Messages in this thread |  | | Date | Sat, 5 Apr 2008 23:29:59 -0700 | From | Andrew Morton <> | Subject | Re: [RFC PATCH] sysfs: refill attribute buffer when reading from offset 0 |
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On Sat, 05 Apr 2008 11:41:22 -0700 Dan Williams <dan.j.williams@intel.com> wrote:
> 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.
This does sound a sensible change.
> 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.
Although that sounds like a rather, umm, optimistic application. I guess if everything's mlocked you might get lucky.
> 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 >
Has this been tested with pread()? That should work - doing an lseek+read is plain dopey.
Can we now remove need_read_fill? Not if we want to support open+lseek+read, I guess - this initial read might not be at offset zero.
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