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    SubjectRe: [PATCH 5.10 002/299] bus: mhi: core: Clear configuration from channel context during reset
    On 2021-05-10 11:17 PM, Manivannan Sadhasivam wrote:
    > Hi Pavel,
    >
    > On Mon, May 10, 2021 at 10:56:50PM +0200, Pavel Machek wrote:
    >> Hi!
    >>
    >> > From: Bhaumik Bhatt <bbhatt@codeaurora.org>
    >> >
    >> > commit 47705c08465931923e2f2b506986ca0bdf80380d upstream.
    >> >
    >> > When clearing up the channel context after client drivers are
    >> > done using channels, the configuration is currently not being
    >> > reset entirely. Ensure this is done to appropriately handle
    >> > issues where clients unaware of the context state end up calling
    >> > functions which expect a context.
    >>
    >> > +++ b/drivers/bus/mhi/core/init.c
    >> > @@ -544,6 +544,7 @@ void mhi_deinit_chan_ctxt(struct mhi_con
    >> > + u32 tmp;
    >> > @@ -554,7 +555,19 @@ void mhi_deinit_chan_ctxt(struct mhi_con
    >> ...
    >> > + tmp = chan_ctxt->chcfg;
    >> > + tmp &= ~CHAN_CTX_CHSTATE_MASK;
    >> > + tmp |= (MHI_CH_STATE_DISABLED << CHAN_CTX_CHSTATE_SHIFT);
    >> > + chan_ctxt->chcfg = tmp;
    >> > +
    >> > + /* Update to all cores */
    >> > + smp_wmb();
    >> > }
    >>
    >> This is really interesting code; author was careful to make sure chcfg
    >> is updated atomically, but C compiler is free to undo that. Plus, this
    >> will make all kinds of checkers angry.
    >>
    >> Does the file need to use READ_ONCE and WRITE_ONCE?
    >>
    >
    > Thanks for looking into this.
    >
    > I agree that the order could be mangled between chcfg read & write and
    > using READ_ONCE & WRITE_ONCE seems to be a good option.
    >
    > Bhaumik, can you please submit a patch and tag stable?
    >
    > Thanks,
    > Mani
    >
    >> Best regards,
    >> Pavel
    >> --
    >> DENX Software Engineering GmbH, Managing Director: Wolfgang Denk
    >> HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

    Hi Pavel/Mani,

    Hemant and I went over this patch and we noticed this particular
    function is
    already being called with the channel mutex lock held. This would take
    care of
    the atomicity and we also probably don't need the smp_wmb() barrier as
    the mutex
    unlock will implicitly take care of it.

    To the point of compiler re-ordering, we would need some help to
    understand the
    purpose of READ_ONCE()/WRITE_ONCE() for these dependent statements:

    > + tmp = chan_ctxt->chcfg;
    > + tmp &= ~CHAN_CTX_CHSTATE_MASK;
    > + tmp |= (MHI_CH_STATE_DISABLED << CHAN_CTX_CHSTATE_SHIFT);
    > + chan_ctxt->chcfg = tmp;

    Since RMW operation means that the chan_ctxt->chcfg is copied to a local
    variable (tmp) and the _same_ is being written back to chan_ctxt->chcfg,
    can
    compiler reorder these dependent statements and cause a different
    result?

    Thanks,
    Bhaumik
    ---
    The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
    Forum,
    a Linux Foundation Collaborative Project

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