Re: [BUG] bus: mhi: possible deadlock in mhi_pm_disable_transition() and mhi_async_power_up()

[Manivannan Sadhasivam]

Hi,

Thanks for the report!

On Sat, Jan 29, 2022 at 10:56:30AM +0800, Jia-Ju Bai wrote:
> Hello,
> 
> My static analysis tool reports a possible deadlock in the mhi driver in
> Linux 5.10:
> 
> mhi_async_power_up()
>   mutex_lock(&mhi_cntrl->pm_mutex); --> Line 933 (Lock A)
>   wait_event_timeout(mhi_cntrl->state_event, ...) --> Line 985 (Wait X)
>   mutex_unlock(&mhi_cntrl->pm_mutex); --> Line 1040 (Unlock A)
> 
> mhi_pm_disable_transition()
>   mutex_lock(&mhi_cntrl->pm_mutex); --> Line 463 (Lock A)
>   wake_up_all(&mhi_cntrl->state_event); --> Line 474 (Wake X)
>   mutex_unlock(&mhi_cntrl->pm_mutex); --> Line 524 (Unlock A)
>   wake_up_all(&mhi_cntrl->state_event); --> Line 526 (Wake X)
> 
> When mhi_async_power_up() is executed, "Wait X" is performed by holding
> "Lock A". If mhi_pm_disable_transition() is concurrently executed at this
> time, "Wake X" cannot be performed to wake up "Wait X" in
> mhi_async_power_up(), because "Lock A" is already hold by
> mhi_async_power_up(), causing a possible deadlock.
> I find that "Wait X" is performed with a timeout, to relieve the possible
> deadlock; but I think this timeout can cause inefficient execution.
> 

As per the MHI design, we can be sure that mhi_pm_disable_transition() won't be
called until wait_event_timeout() completes in mhi_async_power_up(). So this
deadlock is not possible in practical.

Thanks,
Mani

> I am not quite sure whether this possible problem is real and how to fix it
> if it is real.
> Any feedback would be appreciated, thanks :)
> 
> 
> Best wishes,
> Jia-Ju Bai
> 
> 
>