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Date:   Fri, 16 Jul 2021 12:15:12 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
To:     Bhaumik Bhatt <bbhatt@codeaurora.org>
Cc:     Jia-Ju Bai <baijiaju1990@gmail.com>, 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()
Message-ID: <20210716064512.GE3323@workstation>
References: <0a213c92-32d5-efc8-079b-dd20d5ecfe20@gmail.com>
 <2fbeeb8dc647dc88e69ec6624c2078d1@codeaurora.org>
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On Thu, Jul 15, 2021 at 09:57:22AM -0700, Bhaumik Bhatt wrote:
> 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.

Then this shouldn't be added in first place... :/

> 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.
> 

But client can be unloaded during M0 event!

Anyway, I don't think the deadlock scenario is valid because of the usage
of "read_lock_bh()". So if "mhi_send_cmd()" has acquired
"spin_lock_bh(&mhi_cmd->lock)", it can always acquire
"read_lock_bh(&mhi_cntrl->pm_lock)" as multiple readers can acquire the
read lock.

Deadlock would only occur if one of the functions take write lock.

Thanks for auditing.

Regards,
Mani

> 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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