Re: [PATCH net] can: isotp: isotp_rcv_cf(): fix so->rx race problem

[Oliver Hartkopp]

Hi Marc,

On 07.02.22 09:11, Marc Kleine-Budde wrote:
> On 28.01.2022 15:48:05, Oliver Hartkopp wrote:
> > Hello Marc, hello William,
> >
> > On 28.01.22 09:46, Marc Kleine-Budde wrote:
> > > On 28.01.2022 09:32:40, Oliver Hartkopp wrote:
> > > > On 28.01.22 09:07, Marc Kleine-Budde wrote:
> > > > > On 28.01.2022 08:56:19, Oliver Hartkopp wrote:
> > > > > > I've seen the frame processing sometimes freezes for one second when
> > > > > > stressing the isotp_rcv() from multiple sources. This finally freezes
> > > > > > the entire softirq which is either not good and not needed as we only
> > > > > > need to fix this race for stress tests - and not for real world usage
> > > > > > that does not create this case.
> > > > >
> > > > > Hmmm, this doesn't sound good. Can you test with LOCKDEP enabled?
> >
> >
> > > > #
> > > > # Lock Debugging (spinlocks, mutexes, etc...)
> > > > #
> > > > CONFIG_LOCK_DEBUGGING_SUPPORT=y
> > > > # CONFIG_PROVE_LOCKING is not set
> > > CONFIG_PROVE_LOCKING=y
> >
> > Now enabled even more locking (seen relevant kernel config at the end).
> >
> > It turns out that there is no visible difference when using spin_lock() or
> > spin_trylock().
> >
> > I only got some of these kernel log entries
> >
> > Jan 28 11:13:14 silver kernel: [ 2396.323211] perf: interrupt took too long
> > (2549 > 2500), lowering kernel.perf_event_max_sample_rate to 78250
> > Jan 28 11:25:49 silver kernel: [ 3151.172773] perf: interrupt took too long
> > (3188 > 3186), lowering kernel.perf_event_max_sample_rate to 62500
> > Jan 28 11:45:24 silver kernel: [ 4325.583328] perf: interrupt took too long
> > (4009 > 3985), lowering kernel.perf_event_max_sample_rate to 49750
> > Jan 28 12:15:46 silver kernel: [ 6148.238246] perf: interrupt took too long
> > (5021 > 5011), lowering kernel.perf_event_max_sample_rate to 39750
> > Jan 28 13:01:45 silver kernel: [ 8907.303715] perf: interrupt took too long
> > (6285 > 6276), lowering kernel.perf_event_max_sample_rate to 31750
> >
> > But I get these sporadically anyway. No other LOCKDEP splat.
> >
> > At least the issue reported by William should be fixed now - but I'm still
> > unclear whether spin_lock() or spin_trylock() is the best approach here in
> > the NET_RX softirq?!?
>
> With the !spin_trylock() -> return you are saying if something
> concurrent happens, drop it. This doesn't sound correct.

Yes, I had the same feeling and did some extensive load tests using both 
variants.

It turned out the standard spin_lock() works excellent to fix the issue.

Thanks for taking it for upstream here:
https://lore.kernel.org/linux-can/20220209074818.3ylfz4zmuhit7orc@xxxxxxxxxxxxxx/T/#t

Best regards,
Oliver