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    SubjectRe: [BUG] bus: mhi: possible ABBA deadlock in mhi_pm_m0_transition() and mhi_send_cmd()
    On 2021-07-15 02:45 AM, Jia-Ju Bai wrote:
    > Hello,
    >
    > I find there is a possible ABBA deadlock in the MHI driver in Linux
    > 5.10:
    >
    > In mhi_pm_m0_transition():
    > 262:     read_lock_bh(&mhi_cntrl->pm_lock);
    > 281:     spin_lock_irq(&mhi_cmd->lock);
    >
    > In mhi_send_cmd():
    > 1181:   spin_lock_bh(&mhi_cmd->lock);
    > 1207:   read_lock_bh(&mhi_cntrl->pm_lock);
    >
    > When mhi_pm_m0_transition() and mhi_send_cmd() are concurrently
    > executed, the deadlock can occur.
    >
    > I check the code and find a possible case of such concurrent execution:
    >
    > #CPU1:
    > mhi_poll (mhi_event->process_event(...))
    >   mhi_process_ctrl_ev_ring
    >     mhi_pm_m0_transition
    >
    > #CPU2:
    > mhi_prepare_for_transfer
    >   mhi_prepare_channel
    >     mhi_send_cmd
    >
    > Note that mhi_poll() and mhi_prepare_for_transfer() are both exported
    > by EXPORT_SYMBOL.
    > Thus, I guess these two functions could be concurrently called by a MHI
    > driver.
    >
    > I am not quite sure whether this possible deadlock is real and how to
    > fix it if it is real.
    > Any feedback would be appreciated, thanks :)
    >
    >
    > Best wishes,
    > Jia-Ju Bai

    Few pointers from your example:

    1. mhi_poll() is currently not used by any client upstream yet.
    2. Polling is not to be used for single event ring (shared control +
    data) cases
    since it is meant to be for client drivers with dedicated data packets
    only.
    3. mhi_send_cmd() will always be called after an mhi_pm_m0_transition()
    has
    completed by design since we wait for the device to be held in M0 prior
    to it.

    Would like to know what Mani and Hemant have to say. I don't think we
    can run in
    to the scenario from your example so we should be safe.

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

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