On 06/07/20(Mon) 15:44, Vitaliy Makkoveev wrote:
> > On 6 Jul 2020, at 12:17, Martin Pieuchot <m...@openbsd.org> wrote:
> > Assertions and documentation are more important than preventing races
> > because they allow to build awareness and elegant solutions instead of
> > hacking diffs until stuff work without knowing why.
> > 
> > There are two cases where `ifp' are inserted into `ifnet':
> > 1. by autoconf during boot or hotplug
> > 2. by cloning ioctl
> > 
> > In the second case it is always about pseudo-devices.  So the assertion
> > should be conditional like:
> > 
> >     if (ISSET(ifp->if_xflags, IFXF_CLONED))
> >             rw_assert_wrlock(&ifc_lock);
> > 
> > In other words this fixes serializes insertions/removal on the global
> > list `ifnet', the KERNEL_LOCK() being still required for reading it.
> > 
> > Is there any other data structure which ends up being protected by this
> > approach and could be documented?
> We should be sure there is no multiple `ifnet’s in `if_list’ with the same
> `if_xname’.

That's a symptom of a bug.  Checking for a symptom won't prevent another
type of corruption, maybe next time it will be a corrupted pointer?

>             And the assertion you proposed looks not obvious here.

Why, is it because of the if() check?  That's required unless we change
put all if_attach() functions under the lock which would require changing
all driver in-tree.  However since drivers for physical devices are being
attached without having multiple CPUs running there's no possible race.

> Assertion like below looks more reasonable but introduces performance
> impact.

We should first aim for correctness then performance.  In this case,
performance is not even an issue because interfaces are not created
often compared to the rate of processing packets.

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