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Message-ID: <1351717412.2706.28.camel@bwh-desktop.uk.solarflarecom.com> Date: Wed, 31 Oct 2012 21:03:32 +0000 From: Ben Hutchings <bhutchings@...arflare.com> To: "Michael S. Tsirkin" <mst@...hat.com> CC: <netdev@...r.kernel.org>, Herbert Xu <herbert@...dor.hengli.com.au> Subject: Re: skb_linearize On Wed, 2012-10-31 at 21:15 +0200, Michael S. Tsirkin wrote: > On Sun, Sep 16, 2012 at 04:07:12PM +0100, Ben Hutchings wrote: > > On Sun, 2012-09-16 at 12:17 +0300, Michael S. Tsirkin wrote: > > > I notice that dev_hard_start_xmit might invoke > > > __skb_linearize e.g. if device does not support NETIF_F_SG. > > > > > > This in turn onvokes __pskb_pull_tail, and > > > documentation of __pskb_pull_tail says: > > > &sk_buff MUST have reference count of 1. > > > > > > I am guessing 'reference count' means users in this context, right? > > > IIUC this is because it modifies skb in a way that > > > isn't safe if anyone else is looking at the skb. > > > > > > > > > However, I don't see what guarantees that reference > > > count is 1 when dev_hard_start_xmit invokes > > > linearize. In particular it calls dev_queue_xmit_nit > > > which could queue packets on a network tap. > > > > > > Could someone help me understand please? > > > > Reference count here means references to struct sk_buff itself. The > > header area and data fragments are allowed to be shared. > > > > dev_queue_xmit_nit() clones the skb for each tap, so the reference count > > on the original skb remains 1. > > > > Ben. > > Interesting. But don't skb clones share the fragment list? Yes. > Maybe I misunderstand? If they do it looks like the following race > would be possible: > > - skb is cloned and queued e.g. at socket receive queue. > dataref becomes 2. > - On CPU 1, skb_copy_datagram_iovec is called on clone 1, is reads nr_frags and sees > value > 1. > - On CPU 2, __skb_linearize is now called on clone 2, it modified the > skb so nr_frags is now 0, and does put_page for all frags > 1. > - On CPU 1, skb_copy_datagram_iovec will now use the previously read > nr_frags > 1 and access a fragment page that was already freed. > > What did I miss? __skb_linearize() calls __pskb_pull_tail(), which starts with: if (eat > 0 || skb_cloned(skb)) { if (pskb_expand_head(skb, 0, eat > 0 ? eat + 128 : 0, GFP_ATOMIC)) return NULL; } pskb_expand_head() will then create a new unshared head area for the skb being linearised, and will add a reference to each fragment page (skb_frag_ref()). __pskb_pull_tail() unreferences the pages later, as you say, but this all cancels out. (pskb_expand_head() will also cancel zero-copy (skb_orphan_frags() -> skb_copy_ubufs()), which seems like it should be done only after the head area has been unshared. But skb_clone() will also do that, so I don't know how the zero-copy flag would still be set on a cloned skb.) Ben. -- Ben Hutchings, Staff Engineer, Solarflare Not speaking for my employer; that's the marketing department's job. They asked us to note that Solarflare product names are trademarked. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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