linux-kernel - Confused by smp_read_barrier_depends() in rxrpc_rotate_tx

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Message-ID: <20080530124554.GA14312@linux.vnet.ibm.com>
Date:	Fri, 30 May 2008 05:45:54 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	dhowells@...hat.com
Cc:	linux-kernel@...r.kernel.org
Subject: Confused by smp_read_barrier_depends() in rxrpc_rotate_tx_window()

Hello, Dave!

I confess to being confused by the smp_read_barrier_depends() in
rxrpc_rotate_tx_window().  It looks like it is ordering the prior
fetch of tail from call->acks_tail with the subsequent use of
tail as an index into the call->acks_window[] array, but then the
code does an assignment to call->acks_tail a few lines later.

If we hold a lock protecting call->acks_tail, why do we need the
smp_read_barrier_depends()?  If we don't hold such a lock, why
is the assignment to call->acks_tail safe?

Help?

						Thanx, Paul
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