linux-kernel - Re: [PATCH RFC tip/core/rcu 01/14] rcu/nocb: Atomic ->len field in rcu

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: <20190804145051.GG2349@hirez.programming.kicks-ass.net>
Date:   Sun, 4 Aug 2019 16:50:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH RFC tip/core/rcu 01/14] rcu/nocb: Atomic ->len field in
 rcu_segcblist structure

On Fri, Aug 02, 2019 at 08:14:48AM -0700, Paul E. McKenney wrote:
> +/*
> + * Exchange the numeric length of the specified rcu_segcblist structure
> + * with the specified value.  This can cause the ->len field to disagree
> + * with the actual number of callbacks on the structure.  This exchange is
> + * fully ordered with respect to the callers accesses both before and after.
> + */
> +long rcu_segcblist_xchg_len(struct rcu_segcblist *rsclp, long v)
> +{
> +#ifdef CONFIG_RCU_NOCB_CPU
> +	return atomic_long_xchg(&rsclp->len, v);
> +#else
> +	long ret = rsclp->len;
> +
> +	smp_mb(); /* Up to the caller! */
> +	WRITE_ONCE(rsclp->len, v);
> +	smp_mb(); /* Up to the caller! */
> +	return ret;
> +#endif
> +}

That one's weird; for matching semantics the load needs to be between
the memory barriers.