Date: Fri, 22 Mar 2013 12:55:51 -0700
From: Andi Kleen <ak@linux.intel.com>
To: jbaron@redhat.com
Cc: linux-kernel@vger.kernel.org
Subject: racy jump label users
Message-ID: <20130322195551.GQ19692@tassilo.jf.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
User-Agent: Mutt/1.5.21 (2010-09-15)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1132
Lines: 37

Jason,

I noticed that a lot of the jump label users are racy,
because they implement something like this

static void sched_feat_disable(int i)
{
        if (static_key_enabled(&sched_feat_keys[i]))
                static_key_slow_dec(&sched_feat_keys[i]);
}

static void sched_feat_enable(int i)
{
        if (!static_key_enabled(&sched_feat_keys[i]))
                static_key_slow_inc(&sched_feat_keys[i]);
}

with no extra locking, controlled by sysfs. If two
CPUs do this in parallel the reference can be set multiple
times, which gives very unexpected semantics for a sysfs boolean.

Most likely you need a static_key_slow_setstate()
that does the check and set inside the jump label lock.

I understand that for inside kernel use reference
counts are the right semantics, but they are not so
good for sysfs interfaces.

-Andi

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