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    SubjectRe: [PATCH 05/28] fs: introduce new ->get_poll_head and ->poll_mask methods
    On Wed, Mar 21, 2018 at 08:40:09AM +0100, 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 <hch@lst.de>

    Looks ok,
    Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

    --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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