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Date: Thu, 15 Oct 2020 17:29:05 +0200 From: Daniel Vetter <daniel.vetter@...ll.ch> To: Jason Gunthorpe <jgg@...pe.ca> Cc: Dan Williams <dan.j.williams@...el.com>, DRI Development <dri-devel@...ts.freedesktop.org>, LKML <linux-kernel@...r.kernel.org>, KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>, Linux ARM <linux-arm-kernel@...ts.infradead.org>, linux-samsung-soc <linux-samsung-soc@...r.kernel.org>, "open list:DMA BUFFER SHARING FRAMEWORK" <linux-media@...r.kernel.org>, linux-s390 <linux-s390@...r.kernel.org>, Daniel Vetter <daniel.vetter@...el.com>, Kees Cook <keescook@...omium.org>, Andrew Morton <akpm@...ux-foundation.org>, John Hubbard <jhubbard@...dia.com>, Jérôme Glisse <jglisse@...hat.com>, Jan Kara <jack@...e.cz>, Arnd Bergmann <arnd@...db.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, David Hildenbrand <david@...hat.com>, "Rafael J. Wysocki" <rafael.j.wysocki@...el.com> Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource framework On Thu, Oct 15, 2020 at 2:09 AM Jason Gunthorpe <jgg@...pe.ca> wrote: > > On Fri, Oct 09, 2020 at 11:28:54AM -0700, Dan Williams wrote: > > On Fri, Oct 9, 2020 at 7:32 AM Jason Gunthorpe <jgg@...pe.ca> wrote: > > > > > > On Fri, Oct 09, 2020 at 04:24:45PM +0200, Daniel Vetter wrote: > > > > On Fri, Oct 9, 2020 at 2:31 PM Jason Gunthorpe <jgg@...pe.ca> wrote: > > > > > > > > > > On Fri, Oct 09, 2020 at 09:59:31AM +0200, Daniel Vetter wrote: > > > > > > > > > > > +struct address_space *iomem_get_mapping(void) > > > > > > +{ > > > > > > + return iomem_inode->i_mapping; > > > > > > > > > > This should pair an acquire with the release below > > > > > > > > > > > + /* > > > > > > + * Publish /dev/mem initialized. > > > > > > + * Pairs with smp_load_acquire() in revoke_iomem(). > > > > > > + */ > > > > > > + smp_store_release(&iomem_inode, inode); > > > > > > > > > > However, this seems abnormal, initcalls rarely do this kind of stuff > > > > > with global data.. > > > > > > > > > > The kernel crashes if this fs_initcall is raced with > > > > > iomem_get_mapping() due to the unconditional dereference, so I think > > > > > it can be safely switched to a simple assignment. > > > > > > > > Ah yes I checked this all, but forgot to correctly annotate the > > > > iomem_get_mapping access. For reference, see b34e7e298d7a ("/dev/mem: > > > > Add missing memory barriers for devmem_inode"). > > > > > > Oh yikes, so revoke_iomem can run concurrently during early boot, > > > tricky. > > > > It runs early because request_mem_region() can run before fs_initcall. > > Rather than add an unnecessary lock just arrange for the revoke to be > > skipped before the inode is initialized. The expectation is that any > > early resource reservations will block future userspace mapping > > attempts. > > Actually, on this point a simple WRITE_ONCE/READ_ONCE pairing is OK, > Paul once explained that the pointer chase on the READ_ONCE side is > required to be like an acquire - this is why rcu_dereference is just > READ_ONCE Hm so WRITE_ONCE doesn't have any barriers, and we'd need that for updating the pointer. That would leave things rather inconsistent, so I think I'll just leave it as-is for symmetry reasons. None of this code matters for performance anyway, so micro-optimizing barriers seems a bit silly. -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
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