Re: [PATCH] Fix a race in pid generation that causes pids to be reused immediately.

Eric Dumazet
2010-06-15T04:38:54Z

Le lundi 14 juin 2010 à 21:21 -0700, Andrew Morton a écrit :
> On Tue, 15 Jun 2010 13:26:08 +1000 Paul Mackerras <paulus@samba.org> wrote:
>
> > On Mon, Jun 14, 2010 at 06:55:56PM -0700, Andrew Morton wrote:
> >
> > > > kernel/sched_clock.c: if (cmpxchg64(&scd->clock, old_clock, clock) != old_cloc
> > >
> > > I guess that'll flush out any stragglers.
> >
> > And break most non-x86 32-bit architectures, including 32-bit powerpc.
>
> If CONFIG_SMP=y, yes. On UP there's a generic implementation
> (include/asm-generic/cmpxchg-local.h, include/asm-generic/cmpxchg.h)
>
> > Fortunately that code is only used if CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
> > is set, and it looks like only x86 and ia64 set it.
> >
>
> If that happens then the best fix is for those architectures to get
> themselves a cmpxchg64(). Unless for some reason it's simply
> unimplementable? Worst case I guess one could use a global spinlock.
> Second-worst-case: hashed spinlocks.

Hmm, this reminds me a patch I had somewhere, not yet sent to David :)

1) cmpxchg() was not available for all arches, maybe
atomic_long_cmpxchg() is ?

2) I am not sure atomic_long_t quarantee that all 32 or 64 bits of the
underlying long container are available.

Thanks !

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index a291edb..335ca89 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1268,17 +1268,18 @@ skip_hashing:

void rt_bind_peer(struct rtable *rt, int create)
{
- static DEFINE_SPINLOCK(rt_peer_lock);
struct inet_peer *peer;

peer = inet_getpeer(rt->rt_dst, create);

- spin_lock_bh(&rt_peer_lock);
- if (rt->peer == NULL) {
- rt->peer = peer;
+#if defined(__HAVE_ARCH_CMPXCHG)
+ if (!cmpxchg(&rt->peer, NULL, peer))
peer = NULL;
- }
- spin_unlock_bh(&rt_peer_lock);
+#else
+ BUILD_BUG_ON(sizeof(rt->peer) != sizeof(atomic_long_t));
+ if (!atomic_long_cmpxchg((atomic_long_t *)&rt->peer, 0, (long)peer))
+ peer = NULL;
+#endif
if (peer)
inet_putpeer(peer);
}