On Monday March 6, [email protected] wrote:
> > Somewhere in among the comments (thanks), I realised that I was only
> > closing half the race. I had tried to make sure there were no stray
> > references to any dentries, but there is still the inode which is
> > being iput which can cause problem.
> >
> > The following patch takes a totally different approach, is based on an
> > idea from Jan Kara, and is much less intrusive.
> >
> > We:
> > - keep track of "who" is calling prune_dcache, and when a filesystem
> > is being unmounted (s_root == NULL) we only allow the unmount thread
> > to prune dentries.
> > - keep track of how many dentries are in the process of having
> > dentry_iput called on them for pruning
> > - don't allow umount to proceed until that count hits zero
> > - bias the count this way and that to make sure we get a wake_up at
> > the right time
> > - reuse 's_wait_unfrozen' to wait on the iput to complete.
> >
> > Again, I'm very keen on feedback. This race is very hard to trigger,
> > so code review is the only real way to evaluate that patch.
> >
> > Thanks,
> > NeilBrown
> >
>
> The code changes look big, have you looked at
> http://marc.theaimsgroup.com/?l=linux-kernel&m=113817279225962&w=2
No I haven't. I like it.
- Holding the semaphore shouldn't be a problem.
- calling down_read_trylock ought to be fast
- I *think* the unwanted calls to prune_dcache are always under
PF_MEMALLOC - they certainly seem to be.
And it is a nice small change.
Have you had any other feedback on this?
>
> Some top of the head feedback below. Will try and do a detailed review later.
>
> > + /* avoid further wakeups */
> > + sb->s_pending_iputs = 65000;
>
> This looks a bit ugly, what is 65000?
Just the first big number that came to by head... probably not needed.
NeilBrown
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