On 2022/2/2 1:15, Daniel Thompson wrote:
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.
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 :)
Interesting find but I think it would be better to run your tool
against more recent kernels to confirm any problem reports. In this
case the code you mention looks like it was removed in v5.17-rc1
(and should eventually make its way to the stable kernels too).
Hi Daniel,
Thanks for your reply :)
I check Linux v5.17-rc1 code, and find that this possible deadlock does
not exist, due to the changes in commit d651ce8e917f.
However, my tool also reports several other possible deadlocks, which
are caused by waiting with holding mhi_cntrl->pm_mutex.
There are two examples in Linux v5.17-rc1:
#BUG 1
mhi_pm_sys_error_transition()
mutex_lock(&mhi_cntrl->pm_mutex); --> Line 572 (Lock A)
wait_event_timeout(mhi_cntrl->state_event, ...); --> Line 600 (Wait X)
mutex_unlock(&mhi_cntrl->pm_mutex); --> Line 630 (Unlock A)
mhi_pm_disable_transition()
mutex_lock(&mhi_cntrl->pm_mutex); --> Line 464 (Lock A)
mutex_unlock(&mhi_cntrl->pm_mutex); --> Line 496 (Unlock A)
wake_up_all(&mhi_cntrl->state_event); --> Line 498 (Wake X)
#BUG 2
mhi_pm_sys_error_transition()
mutex_lock(&mhi_cntrl->pm_mutex); --> Line 572 (Lock A)
wait_event_timeout(mhi_cntrl->state_event, ...); --> Line 600 (Wait X)
mutex_unlock(&mhi_cntrl->pm_mutex); --> Line 630 (Unlock A)
mhi_power_down()
mutex_lock(&mhi_cntrl->pm_mutex); --> Line 1139 (Lock A)
wake_up_all(&mhi_cntrl->state_event); --> Line 1165 (Wait X)
mutex_unlock(&mhi_cntrl->pm_mutex); --> Line 1168 (Unlock A)
I am not quite sure whether these possible problems are real.
Any feedback would be appreciated, thanks :)
Best wishes,
Jia-Ju Bai
