Subject: Re: [PATCH 13/36] fs: introduce new ->get_poll_head and ->poll_mask methods

From: "Darrick J. Wong" <[email-masked]>
Date: Mon, 19 Mar 2018 20:29:38 -0700

On Mon, Mar 05, 2018 at 01:27:20PM -0800, Christoph Hellwig wrote:
> ->get_poll_head returns the waitqueue that the poll operation is going
> to sleep on.  Note that this means we can only use a single waitqueue
> for the poll, unlike some current drivers that use two waitqueues for
> different events.  But now that we have keyed wakeups and heavily use
> those for poll there aren't that many good reason left to keep the
> multiple waitqueues, and if there are any ->poll is still around, the
> driver just won't support aio poll.
> 
> Signed-off-by: Christoph Hellwig <[email-masked]>

I've been wondering, how does a regular filesystem connect with this?
Also, does anything implement get_poll_head?  It looks to me like an aio
poll provider has to provide both...

--D

> ---
>  Documentation/filesystems/Locking |  7 ++++++-
>  Documentation/filesystems/vfs.txt | 13 +++++++++++++
>  fs/select.c                       | 28 ++++++++++++++++++++++++++++
>  include/linux/fs.h                |  2 ++
>  include/linux/poll.h              | 27 +++++++++++++++++++++++----
>  5 files changed, 72 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
> index 220bba28f72b..6d227f9d7bd9 100644
> --- a/Documentation/filesystems/Locking
> +++ b/Documentation/filesystems/Locking
> @@ -440,6 +440,8 @@ prototypes:
>  	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
>  	int (*iterate) (struct file *, struct dir_context *);
>  	__poll_t (*poll) (struct file *, struct poll_table_struct *);
> +	struct wait_queue_head * (*get_poll_head)(struct file *, __poll_t);
> +	__poll_t (*poll_mask) (struct file *, __poll_t);
>  	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
>  	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
>  	int (*mmap) (struct file *, struct vm_area_struct *);
> @@ -470,7 +472,7 @@ prototypes:
>  };
>  
>  locking rules:
> -	All may block.
> +	All except for ->poll_mask may block.
>  
>  ->llseek() locking has moved from llseek to the individual llseek
>  implementations.  If your fs is not using generic_file_llseek, you
> @@ -498,6 +500,9 @@ in sys_read() and friends.
>  the lease within the individual filesystem to record the result of the
>  operation
>  
> +->poll_mask can be called with or without the waitqueue lock for the waitqueue
> +returned from ->get_poll_head.
> +
>  --------------------------- dquot_operations -------------------------------
>  prototypes:
>  	int (*write_dquot) (struct dquot *);
> diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
> index f608180ad59d..50ee13563271 100644
> --- a/Documentation/filesystems/vfs.txt
> +++ b/Documentation/filesystems/vfs.txt
> @@ -857,6 +857,8 @@ struct file_operations {
>  	ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
>  	int (*iterate) (struct file *, struct dir_context *);
>  	__poll_t (*poll) (struct file *, struct poll_table_struct *);
> +	struct wait_queue_head * (*get_poll_head)(struct file *, __poll_t);
> +	__poll_t (*poll_mask) (struct file *, __poll_t);
>  	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
>  	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
>  	int (*mmap) (struct file *, struct vm_area_struct *);
> @@ -901,6 +903,17 @@ otherwise noted.
>  	activity on this file and (optionally) go to sleep until there
>  	is activity. Called by the select(2) and poll(2) system calls
>  
> +  get_poll_head: Returns the struct wait_queue_head that poll, select,
> +  epoll or aio poll should wait on in case this instance only has single
> +  waitqueue.  Can return NULL to indicate polling is not supported,
> +  or a POLL* value using the POLL_TO_PTR helper in case a grave error
> +  occured and ->poll_mask shall not be called.
> +
> +  poll_mask: return the mask of POLL* values describing the file descriptor
> +  state.  Called either before going to sleep on the waitqueue returned by
> +  get_poll_head, or after it has been woken.  If ->get_poll_head and
> +  ->poll_mask are implemented ->poll does not need to be implement.
> +
>    unlocked_ioctl: called by the ioctl(2) system call.
>  
>    compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
> diff --git a/fs/select.c b/fs/select.c
> index ba91103707ea..cc270d7f6192 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -34,6 +34,34 @@
>  
>  #include <linux/uaccess.h>
>  
> +__poll_t vfs_poll(struct file *file, struct poll_table_struct *pt)
> +{
> +	unsigned int events = poll_requested_events(pt);
> +	struct wait_queue_head *head;
> +
> +	if (unlikely(!file_can_poll(file)))
> +		return DEFAULT_POLLMASK;
> +
> +	if (file->f_op->poll)
> +		return file->f_op->poll(file, pt);
> +
> +	/*
> +	 * Only get the poll head and do the first mask check if we are actually
> +	 * going to sleep on this file:
> +	 */
> +	if (pt && pt->_qproc) {
> +		head = vfs_get_poll_head(file, events);
> +		if (!head)
> +			return DEFAULT_POLLMASK;
> +		if (IS_ERR(head))
> +			return PTR_TO_POLL(head);
> +
> +		pt->_qproc(file, head, pt);
> +	}
> +
> +	return file->f_op->poll_mask(file, events);
> +}
> +EXPORT_SYMBOL_GPL(vfs_poll);
>  
>  /*
>   * Estimate expected accuracy in ns from a timeval.
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 79c413985305..6ea2c0843bb1 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1708,6 +1708,8 @@ struct file_operations {
>  	int (*iterate) (struct file *, struct dir_context *);
>  	int (*iterate_shared) (struct file *, struct dir_context *);
>  	__poll_t (*poll) (struct file *, struct poll_table_struct *);
> +	struct wait_queue_head * (*get_poll_head)(struct file *, __poll_t);
> +	__poll_t (*poll_mask) (struct file *, __poll_t);
>  	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
>  	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
>  	int (*mmap) (struct file *, struct vm_area_struct *);
> diff --git a/include/linux/poll.h b/include/linux/poll.h
> index 7e0fdcf905d2..42e8e8665fb0 100644
> --- a/include/linux/poll.h
> +++ b/include/linux/poll.h
> @@ -74,18 +74,37 @@ static inline void init_poll_funcptr(poll_table *pt, poll_queue_proc qproc)
>  	pt->_key   = ~(__poll_t)0; /* all events enabled */
>  }
>  
> +/*
> + * ->get_poll_head can return a __poll_t in the PTR_ERR, use these macros
> + * to return the value and recover it.  It takes care of the negation as
> + * well as off the annotations.
> + */
> +#define POLL_TO_PTR(mask)	(ERR_PTR(-(__force int)(mask)))
> +#define PTR_TO_POLL(ptr)	((__force __poll_t)-PTR_ERR((ptr)))
> +
>  static inline bool file_can_poll(struct file *file)
>  {
> -	return file->f_op->poll;
> +	return file->f_op->poll ||
> +		(file->f_op->get_poll_head && file->f_op->poll_mask);
>  }
>  
> -static inline __poll_t vfs_poll(struct file *file, struct poll_table_struct *pt)
> +static inline struct wait_queue_head *vfs_get_poll_head(struct file *file,
> +		__poll_t events)
>  {
> -	if (unlikely(!file->f_op->poll))
> +	if (unlikely(!file->f_op->get_poll_head || !file->f_op->poll_mask))
> +		return NULL;
> +	return file->f_op->get_poll_head(file, events);
> +}
> +
> +static inline __poll_t vfs_poll_mask(struct file *file, __poll_t events)
> +{
> +	if (unlikely(!file->f_op->poll_mask))
>  		return DEFAULT_POLLMASK;
> -	return file->f_op->poll(file, pt);
> +	return file->f_op->poll_mask(file, events) & events;
>  }
>  
> +__poll_t vfs_poll(struct file *file, struct poll_table_struct *pt);
> +
>  struct poll_table_entry {
>  	struct file *filp;
>  	__poll_t key;
> -- 
> 2.14.2
> 

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