(ctx, 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	2010-01-06 18:33:27.000000000 -0800
> +++ linux-2.6.mod/include/linux/eventfd.h	2010-01-10 10:57:06.000000000 -0800
> @@ -34,6 +34,9 @@ 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);
> +int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx, wait_queue_t *wait,
> +				  __u64 *cnt);
>  
>  #else /* CONFIG_EVENTFD */
>  
> @@ -61,6 +64,18 @@ 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;
> +}
> +
> +static inline int eventfd_ctx_remove_wait_queue(struct eventfd_ctx *ctx,
> +						wait_queue_t *wait, __u64 *cnt)
> +{
> +	return -ENOSYS;
> +}
> +
>  #endif
>  
>  #endif /* _LINUX_EVENTFD_H */