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Date:   Thu, 15 Jul 2021 09:57:22 -0700
From:   Bhaumik Bhatt <bbhatt@codeaurora.org>
To:     Jia-Ju Bai <baijiaju1990@gmail.com>
Cc:     manivannan.sadhasivam@linaro.org, hemantk@codeaurora.org,
        linux-arm-msm@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] bus: mhi: possible ABBA deadlock in mhi_pm_m0_transition()
 and mhi_send_cmd()
Organization: Qualcomm Innovation Center, Inc.
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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
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