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Wysocki" Content-Type: text/plain; charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Oct 15, 2020 at 2:09 AM Jason Gunthorpe 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 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 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