On Mon, Nov 21, 2016 at 2:27 PM, Reshetova, Elena <elena.reshet...@intel.com> wrote: >> On Mon, Nov 21, 2016 at 04:49:15PM +0100, Peter Zijlstra wrote: >> > > Speaking of non-fitting patterns. This one is quite common in >> > > networking code for refcounters: >> > > >> > > if (atomic_cmpxchg(&cur->refcnt, 1, 0) == 1) {} This is from >> > > net/netfilter/nfnetlink_acct.c, but there are similar ones in other >> > > places. >> > >> > Cute, but weird it doesn't actually decrement if not 1. >> >> Hurgh.. creative refcounting that. The question is how much of that do >> we want to support? It really must not decrement there. > > And one more creative usage: > > http://lxr.free-electrons.com/source/net/ipv4/udp.c#L1940 > > if (!sk || !atomic_inc_not_zero_hint(&sk->sk_refcnt, 2)) > return; > > I didn't even guess anyone is using atomic_inc_not_zero_hint... > But network code keeps surprising me today :) > So, yes, I guess the question is what to do with these cases really?
Many of the calls to non-supported functions can be decomposed into calls to supported functions. The ones that may prove interesting are ones like atomic_cmpxchg(), in which some sort of external locking is going to be required to achieve the same atomicity guarantees provided by cmpxchg, like so: mutex_lock(lock); cnt = refcount_read(ref); if (cnt == val1) { refcount_set(ref, val2); } mutex_unlock(lock); return cnt;