Re: [WIP v2] igc: fix deadlock caused by taking RTNL in RPM resume path

[James Hogan <jhogan@kernel.org>]

On Saturday, 13 August 2022 01:05:41 BST Vinicius Costa Gomes wrote:
> James Hogan <jhogan@kernel.org> writes:
> > On Thursday, 11 August 2022 21:25:24 BST Vinicius Costa Gomes wrote:
> >> It was reported a RTNL deadlock in the igc driver that was causing
> >> problems during suspend/resume.
> >> 
> >> The solution is similar to commit ac8c58f5b535 ("igb: fix deadlock
> >> caused by taking RTNL in RPM resume path").
> >> 
> >> Reported-by: James Hogan <jhogan@kernel.org>
> >> Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
> >> ---
> >> Sorry for the noise earlier, my kernel config didn't have runtime PM
> >> enabled.
> > 
> > Thanks for looking into this.
> > 
> > This is identical to the patch I've been running for the last week. The
> > deadlock is avoided, however I now occasionally see an assertion from
> > netif_set_real_num_tx_queues due to the lock not being taken in some cases
> > via the runtime_resume path, and a suspicious rcu_dereference_protected()
> > warning (presumably due to the same issue of the lock not being taken).
> > See here for details:
> > https://lore.kernel.org/netdev/4765029.31r3eYUQgx@saruman/
> 
> Oh, sorry. I missed the part that the rtnl assert splat was already
> using similar/identical code to what I got/copied from igb.
> 
> So what this seems to be telling us is that the "fix" from igb is only
> hiding the issue,

I suppose the patch just changes the assumption from "lock will never be held 
on runtime resume path" (incorrect, deadlock) to "lock will always be held on 
runtime resume path" (also incorrect, probably racy).

> and we would need to remove the need for taking the
> RTNL for the suspend/resume paths in igc and igb? (as someone else said
> in that igb thread, iirc)

(I'll defer to others on this. I'm pretty unfamiliar with networking code and 
this particular lock.)

Cheers
James