linux-kernel - [PATCH] PCI/P2PDMA: Annotate RCU dereference

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Message-Id: <20230209172953.4597-1-logang@deltatee.com>
Date:   Thu,  9 Feb 2023 10:29:53 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Chaitanya Kulkarni <chaitanyak@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        kernel test robot <lkp@...el.com>
Subject: [PATCH] PCI/P2PDMA: Annotate RCU dereference

A dereference of the __rcu pointer was noticed by sparse:

  drivers/pci/p2pdma.c:199:44: sparse: sparse: dereference of noderef expression

The __rcu pointer should be dereferenced using
rcu_dereference_protected() instead of accessed directly. Its safe
to use rcu_derference_protected() seeing a reference is held on
the pgmap's percpu reference counter and thus it cannot disappear.

Reported-by: kernel test robot <lkp@...el.com>
Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
---

Based on v6.2-rc7

 drivers/pci/p2pdma.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
index 86812d2073ea..9e8205572830 100644
--- a/drivers/pci/p2pdma.c
+++ b/drivers/pci/p2pdma.c
@@ -194,11 +194,13 @@ static const struct attribute_group p2pmem_group = {
 static void p2pdma_page_free(struct page *page)
 {
 	struct pci_p2pdma_pagemap *pgmap = to_p2p_pgmap(page->pgmap);
+	/* safe to dereference while a reference is held to the percpu ref */
+	struct pci_p2pdma *p2pdma =
+		rcu_dereference_protected(pgmap->provider->p2pdma, 1);
 	struct percpu_ref *ref;

-	gen_pool_free_owner(pgmap->provider->p2pdma->pool,
-			    (uintptr_t)page_to_virt(page), PAGE_SIZE,
-			    (void **)&ref);
+	gen_pool_free_owner(p2pdma->pool, (uintptr_t)page_to_virt(page),
+			    PAGE_SIZE, (void **)&ref);
 	percpu_ref_put(ref);
 }


base-commit: 4ec5183ec48656cec489c49f989c508b68b518e3
--
2.30.2