On Tue, Jun 12 2007, Evgeniy Polyakov wrote: > > > > > and putting cloned skb into private field instead of > > > > > original on in spd_fill_page() ends up without kernel hung. > > > > > > > > Why? Seems pointless to allocate a clone just to hold on to the skb, a > > > > reference should be equally good. I would not be opposed to doing it > > > > this way, I just don't see what a clone buys us as compared to just > > > > holding that reference to the skb. > > > > > > Receiving code does not expect shared skbs - too many fields are changed > > > with assumptions that it is a private copy. > > > > Actually the main problem is that tcp_read_sock() unconditionally frees > > the skb, so it wouldn't help if we grabbed a reference to it. I've yet > > to receive an explanation of why it does so, seem awkward and violates > > the whole principle of reference counted objects. Davem?? > > > > So for now, skb_splice_bits() clones the incoming skb to avoid that. I'd > > hope we can get rid of that by fixing tcp_read_sock(), though. > > It does that because it knows, that skb is not allowed to be shared > there. Similar things are being done in udp for example - code changes > internal mebers of skb, since it knows skb is not shared. > > For example generic_make_request() is not allowed to change, say, > bio->bi_sector or bi_destructor, since it does not own a block request, > not matter what bi_cnt is. From another side, ->bi_destructor() can do > whatever it wants with bio without any check for its reference counter.
But generic_make_request() DOES change ->bi_sector, that's how partition remapping works :-). The destructor can of course do whatever it wants, by definition the bio is not referenced at that point (or it would not have been called). So while I think your analogy is quite poor, I do now follow the code (even if I think it's ugly). There's quite a big difference between changing parts of the elements of a structure to just grabbing a reference to it. If the skb cannot be referenced, skb_get() should return NULL. But that aside, I see the issue. I'll just stick to the clone, it works fine as-is (well almost, there's a leak there, but functionally it's ok!). -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
