linux-kernel - Re: [PATCH v9 03/10] asm/rwonce: Introduce [READ|WRITE]_ONCE() support for _

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Message-ID: <CAFULd4YDSRzMAvxunw=vfC-jw3xHPRjg=+X6mDONUXSSnU=7Xw@mail.gmail.com>
Date: Thu, 7 Nov 2024 14:47:12 +0100
From: Uros Bizjak <ubizjak@...il.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Arnd Bergmann <arnd@...db.de>, Joerg Roedel <joro@...tes.org>, 
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>, linux-kernel@...r.kernel.org, 
	iommu@...ts.linux.dev, Robin Murphy <robin.murphy@....com>, vasant.hegde@....com, 
	Kevin Tian <kevin.tian@...el.com>, jon.grimm@....com, santosh.shukla@....com, 
	pandoh@...gle.com, kumaranand@...gle.com, 
	Linux-Arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v9 03/10] asm/rwonce: Introduce [READ|WRITE]_ONCE()
 support for __int128

On Thu, Nov 7, 2024 at 2:37 PM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Thu, Nov 07, 2024 at 11:01:58AM +0100, Arnd Bergmann wrote:
>
> > >> and later:
> > >>
> > >>  * Yes, this permits 64-bit accesses on 32-bit architectures. These will
> > >>  * actually be atomic in some cases (namely Armv7 + LPAE), but for others we
> > >>  * rely on the access being split into 2x32-bit accesses for a 32-bit quantity
> > >>  * (e.g. a virtual address) and a strong prevailing wind.
> > >>
> > >> This is the "strong prevailing wind", mentioned in the patch review at [1].
> > >>
> > >> [1] https://lore.kernel.org/lkml/20241016130819.GJ3559746@nvidia.com/
> >
> > I understand the special case for ARMv7VE. I think the more important
> > comment in that file is
> >
> >   * Use __READ_ONCE() instead of READ_ONCE() if you do not require any
> >   * atomicity. Note that this may result in tears!
>
> That makes sense, let's just use that and there is no need to change
> anything here?
>
> Uros?

Yes, preloading "old" value for try_cmpxchg loop does not need to be
atomic (cmpxchg will fail in case teared value is preloaded and loop
will be retried). So, __READ_ONCE() is perfectly OK to be used in this
series.

Please note that __READ_ONCE() uses  __unqual_scalar_typeof()
operator, so at least patch at [1] to teach __uqual_scalar_typeof()
about __int128 is needed.

[1] https://lore.kernel.org/lkml/CAFULd4Z86uiH+w+1N36kOuhYZ5_ZkQkaEN6nyPh8VNJth3WNhg@mail.gmail.com/

Uros.