On Tue, May 16, 2023 at 10:22 PM -07, John Fastabend wrote: > We noticed some rare sk_buffs were stepping past the queue when system was > under memory pressure. The general theory is to skip enqueueing > sk_buffs when its not necessary which is the normal case with a system > that is properly provisioned for the task, no memory pressure and enough > cpu assigned. > > But, if we can't allocate memory due to an ENOMEM error when enqueueing > the sk_buff into the sockmap receive queue we push it onto a delayed > workqueue to retry later. When a new sk_buff is received we then check > if that queue is empty. However, there is a problem with simply checking > the queue length. When a sk_buff is being processed from the ingress queue > but not yet on the sockmap msg receive queue its possible to also recv > a sk_buff through normal path. It will check the ingress queue which is > zero and then skip ahead of the pkt being processed. > > Previously we used sock lock from both contexts which made the problem > harder to hit, but not impossible. > > To fix instead of popping the skb from the queue entirely we peek the > skb from the queue and do the copy there. This ensures checks to the > queue length are non-zero while skb is being processed. Then finally > when the entire skb has been copied to user space queue or another > socket we pop it off the queue. This way the queue length check allows > bypassing the queue only after the list has been completely processed. > > To reproduce issue we run NGINX compliance test with sockmap running and > observe some flakes in our testing that we attributed to this issue. > > Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()") > Tested-by: William Findlay <will@xxxxxxxxxxxxx> > Suggested-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx> > Signed-off-by: John Fastabend <john.fastabend@xxxxxxxxx> > --- Reviewed-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>