lkml.org 
[lkml]   [2001]   [Jun]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] devfs v181 available
Alexander Viro writes:
> On Mon, 18 Jun 2001, Richard Gooch wrote:
> > Alexander Viro writes:
> > > On Mon, 18 Jun 2001, Richard Gooch wrote:
> > > > - Widened locking in <devfs_readlink> and <devfs_follow_link>
> > >
> > > No, you hadn't. Both vfs_readlink() and vfs_follow_link() are blocking
> > > functions, so BKL is worthless there.
> >
> > Huh? The BKL will protect against other operations which might cause
> > the devfs entry to be unregistered, where those other operations also
> > grab the BKL. So, it's an improvement.
>
> BKL is released as soon as you block. You _do_ regain it when you get
> the next timeslice, but in the meanwhile anything could happen.
>
> > Sure, some operations may cause unregistration without grabbing the
>
> Irrelevant. BKL provides an exclusion only on non-blocking areas.

Yeah, I know all that.

> > BKL, but that's orthogonal (and requires more extensive changes). If
> > this "widening" is of no use, then what use are the existing grabs of
> > the BKL in those functions? You're the one who added them in the first
> > place.
>
> _Moved_ them there from the callers of these functions. And AFAICS
> you do need BKL for get_devfs_entry_...(); otherwise relocation of
> the table will be able to screw you inside of that function. Now, it
> will merrily screw you anyway in a lot of places, but that's another
> story.

OK, so it was another global change.

> BTW, free advice: when you are checking some condition treat the
> result as something that can expire. And don't rely on it past the
> moment when it might expired. E.g. in case of de->registered result
> expires as soon as you do unlock_kernel() _or_ do anything that
> might block.

Yeah, I know. Fear not: I'll be adding proper locks (spinlocks and
semaphores) to the code, continuing the work I started this weekend. I
don't like the BKL anyway (too coarse grained), and hope to see it
removed entirely one day.

Question: assuming data fed to vfs_follow_link() is "safe", does it
need the BKL? I can see that vfs_readlink() obviously doesn't need
it. From reading Documentation/filesystems/Locking I suspect it
doesn't need the BKL, but the way I read it says "follow_link() method
does not *have* the BKL already". But that doesn't explicitely say
whether vfs_follow_link() needs it.

Regards,

Richard....
Permanent: rgooch@atnf.csiro.au
Current: rgooch@ras.ucalgary.ca
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 12:55    [W:0.073 / U:1.476 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site