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: <20140602170327.GV22231@linux.vnet.ibm.com>
Date:	Mon, 2 Jun 2014 10:03:27 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jason Low <jason.low2@...com>
Cc:	Mikulas Patocka <mpatocka@...hat.com>,
	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,
	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, Jun 02, 2014 at 09:50:10AM -0700, Jason Low wrote:
> 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?

And one could use cmpxchg() or atomic_add_return(..., 0) to read a value
out.  Probably at the cost of some performance impact, though.

							Thanx, Paul

> -----
> 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/