Messages in this thread Patch in this message |  | | From | Dan Williams <> | Subject | [RFC PATCH] sysfs: refill attribute buffer when reading from offset 0 | Date | Sat, 05 Apr 2008 11:41:22 -0700 |
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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: NeilBrown <neilb@suse.de> Cc: Tejun Heo <htejun@gmail.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> ---
fs/sysfs/file.c | 5 ++--- 1 files changed, 2 insertions(+), 3 deletions(-)
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c index baa663e..0a26ba8 100644 --- a/fs/sysfs/file.c +++ b/fs/sysfs/file.c @@ -128,7 +128,7 @@ 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; @@ -409,8 +409,7 @@ 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
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