svn commit: r262309 - head/sys/kern

[Mateusz Guzik]

On Thu, Feb 27, 2014 at 09:06:31AM -0500, John Baldwin wrote:
> On Wednesday, February 26, 2014 4:22:00 pm Mateusz Guzik wrote:
> > On Wed, Feb 26, 2014 at 03:29:31PM -0500, John Baldwin wrote:
> > > On Wednesday, February 26, 2014 2:23:48 pm Mateusz Guzik wrote:
> > > > Other thing is that with that change in place we can get rid of
> > > > XLOCK/XUNLOCK around fdfree in fdescfree.
> > > 
> > > I would rather remove the fd_refcnt checks, or do them differently (not
> > > in the loop).  Right now a reader might be confused to think that
> > > fd_refcnt can change within the loop when it cannot and then worry about
> > > unhandled races that don't exist (i.e. if fd_refcnt can change within
> > > the loop, what prevents the individual file objects from being freed out
> > > from under the loop?)
> > > 
> > 
> > But it can change.
> > 
> > kern_proc_filedesc_out calls export_fd_to_sb which drops the lock for
> > each fp and sysctl_kern_proc_ofiledesc drops the lock when dealing with
> > vnodes.
> > 
> > As far as I can say all this is safe - either data is refed (vref on a
> > vnode) or the lock is still held while the data is being read, so by the
> > time fp can be freed it is no longer used.
> 
> Ugh, ok.  Then the change is fine as-is, but I think we have to leave
> the locking in place around fdfree() still as a result.
> 

I don't see why.

refcnt cannot drop as long as something holds fdp lock.

1) So let's say kern_proc_filedesc_out grabs the lock, refcnt is 0.
No files are inspected and the loop is terminated.

2) So let's say refcnt is 1 and fp is being read. Lock is released only
when the function is done with fp. Then fdescfree drops refcnt to 0
and proceeds to free fps. And we are back to 1).

IOW I don't think locking around fdfree is of any use right now,
although I don't feel strongly about removing it.

-- 
Mateusz Guzik <mjguzik gmail.com>