Re: [PATCH v2] ptp: remove ptp->n_vclocks check logic in ptp_vclock_in_use()

[Jeongjun Park]

Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Wed, 21 May 2025 01:07:17 +0900 Jeongjun Park wrote:
> > The reason why this is appropriate is that any path that uses
> > ptp->n_vclocks must unconditionally check if ptp->n_vclocks is greater
> > than 0 before unregistering vclocks, and all functions are already
> > written this way. And in the function that uses ptp->n_vclocks, we
> > already get ptp->n_vclocks_mux before unregistering vclocks.
>
> What about ptp_clock_freerun()? We seem to call it for clock ops
> like settime and it does not check n_vclocks.

ptp_clock_freerun() calls ptp_vclock_in_use() to check n_vclocks.

>
> > Therefore, we need to remove the redundant check for ptp->n_vclocks in
> > ptp_vclock_in_use() to prevent recursive locking.
>
> IIUC lockdep is complaining that we are trying to lock the vclock's
> n_vclocks_mux, while we already hold that lock for the real clock's
> instance. It doesn't understand that the two are in a fixed hierarchy
> so the deadlock is not possible.
>
> If my understanding is correct could you please clearly state in the
> commit message that this is a false positive? And if so isn't a better
> fix to _move_ the !ptp->is_virtual_clock check before the lock in
> ptp_vclock_in_use()? that way we preserve current behavior for real
> clocks, but vclocks can return early and avoid confusing lockdep?
> --
> pw-bot: cr

Your right! This deadlock report seems to be a false positive. It seems
appropriate to add a description of this false positive to the commit
message.

However, it is not appropriate to move the code that checks
ptp->is_virtual_clock. If you need to check n_vclocks when checking
whether ptp virtual clock is in use, it means that caller function has
already performed work related to n_vclocks, and in this case, it is
appropriate to perform n_vclocks check and n_vclocks_mux lock in caller
function.

Therefore, considering the overall structure, it is more appropriate to
remove unnecessary locks and n_vclocks checks in ptp_vclock_in_use().