linux-kernel - sequence lock in Linux

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Message-ID: <20100611194016.GA5213@Krystal>
Date:	Fri, 11 Jun 2010 15:40:16 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org
Subject: sequence lock in Linux

Hi Paul,

(CCing lkml)

Is it just me, or the following code:

static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
{
        unsigned ret;

repeat:
        ret = sl->sequence;
        smp_rmb();
        if (unlikely(ret & 1)) {
                cpu_relax();
                goto repeat;
        }

        return ret;
}

could use a ACCESS_ONCE() around the sl->sequence read ? I'm concerned about the
compiler generating code that reads the sequence number chunkwise.

The same apply to all other reads of the sequence number in seqlock.h (including
the retry code).

Thoughts ?

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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