On Wed, Jun 13, 2018 at 10:50:05PM +0000, Dexuan Cui wrote: > > From: Bjorn Helgaas <helgaas@xxxxxxxxxx> > > 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
