Re: [BUG] android: possible ABBA deadlock in print_binder_proc() and binder_get_txn_from_and_acq_inner()

[Todd Kjos]

On Mon, Sep 6, 2021 at 7:30 PM Jia-Ju Bai <baijiaju1990@xxxxxxxxx> wrote:
>
> Hello,
>
> My static analysis tool reports a possible ABBA deadlock in the android
> driver in Linux 5.10:
>
> print_binder_proc()
>    binder_inner_proc_lock()
>      _binder_inner_proc_lock()
>        spin_lock(&proc->inner_lock); --> Line 662 (Lock A)
>    print_binder_thread_ilocked()
>      print_binder_transaction_ilocked()
>        spin_lock(&t->lock); --> Line 5531 (Lock B)
>
> binder_get_txn_from_and_acq_inner()
>    binder_get_txn_from()
>      spin_lock(&t->lock); --> Line 1833 (Lock B)
>    binder_inner_proc_lock()
>      _binder_inner_proc_lock()
>        spin_lock(&proc->inner_lock); --> Line 662 (Lock A)

Hmm. What kernel version are you looking at? These line numbers don't
match what I see in mainline. Also, the sequence for
binder_get_txn_from_and_acq_inner() looks like this for me:

binder_get_txn_from_and_acq_inner()
    binder_get_txn_from()
      spin_lock(&t->lock); --> Line 1427 (Lock B)
      spin_unlock(&t->lock); --> Line 1431 (Lock B)
    binder_inner_proc_lock()
      _binder_inner_proc_lock()
         spin_lock(&proc->inner_lock); --> Line 256 (Lock A)

Which doesn't have the deadlock hazard.

>
> When print_binder_proc() and binder_get_txn_from_and_acq_inner() are
> concurrently executed, the deadlock can occur.
>
> 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 :)
>
> Reported-by: TOTE Robot <oslab@xxxxxxxxxxxxxxx>
>
>
> Best wishes,
> Jia-Ju Bai