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

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Date:	Tue, 15 Jun 2010 06:38:46 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Paul Mackerras <paulus@...ba.org>, tytso@....edu,
	Salman <sqazi@...gle.com>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	tytso@...gle.com, torvalds@...ux-foundation.org, walken@...gle.com,
	Chen Liqin <liqin.chen@...plusct.com>,
	Lennox Wu <lennox.wu@...il.com>
Subject: Re: [PATCH] Fix a race in pid generation that causes pids to be
 reused immediately.

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@...ba.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);
 }


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