linux-kernel - Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1401727810.7440.34.camel@j-VirtualBox>
Date:	Mon, 02 Jun 2014 09:50:10 -0700
From:	Jason Low <jason.low2@...com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>, jejb@...isc-linux.org,
	deller@....de, John David Anglin <dave.anglin@...l.net>,
	linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com, chegu_vinod@...com, Waiman.Long@...com,
	tglx@...utronix.de, riel@...hat.com, akpm@...ux-foundation.org,
	davidlohr@...com, hpa@...or.com, andi@...stfloor.org, aswin@...com,
	scott.norton@...com
Subject: Re: [PATCH v2] introduce atomic_pointer to fix a race condition in
 cancelable mcs spinlocks

On Mon, 2014-06-02 at 12:00 -0400, Mikulas Patocka wrote:
> If you write to some variable with ACCESS_ONCE and use cmpxchg or xchg at
> the same time, you break it. ACCESS_ONCE doesn't take the hashed spinlock,
> so, in this case, cmpxchg or xchg isn't really atomic at all.

So if the problem is using ACCESS_ONCE writes with cmpxchg and xchg at
the same time, would the below change address this problem?

-----
diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
index 838dc9e..8396721 100644
--- a/kernel/locking/mcs_spinlock.c
+++ b/kernel/locking/mcs_spinlock.c
@@ -71,7 +71,7 @@ bool osq_lock(struct optimistic_spin_queue **lock)
 	if (likely(prev == NULL))
 		return true;
 
-	ACCESS_ONCE(prev->next) = node;
+	xchg(&prev->next, node);
 
 	/*
 	 * Normally @prev is untouchable after the above store; because at that
@@ -144,7 +144,7 @@ unqueue:
 	 */
 
 	ACCESS_ONCE(next->prev) = prev;
-	ACCESS_ONCE(prev->next) = next;
+	xchg(&prev->next, next);
 
 	return false;
 }


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/