linux-kernel - [Question] bluetooth/{bnep,cmtp,hidp}: memory barriers

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Message-ID: <20180730031030.GA9430@andrea>
Date:   Mon, 30 Jul 2018 05:10:30 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Marcel Holtmann <marcel@...tmann.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     linux-bluetooth@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Jeffy Chen <jeffy.chen@...k-chips.com>,
        Brian Norris <briannorris@...omium.org>,
        AL Yu-Chen Cho <acho@...e.com>
Subject: [Question] bluetooth/{bnep,cmtp,hidp}: memory barriers

Hi,

I'm currently puzzled by the the three calls to smp_mb__before_atomic()
in bnep_session(), cmtp_session() and hidp_session_run() respectively:

On the one hand, these barriers provide no guarantee on the subsequent
atomic_read(s->terminate) (as the comments preceding the barriers seem
to suggest), because atomic_read() is not a read-modify-write.

On the other hand, I'm currently unable to say *why such an "mb" would
be required: not being too familiar with this code, I figured I should
ask before sending a patch.  ;-)

  Andrea