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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu, 15 Oct 2020 09:55:38 +0200
Message-ID: <CAKMK7uHwi8_zurd59aX7ZDst+TGFB3TxyzPiWc8fmVvmomskMw@mail.gmail.com>
Subject: Re: [PATCH v2 14/17] resource: Move devmem revoke code to resource framework
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
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        "open list:DMA BUFFER SHARING FRAMEWORK" 
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On Thu, Oct 15, 2020 at 9:52 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> On Thu, Oct 15, 2020 at 2:09 AM Jason Gunthorpe <jgg@ziepe.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@ziepe.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@ziepe.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
>
> Indeed this changed with the sm_read_barrier_depends() removal a year
> ago. Before that READ_ONCE and rcu_dereference where not actually the
> same. I guess I'll throw a patch on top to switch that over too.

Actually 2019 landed just the clean-up, the read change landed in 2017 already:

commit 76ebbe78f7390aee075a7f3768af197ded1bdfbb
Author: Will Deacon <will@kernel.org>
Date:   Tue Oct 24 11:22:47 2017 +0100

   locking/barriers: Add implicit smp_read_barrier_depends() to READ_ONCE()

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch