Re: [PATCH] PCI/P2PDMA: finish RCU conversion of pdev->p2pdma

[Logan Gunthorpe]

On 2021-07-01 3:56 a.m., Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> Cc: Ira Weiny <ira.weiny@xxxxxxxxx>
> Cc: Logan Gunthorpe <logang@xxxxxxxxxxxx>
> Cc: Bjorn Helgaas <bhelgaas@xxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> Cc: linux-pci@xxxxxxxxxxxxxxx

Reviewed-by: Logan Gunthorpe <logang@xxxxxxxxxxxx>

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@xxxxxxxxxxxxxx/

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