From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-18.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 348EAC43460 for ; Tue, 11 May 2021 16:05:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 07CA36191C for ; Tue, 11 May 2021 16:05:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231551AbhEKQG5 (ORCPT ); Tue, 11 May 2021 12:06:57 -0400 Received: from us-smtp-delivery-124.mimecast.com ([216.205.24.124]:44172 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231462AbhEKQG4 (ORCPT ); Tue, 11 May 2021 12:06:56 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1620749149; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=hrrdEBLqCOmDZJ+zOkXaPF7O/7nbLIRMtWhGOzsY1Zc=; b=VWa2s+HwWpIWWJZ/Mxn6LNjjw07GiLIJOYVupnNLLEZQ3UmkJvF6f8N3ZyqQib/44gGkUG 59OEIonBN6Ch9KGHK0T9kDhtEri7CuFUY4xxZqWiLQ1g8cJsPXiIgje77i273RiR4hQc4M IJn9MECMDwER/zy6vbOwi5mbJMjYmFI= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-411-IY58VNbdOuCPlC-_betBlg-1; Tue, 11 May 2021 12:05:47 -0400 X-MC-Unique: IY58VNbdOuCPlC-_betBlg-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 4CE848186E1; Tue, 11 May 2021 16:05:44 +0000 (UTC) Received: from [10.3.115.19] (ovpn-115-19.phx2.redhat.com [10.3.115.19]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0AD745D9F2; Tue, 11 May 2021 16:05:40 +0000 (UTC) Subject: Re: [PATCH 02/16] PCI/P2PDMA: Avoid pci_get_slot() which sleeps To: John Hubbard , Logan Gunthorpe , linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org, linux-block@vger.kernel.org, linux-pci@vger.kernel.org, linux-mm@kvack.org, iommu@lists.linux-foundation.org Cc: Stephen Bates , Christoph Hellwig , Dan Williams , Jason Gunthorpe , =?UTF-8?Q?Christian_K=c3=b6nig?= , Matthew Wilcox , Daniel Vetter , Jakowski Andrzej , Minturn Dave B , Jason Ekstrand , Dave Hansen , Xiong Jianxin , Bjorn Helgaas , Ira Weiny , Robin Murphy References: <20210408170123.8788-1-logang@deltatee.com> <20210408170123.8788-3-logang@deltatee.com> From: Don Dutile Message-ID: Date: Tue, 11 May 2021 12:05:40 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 5/2/21 1:35 AM, John Hubbard wrote: > On 4/8/21 10:01 AM, Logan Gunthorpe wrote: >> In order to use upstream_bridge_distance_warn() from a dma_map function, >> it must not sleep. However, pci_get_slot() takes the pci_bus_sem so it >> might sleep. >> >> In order to avoid this, try to get the host bridge's device from >> bus->self, and if that is not set, just get the first element in the >> device list. It should be impossible for the host bridge's device to >> go away while references are held on child devices, so the first element >> should not be able to change and, thus, this should be safe. >> >> Signed-off-by: Logan Gunthorpe >> --- >>   drivers/pci/p2pdma.c | 14 ++++++++++++-- >>   1 file changed, 12 insertions(+), 2 deletions(-) >> >> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c >> index bd89437faf06..473a08940fbc 100644 >> --- a/drivers/pci/p2pdma.c >> +++ b/drivers/pci/p2pdma.c >> @@ -311,16 +311,26 @@ static const struct pci_p2pdma_whitelist_entry { >>   static bool __host_bridge_whitelist(struct pci_host_bridge *host, >>                       bool same_host_bridge) >>   { >> -    struct pci_dev *root = pci_get_slot(host->bus, PCI_DEVFN(0, 0)); >>       const struct pci_p2pdma_whitelist_entry *entry; >> +    struct pci_dev *root = host->bus->self; >>       unsigned short vendor, device; >>   +    /* >> +     * This makes the assumption that the first device on the bus is the >> +     * bridge itself and it has the devfn of 00.0. This assumption should >> +     * hold for the devices in the white list above, and if there are cases >> +     * where this isn't true they will have to be dealt with when such a >> +     * case is added to the whitelist. > > Actually, it makes the assumption that the first device *in the list* > (the host->bus-devices list) is 00.0.  The previous code made the > assumption that you wrote. > > By the way, pre-existing code comment: pci_p2pdma_whitelist[] seems > really short. From a naive point of view, I'd expect that there must be > a lot more CPUs/chipsets that can do pci p2p, what do you think? I > wonder if we have to be so super strict, anyway. It just seems extremely > limited, and I suspect there will be some additions to the list as soon > as we start to use this. > > >> +     */ >>       if (!root) >> +        root = list_first_entry_or_null(&host->bus->devices, >> +                        struct pci_dev, bus_list); > > OK, yes this avoids taking the pci_bus_sem, but it's kind of cheating. > Why is it OK to avoid taking any locks in order to retrieve the > first entry from the list, but in order to retrieve any other entry, you > have to aquire the pci_bus_sem, and get a reference as well? Something > is inconsistent there. > > The new version here also no longer takes a reference on the device, > which is also cheating. But I'm guessing that the unstated assumption > here is that there is always at least one entry in the list. But if > that's true, then it's better to show clearly that assumption, instead > of hiding it in an implicit call that skips both locking and reference > counting. > > You could add a new function, which is a cut-down version of pci_get_slot(), > like this, and call this from __host_bridge_whitelist(): > > /* >  * A special purpose variant of pci_get_slot() that doesn't take the pci_bus_sem >  * lock, and only looks for the 00.0 bus-device-function. Once the PCI bus is >  * up, it is safe to call this, because there will always be a top-level PCI >  * root device. >  * >  * Other assumptions: the root device is the first device in the list, and the >  * root device is numbered 00.0. >  */ > struct pci_dev *pci_get_root_slot(struct pci_bus *bus) > { >     struct pci_dev *root; >     unsigned devfn = PCI_DEVFN(0, 0); > >     root = list_first_entry_or_null(&bus->devices, struct pci_dev, >                     bus_list); >     if (root->devfn == devfn) >         goto out; > ... add a flag (set for p2pdma use)  to the function to print out what the root->devfn is, and what the device is so the needed quirk &/or modification can added to handle when this assumption fails; or make it a prdebug that can be flipped on for this failing situation, again, to add needed change to accomodate. >     root = NULL; >  out: >     pci_dev_get(root); >     return root; > } > EXPORT_SYMBOL(pci_get_root_slot); > > ...I think that's a lot clearer to the reader, about what's going on here. > > Note that I'm not really sure if it *is* safe, I would need to ask other > PCIe subsystem developers with more experience. But I don't think anyone > is trying to make p2pdma calls so early that PCIe buses are uninitialized. > > >> + >> +    if (!root || root->devfn) >>           return false; >>         vendor = root->vendor; >>       device = root->device; >> -    pci_dev_put(root); and the reason to remove the dev_put is b/c it can sleep as well? is that ok, given the dev_get that John put into the new pci_get_root_slot()? ... seems like a locking version with no get/put's is needed, or, fix the host-bridge setups so no !NULL self pointers. >>         for (entry = pci_p2pdma_whitelist; entry->vendor; entry++) { >>           if (vendor != entry->vendor || device != entry->device) >> > > thanks,