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Wed, 13 Jun 2018 22:50:06 +0000 Received: from KL1P15301MB0006.APCP153.PROD.OUTLOOK.COM ([fe80::91a9:30c3:c99f:a8bc]) by KL1P15301MB0006.APCP153.PROD.OUTLOOK.COM ([fe80::91a9:30c3:c99f:a8bc%6]) with mapi id 15.20.0884.010; Wed, 13 Jun 2018 22:50:06 +0000 From: Dexuan Cui To: Bjorn Helgaas CC: 'Bjorn Helgaas' , Haiyang Zhang , 'Lorenzo Pieralisi' , "'linux-pci@vger.kernel.org'" , KY Srinivasan , Stephen Hemminger , "'olaf@aepfle.de'" , "'apw@canonical.com'" , "'jasowang@redhat.com'" , "'linux-kernel@vger.kernel.org'" , "'driverdev-devel@linuxdriverproject.org'" , "'vkuznets@redhat.com'" , "'marcelo.cerri@canonical.com'" Subject: RE: [PATCH] PCI: hv: Fix a __local_bh_enable_ip warning in hv_compose_msi_msg() Thread-Topic: [PATCH] PCI: hv: Fix a __local_bh_enable_ip warning in hv_compose_msi_msg() Thread-Index: AdPyKucLHxE0xn3+RTSKXedSJeHHcgCNtVtQAmSYQFABWDo6wAADvceAAAAiUoA= Date: Wed, 13 Jun 2018 22:50:05 +0000 Message-ID: References: <20180613221509.GB12463@bjorn-Precision-5520> In-Reply-To: <20180613221509.GB12463@bjorn-Precision-5520> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: msip_labels: MSIP_Label_f42aa342-8706-4288-bd11-ebb85995028c_Enabled=True; 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received-spf: None (protection.outlook.com: microsoft.com does not designate permitted sender hosts) x-microsoft-antispam-message-info: ZUEgvla3YjkuTjdejoFNVi/sPGukdLwqAP1JcJU+3ngV/nuXnj2wpejekP90eotow8g4afz+kYvsCE0YUTUIlEI8235mBVlW+hpqA21zRP2jgEfWHlWOsm1i7enVJmbgmgyf+Y7AZLkvA5B95mS9toJLSaS6IhTqfp08n3RbI/qCLV47ywALmLrzEWSWHZDX spamdiagnosticoutput: 1:99 spamdiagnosticmetadata: NSPM Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: microsoft.com X-MS-Exchange-CrossTenant-Network-Message-Id: 6dbe3ba9-bbfa-4f18-280e-08d5d18001eb X-MS-Exchange-CrossTenant-originalarrivaltime: 13 Jun 2018 22:50:05.6617 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47 X-MS-Exchange-Transport-CrossTenantHeadersStamped: KL1P15301MB0037 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: Bjorn Helgaas > 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? >=20 > 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! =20 > You might, however, fix the commit log: >=20 > This is not an issue because hv_pci_onchannelcallback() is not slow, > and it not a hot path. >=20 > 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. =20 > 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()? >=20 > Bjorn Actually in my original internal version of the patch, I did use=20 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()/enab= le(). However, recently I found that if we enable CONFIG_PROVE_LOCKING=3Dy, the local_bh_enable() can trigger a warning because the function=20 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. :-) Thanks, -- Dexuan