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 <[email protected]> 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 <[email protected]> > > Acked-by: Bjorn Helgaas <[email protected]> > > Reviewed-by: Logan Gunthorpe <[email protected]> > > --- > > 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/[email protected]/ > > > + 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(); >
