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

[Marc Kleine-Budde]

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.

Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
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