inet: frags: Turn fqdir->dead into an int for old Alphas

From: Herbert Xu <herbert@gondor.apana.org.au>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>,
	"Paul E. McKenney" <paulmck@linux.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Fengguang Wu <fengguang.wu@intel.com>, LKP <lkp@01.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Netdev <netdev@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Andrea Parri <andrea.parri@amarulasolutions.com>,
	Luc Maranget <luc.maranget@inria.fr>,
	Jade Alglave <j.alglave@ucl.ac.uk>
Subject: inet: frags: Turn fqdir->dead into an int for old Alphas
Date: Fri, 7 Jun 2019 22:09:49 +0800	[thread overview]
Message-ID: <20190607140949.tzwyprrhmqdx33iu@gondor.apana.org.au> (raw)
In-Reply-To: <CAHk-=wgGnCw==uY8radrB+Tg_CEmzOtwzyjfMkuh7JmqFh+jzQ@mail.gmail.com>

On Tue, Jun 04, 2019 at 09:04:55AM -0700, Linus Torvalds wrote:
>
> In fact, the alpha port was always subtly buggy exactly because of the
> "byte write turns into a read-and-masked-write", even if I don't think
> anybody ever noticed (we did fix cases where people _did_ notice,
> though, and we might still have some cases where we use 'int' for
> booleans because of alpha issues.).

This is in fact a real bug in the code in question that no amount
of READ_ONCE/WRITE_ONCE would have caught.  The field fqdir->dead is
declared as boolean so writing to it is not atomic (on old Alphas).

I don't think it currently matters because padding would ensure
that it is in fact 64 bits long.  However, should someone add another
char/bool/bitfield in this struct in future it could become an issue.

So let's fix it.

---8<--
The field fqdir->dead is meant to be written (and read) atomically.
As old Alpha CPUs can't write a single byte atomically, we need at
least an int for it to work.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/include/net/inet_frag.h b/include/net/inet_frag.h
index e91b79ad4e4a..8c458fba74ad 100644
--- a/include/net/inet_frag.h
+++ b/include/net/inet_frag.h
@@ -14,7 +14,9 @@ struct fqdir {
 	int			max_dist;
 	struct inet_frags	*f;
 	struct net		*net;
-	bool			dead;
+
+	/* We can't use boolean because this needs atomic writes. */
+	int			dead;
 
 	struct rhashtable       rhashtable ____cacheline_aligned_in_smp;
 
diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 35e9784fab4e..05aa7c145817 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -193,7 +193,7 @@ void fqdir_exit(struct fqdir *fqdir)
 {
 	fqdir->high_thresh = 0; /* prevent creation of new frags */
 
-	fqdir->dead = true;
+	fqdir->dead = 1;
 
 	/* call_rcu is supposed to provide memory barrier semantics,
 	 * separating the setting of fqdir->dead with the destruction
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
