On 21 Feb 2013 16:44:41 +0100 Bodo Stroesser <bstroesser@xxxxxxxxxxxxxx>
wrote:
> On 21 Feb 2013 00:55:00 +0100 neilb@xxxxxxx wrote:
>
> > On 20 Feb 2013 14:57:07 +0100 bstroesser@xxxxxxxxxxxxxx wrote:
> >
> > > On 20 Feb 2013 04:09:00 +0100 neilb@xxxxxxx wrote:
>
> snip
>
> > > > Maybe:
> > > >
> > > > switch(cache_make_upcall(detail, h)) {
> > > > case -EINVAL:
> > > > if (rv) {
> > > > set_bit(CACHE_NEGATIVE, &h->flags);
> > > > cache_fresh_locked(h, get_seconds() + CACHE_NEW_EXPIRY);
> > > > rv = -ENOENT;
> > > > }
> > > > /* FALLTHROUGH */
> > > > case -EAGAIN:
> > > > cache_fresh_unlocked(h, detail);
> > > > }
> > >
> > > I agree, your patch is obviously better than the mine.
> > > But let me suggest one little change: I would like to substitute
> > > cache_fresh_unlocked() by clear_bit() and cache_revisit_request(),
> > > as the call to cache_dequeue() in cache_fresh_unlocked() seems to
> > > be obsolete here:
> >
> > It is exactly this sort of thinking (on my part) that got us into this mess
> > in the first place. I reasoned that the full locking/testing/whatever wasn't
> > necessary and took a short cut. It wasn't a good idea.
>
> Maybe I'm totally wrong, but AFAICS, calling cache_dequeue() here in extreme
> situations (two threads in parallel calling check_cache() while a first reader
> is going to open cache access) could again cause a race (?)
Can you explain the race you see?
>
> BTW: if there is a reader for a cache, is there any protection against many
> upcalls being queued for the same cache entry?
The CACHE_PENDING flag should provide that protection. We only queue an
upcall after "test_and_set", and always dequeue after "clear_bit".
NeilBrown
>
> Bodo
>
> >
> > Given that this is obviously difficult code to get right, we should make it
> > as easy to review as possible. Have "cache_fresh_unlocked" makes it more
> > obviously correct, and that is a good thing.
> > Maybe cache_dequeue isn't needed here, but it won't hurt so I'd much rather
> > have the clearer code.
> > In fact, I'd also like to change
> >
> > if (test_and_clear_bit(CACHE_PENDING, &ch->flags))
> > cache_dequeue(current_detail, ch);
> > cache_revisit_request(ch);
> >
> > near the end of cache_clean to call cache_fresh_unlocked().
> >
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