Re: [PATCH net] ptp: prevent possible ABBA deadlock in ptp_clock_freerun()

Hello,

Jakub Kicinski [off-list ref] wrote:

On Sat,  5 Jul 2025 23:50:31 +0900 Jeongjun Park wrote:

quoted

ABBA deadlock occurs in the following scenario:

       CPU0                           CPU1
       ----                           ----
  n_vclocks_store()
    lock(&ptp->n_vclocks_mux) [1]
                                     pc_clock_adjtime()
                                       lock(&clk->rwsem) [2]
                                       ...
                                       ptp_clock_freerun()
                                         ptp_vclock_in_use()
                                           lock(&ptp->n_vclocks_mux) [3]
    ptp_clock_unregister()
      posix_clock_unregister()
        lock(&clk->rwsem) [4]

To solve this with minimal patches, we should change ptp_clock_freerun()
to briefly release the read lock before calling ptp_vclock_in_use() and
then re-lock it when we're done.

Dropping locks randomly is very rarely the correct fix.

Of course, we can change it to lock clk->rwsem before calling
ptp_clock_unregister(), but it would require a lot of code modifications,
and posix_clock_unregister() would also have to be modified, so I don't
think it's very appropriate.

That's why I suggested a way to briefly release the lock in
ptp_clock_freerun().

Either way - you forgot to CC Vladimir, again.

No need to reference Vladimir, as this bug is a structural issue that has
been around since the n_vclocks feature was added, as indicated in the
Fixes tag.

--
pw-bot: cr

Regards,

Jeongjun Park