> From: Lorenzo Pieralisi <lorenzo.pieralisi@xxxxxxx> > Sent: Friday, June 29, 2018 02:39 > To: Dexuan Cui <decui@xxxxxxxxxxxxx> > 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. Now my understanding is: 1) When hv_compose_msi_msg() is called with local irq ENABLED by the upper level irq code, the current code is good and the lockdep warning is not triggered. 2) When hv_compose_msi_msg() is called with local irq DISABLED by the upper level irq code, the current code *is* buggg: local_bh_enable() can potentially call do_softirq(), which is not supposed to run when local irq is DISABLED. I think the lockdep warning is triggered for this reason. In summary, now I realized the warning is not spurious, and here at the first place I should not use local_bh_disable()/enable(), which are not supposed to be used when local irq can be DISABLED. > 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. > > Lorenzo Now I realized there *is* a bug. I'm going to send a v2 with a new changelog, though the changed code will remain the same. Thanks, -- Dexuan
