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Date: Fri, 1 Jan 2010 10:56:15 -0800 (PST)
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Borislav Petkov <petkovbb@...glemail.com>,
David Airlie <airlied@...ux.ie>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Greg KH <greg@...ah.com>, Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] sysfs: Cache the last sysfs_dirent to improve readdir
scalability
On Fri, 1 Jan 2010, Eric W. Biederman wrote:
>
> When sysfs_readdir stops short we now cache the next sysfs_dirent to
> return to user space in filp->private_data. There is no impact on the
> rest of sysfs by doing this and in the common case it allows us to
> pick up exactly where we left off with no seeking.
>
> Additionally I drop and regrab the sysfs_mutex around filldir to avoid
> a page fault arbitrarily increasing the hold time on the sysfs_mutex.
Ok, looks mostly sane, but a few things look odd.
>
> - if (filp->f_pos == 0) {
> + if (!pos && filp->f_pos == 0) {
> ino = parent_sd->s_ino;
> if (filldir(dirent, ".", 1, filp->f_pos, ino, DT_DIR) == 0)
> filp->f_pos++;
> }
> - if (filp->f_pos == 1) {
> + if (!pos && filp->f_pos == 1) {
> if (parent_sd->s_parent)
> ino = parent_sd->s_parent->s_ino;
> else
> @@ -847,29 +879,35 @@ static int sysfs_readdir(struct file * filp, void * dirent, filldir_t filldir)
> if (filldir(dirent, "..", 2, filp->f_pos, ino, DT_DIR) == 0)
> filp->f_pos++;
> }
> - if ((filp->f_pos > 1) && (filp->f_pos < INT_MAX)) {
> - mutex_lock(&sysfs_mutex);
> -
> - /* Skip the dentries we have already reported */
> - pos = parent_sd->s_dir.children;
> - while (pos && (filp->f_pos > pos->s_ino))
> - pos = pos->s_sibling;
> + /* EOF? */
> + if (!pos && filp->f_pos > 2)
> + return 0;
These are all incorrect in the presense of 'lseek'. You can't do that
if (!pos && "test filp->f_pos")
thing, because you get all the wrong results for both the case of an lseek
before doing any readdir (which is undefined behavior, so I guess that's
technically ok) _and_ for the 'lseek back to zero _after_ doing a readdir'
case (which is _not_ undefined behavior!
It looks like it might be easy to fix by making a sysfs_llseek() function
that does something like
.. sysfs_llseek(..)
{
mutex_lock(&sysfs_mutex);
sysfs_release();
filp->private_data = NULL;
mutex_unlock(&sysfs_mutex);
return generic_file_llseek(..);
}
or similar. Except themn you'll need to change the EOF condition testing
and turn it into a re-validation event. Or maybe do the re-validation in
sysfs_llseek() itself, rather than just dropping the cached data.
Hmm? I haven't thought it through very deeply, so maybe I'm missing
something.
Linus
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