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    Subject[PATCH 30/36] sysfs: refill attribute buffer when reading from offset 0
    Date
    From: Dan Williams <dan.j.williams@intel.com>

    Requiring userspace to close and re-open sysfs attributes has been the
    policy since before 2.6.12. It allows userspace to get a consistent
    snapshot of kernel state and consume it with incremental reads and seeks.

    Now, if the file position is zero the kernel assumes userspace wants to see
    the new value. The application for this change is to allow a userspace
    RAID metadata handler to check the state of an array without causing any
    memory allocations. Thus not causing writeback to a raid array that might
    be blocked waiting for userspace to take action.

    Cc: Neil Brown <neilb@suse.de>
    Acked-by: Tejun Heo <htejun@gmail.com>
    Signed-off-by: Dan Williams <dan.j.williams@intel.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
    ---
    Documentation/filesystems/sysfs.txt | 9 +++++++--
    fs/sysfs/file.c | 5 ++---
    2 files changed, 9 insertions(+), 5 deletions(-)

    diff --git a/Documentation/filesystems/sysfs.txt b/Documentation/filesystems/sysfs.txt
    index 4598ef7..7f27b8f 100644
    --- a/Documentation/filesystems/sysfs.txt
    +++ b/Documentation/filesystems/sysfs.txt
    @@ -176,8 +176,10 @@ implementations:
    Recall that an attribute should only be exporting one value, or an
    array of similar values, so this shouldn't be that expensive.

    - This allows userspace to do partial reads and seeks arbitrarily over
    - the entire file at will.
    + This allows userspace to do partial reads and forward seeks
    + arbitrarily over the entire file at will. If userspace seeks back to
    + zero or does a pread(2) with an offset of '0' the show() method will
    + be called again, rearmed, to fill the buffer.

    - On write(2), sysfs expects the entire buffer to be passed during the
    first write. Sysfs then passes the entire buffer to the store()
    @@ -192,6 +194,9 @@ implementations:

    Other notes:

    +- Writing causes the show() method to be rearmed regardless of current
    + file position.
    +
    - The buffer will always be PAGE_SIZE bytes in length. On i386, this
    is 4096.

    diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
    index a859c32..ade9a7e 100644
    --- a/fs/sysfs/file.c
    +++ b/fs/sysfs/file.c
    @@ -129,7 +129,7 @@ sysfs_read_file(struct file *file, char __user *buf, size_t count, loff_t *ppos)
    ssize_t retval = 0;

    mutex_lock(&buffer->mutex);
    - if (buffer->needs_read_fill) {
    + if (buffer->needs_read_fill || *ppos == 0) {
    retval = fill_read_buffer(file->f_path.dentry,buffer);
    if (retval)
    goto out;
    @@ -410,8 +410,7 @@ static int sysfs_release(struct inode *inode, struct file *filp)
    * return POLLERR|POLLPRI, and select will return the fd whether
    * it is waiting for read, write, or exceptions.
    * Once poll/select indicates that the value has changed, you
    - * need to close and re-open the file, as simply seeking and reading
    - * again will not get new data, or reset the state of 'poll'.
    + * need to close and re-open the file, or seek to 0 and read again.
    * Reminder: this only works for attributes which actively support
    * it, and it is not possible to test an attribute from userspace
    * to see if it supports poll (Neither 'poll' nor 'select' return
    --
    1.5.4.5


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