Re: [PATCH] PCI: hv: Fix a __local_bh_enable_ip warning in hv_compose_msi_msg()

[Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>]

On Wed, Jun 13, 2018 at 10:50:05PM +0000, Dexuan Cui wrote:
> > From: Bjorn Helgaas <helgaas@kernel.org>
> > Sent: Wednesday, June 13, 2018 15:15
> > > ...
> > > It looks Lorenzo's pci.git tree has not been updated for 3+ weeks.
> > > I guess Lorenzo may be on vacation.
> > >
> > > @Bjorn, can this patch go through your tree?
> > > Should I resubmit it?
> > 
> > No need to resubmit it, Lorenzo has been out for a bit, but I'm sure
> > he'll pick this up as he catches up.
> OK, I see. Thanks!
>  
> > You might, however, fix the commit log:
> > 
> >   This is not an issue because hv_pci_onchannelcallback() is not slow,
> >   and it not a hot path.
> > 
> > This has at least one typo (I think you mean "and *is* not a hot
> > path").
> Sorry -- yes, it's a typo. I hope Lorenzo can help to fix this, or I can
> resubmit it if Lorenzo or you want me to do it.
>  
> > I also don't understand the sentence as a whole because the
> > hv_pci_onchannelcallback() comment says it's called whenever the host
> > sends a packet to this channel, and that *does* sound like a hot path.
> Sorry for not making it clear.
> The host only sends a packet into the channel of the guest when there
> is a change of device configuration (i.e. hot add or remove a device), or
> the host is responding to the guest's request.
> 
> The change of device configuration is only triggered on-demand by the
> administrator on the host, and the guest's requests are one-off when
> the device is probed.
> 
> So IMO the callback is not a hot path.
> 
> > I also don't understand the "hv_pci_onchannelcallback() is not slow"
> > part.  In other words, you're saying hv_pci_onchannelcallback() is
> > fast and it's not a hot path.  And apparently this has something to do
> > with the difference between local_bh_disable() and local_irq_save()?
> > 
> > Bjorn
> Actually in my original internal version of the patch, I did use 
> local_irq_save/restore().
> 
> hv_pci_onchannelcallback() itself runs fast, but here since it's in a
> loop (i.e. the while (!try_wait_for_completion(&comp.comp_pkt.host_event)
> loop), IIRC I was asked if I really need local_irq_save/restore(),
> and I answered "not really", so later I switched to local_bh_disable()/enable().
> 
> However, recently I found that if we enable CONFIG_PROVE_LOCKING=y,
> the local_bh_enable() can trigger a warning because the function 
> hv_compose_msi_msg() can be called with local IRQs disabled (BTW,
> hv_compose_msi_msg() can also be called with local IRQS enabled in
> another code path):
> 
> IRQs not enabled as expected
>   WARNING: CPU: 0 PID: 408 at kernel/softirq.c:162 __local_bh_enable_ip
> 
> Despite the warning, the code itself can still work correctly, but IMO we'd
> better switch back to local_irq_save/restore(), and hence I made the patch.
> 
> I hope the explanation sounds reasonable. :-)

Sorry for the delay in replying. I need to understand if you are
preventing a spurious lockdep warning or you are fixing a kernel
bug. From your commit log, I assume the former option but I do
not think that's what you are really doing.

Apart from the commit log typos fixes I would like a log that
explains *why* this is not a kernel bug fix rather than a harmless
lockdep warning prevention.

Thanks,
Lorenzo