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 Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id A6A27C53200 for ; Wed, 29 Jul 2026 10:08:46 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 0BCB510E00E; Wed, 29 Jul 2026 10:08:46 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=pass (2048-bit key; unprotected) header.d=kernel.org header.i=@kernel.org header.b="mG1EC1Me"; dkim-atps=neutral Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by gabe.freedesktop.org (Postfix) with ESMTPS id 6688810E00E for ; Wed, 29 Jul 2026 10:08:45 +0000 (UTC) Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 93B59600B1; Wed, 29 Jul 2026 10:08:44 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id BA1A81F000E9; Wed, 29 Jul 2026 10:08:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785319724; bh=R4pDNKLqHIaBBWNtEZelWW2DvKRxTtOSWeLWqmFNNxo=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=mG1EC1MeG6CYmME+U3L7b4yQqwCJw23+E759aBZ4ccpLnQkxENlO5ropRj03sGmEe g9UWSnbGmywcXZgcLdl1ZLvc0X2n4C50Ys6coE1s+0QdqTmzOb0tssZNnT0cxKJ3rO T38H/QRf/VEX5IsIiwDKpxi3IanyOpBvsY8EPpgPih3Wu661jJkvHjrkSB29vUm/mv X6mQjmjxMJuUGgyiUp1kjuHSMcREJmJ/yOE3QKP+4Br9zT4BFbqDUeyZ3sNCYzWwY9 Hc+JTQg9VlfXn/55qCNRboFkKIxzh0RipD5Gj0tx1hXIf1STPdXCcNVFIV8mauFyJu g3sS2tjv7v0sw== Date: Wed, 29 Jul 2026 13:08:40 +0300 From: Leon Romanovsky To: Alex Williamson Cc: Matt Evans , Jason Gunthorpe , Alex Mastro , Christian =?iso-8859-1?Q?K=F6nig?= , Bjorn Helgaas , Logan Gunthorpe , Kevin Tian , Pranjal Shrivastava , Longfang Liu , Mahmoud Adam , David Matlack , =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , Sumit Semwal , Ankit Agrawal , Alistair Popple , Vivek Kasireddy , linux-kernel@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, kvm@vger.kernel.org, linux-pci@vger.kernel.org Subject: Re: [PATCH v5 1/9] PCI/P2PDMA: Split pool-related cleanup out of pci_p2pdma_release() Message-ID: <20260729100840.GM12003@unreal> References: <20260715174737.15287-1-matt@ozlabs.org> <20260715174737.15287-2-matt@ozlabs.org> <20260728163343.2a39a8d1@shazbot.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260728163343.2a39a8d1@shazbot.org> X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" On Tue, Jul 28, 2026 at 04:33:43PM -0600, Alex Williamson wrote: > Hi Leon, > > Please see below... > > On Wed, 15 Jul 2026 18:47:24 +0100 > Matt Evans wrote: > > > Preparing for a refactor in a subsequent patch, split the pool-related > > release code into a new pci_p2pdma_release_pool() function. > > > > This allows future compile-time selection of a null implementation for > > pci_p2pdma_release_pool(), when p2pdma.c is refactored into core- and > > P2P-related files. > > > > Signed-off-by: Matt Evans > > Acked-by: Bjorn Helgaas > > Reviewed-by: Logan Gunthorpe > > --- > > drivers/pci/p2pdma.c | 24 ++++++++++++++---------- > > 1 file changed, 14 insertions(+), 10 deletions(-) > > > > diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c > > index b2d5266f8653..498bca257419 100644 > > --- a/drivers/pci/p2pdma.c > > +++ b/drivers/pci/p2pdma.c > > @@ -226,6 +226,17 @@ static const struct dev_pagemap_ops p2pdma_pgmap_ops = { > > .folio_free = p2pdma_folio_free, > > }; > > > > +static void pci_p2pdma_release_pool(struct pci_dev *pdev, > > + struct pci_p2pdma *p2pdma) > > +{ > > + if (!p2pdma->pool) > > + return; > > + > > + synchronize_rcu(); > > Sashiko notes[1] a high, preexisting issue here that looks like it was > introduced via 372d6d1b8ae3 ("PCI/P2PDMA: Refactor to separate core P2P > functionality from memory allocation"). That commit creates a two-tier > scheme where the optional memory allocation capabilities, such as the > pool, live above a core layer. Prior to that commit, this synchronize > RCU call was unconditional. Making it conditional on the pool suggests > it was only considered relevant to the optional layer. > > However, map_types, which remains in the core layer, is RCU referenced. > Was the synchronize_rcu() call here miscategorized into the optional > tier? Should it instead have remained unconditional? I remember this concern when I wrote the patches and wanted to remove RCU entirely. I revisited the Sashiko report, but reached the same conclusion again. The "bug" reported by Sashiko does not exist. The two-layer split ensures that p2p is bound to the driver's lifecycle. As a result, pdev->p2pdma is assigned and cleared only once during the lifetime of pdev. In this case, the RCU primitives are effectively NOPs, since nothing will ever update that pointer. I still needed to keep rcu_dereference() in pci_p2pdma_map_type() to satisfy static analyzers, which would otherwise complain about accessing an RCU-protected pointer without the proper annotations. RCU is used only in the sysfs flow. Thanks > > We might need a precursor Fixes: patch to this series that makes it > unconditional in pci_p2pdma_release(), so that it remains there with > this refactor rather than becoming part of this new pool release > function. Thanks, > > Alex > > [1]https://lore.kernel.org/all/20260715180521.3CDD61F00A3A@smtp.kernel.org/ > > > + gen_pool_destroy(p2pdma->pool); > > + sysfs_remove_group(&pdev->dev.kobj, &p2pmem_group); > > +} > > + > > static void pci_p2pdma_release(void *data) > > { > > struct pci_dev *pdev = data; > > @@ -237,15 +248,8 @@ static void pci_p2pdma_release(void *data) > > > > /* Flush and disable pci_alloc_p2p_mem() */ > > pdev->p2pdma = NULL; > > - if (p2pdma->pool) > > - synchronize_rcu(); > > + pci_p2pdma_release_pool(pdev, p2pdma); > > xa_destroy(&p2pdma->map_types); > > - > > - if (!p2pdma->pool) > > - return; > > - > > - gen_pool_destroy(p2pdma->pool); > > - sysfs_remove_group(&pdev->dev.kobj, &p2pmem_group); > > } > > > > /** > > @@ -946,8 +950,8 @@ void *pci_alloc_p2pmem(struct pci_dev *pdev, size_t size) > > struct pci_p2pdma *p2pdma; > > > > /* > > - * Pairs with synchronize_rcu() in pci_p2pdma_release() to > > - * ensure pdev->p2pdma is non-NULL for the duration of the > > + * Pairs with synchronize_rcu() in pci_p2pdma_release_pool() > > + * to ensure pdev->p2pdma is non-NULL for the duration of the > > * read-lock. > > */ > > rcu_read_lock(); >