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To:     Eric Dumazet <eric.dumazet@gmail.com>,
        Bjorn Helgaas <bhelgaas@google.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Ira Weiny <ira.weiny@intel.com>,
        Christoph Hellwig <hch@lst.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        "Rafael J. Wysocki" <rafael@kernel.org>, linux-pci@vger.kernel.org
References: <20210701095621.3129283-1-eric.dumazet@gmail.com>
From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <986ec985-a42c-9107-564d-5878a669388b@deltatee.com>
Date:   Mon, 5 Jul 2021 10:11:55 -0600
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Subject: Re: [PATCH] PCI/P2PDMA: finish RCU conversion of pdev->p2pdma
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On 2021-07-01 3:56 a.m., Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> While looking at pci_alloc_p2pmem() I found rcu protection
> was not properly applied there, as pdev->p2pdma was
> potentially read multiple times.
> 
> I decided to fix pci_alloc_p2pmem(), add __rcu qualifier
> to p2pdma field of struct pci_dev, and fix all
> other accesses to this field with proper rcu verbs.
> 
> Fixes: 1570175abd16 ("PCI/P2PDMA: track pgmap references per resource, not globally")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Ira Weiny <ira.weiny@intel.com>
> Cc: Logan Gunthorpe <logang@deltatee.com>
> Cc: Bjorn Helgaas <bhelgaas@google.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Cc: linux-pci@vger.kernel.org

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

For history, though, Dan had originally suggested the full RCU
protection is not necessary and it was only a barrier to force the NULL
check on teardown to resolve:

https://lore.kernel.org/nvdimm/CAPcyv4jZiK+OHjwNqDARv4g326AQZx7N_Lmxj1Zux_bX1T2CLQ@mail.gmail.com/

Things may have changed since then and other uses might be racing with
the teardown, so having it marked __rcu and fully protected sounds like
a good idea to me.

Logan