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Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock'

From "Rafael J. Wysocki" <rjw@rjwysocki.net>
Newsgroups linux.kernel
Subject Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock'
Date 2015-09-27 15:50 +0200
Message-ID <qdkuZ-kt-5@gated-at.bofh.it> (permalink)
References <qcEm5-746-7@gated-at.bofh.it> <qcHWF-3PA-1@gated-at.bofh.it> <qd3Nv-1lI-7@gated-at.bofh.it>
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On Saturday, September 26, 2015 12:52:08 PM James Bottomley wrote:
> On Fri, 2015-09-25 at 22:58 +0200, Rafael J. Wysocki wrote:
> > On Friday, September 25, 2015 01:25:49 PM Viresh Kumar wrote:
> > > On 25 September 2015 at 13:33, Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
> > > > You're going to change that into bool in the next patch, right?
> > > 
> > > Yeah.
> > > 
> > > > So what if bool is a byte and the field is not word-aligned
> > > 
> > > Its between two 'unsigned long' variables today, and the struct isn't packed.
> > > So, it will be aligned, isn't it?
> > > 
> > > > and changing
> > > > that byte requires a read-modify-write.  How do we ensure that things remain
> > > > consistent in that case?
> > > 
> > > I didn't understood why a read-modify-write is special here? That's
> > > what will happen
> > > to most of the non-word-sized fields anyway?
> > > 
> > > Probably I didn't understood what you meant..
> > 
> > Say you have three adjacent fields in a structure, x, y, z, each one byte long.
> > Initially, all of them are equal to 0.
> > 
> > CPU A writes 1 to x and CPU B writes 2 to y at the same time.
> > 
> > What's the result?
> 
> I think every CPU's  cache architecure guarantees adjacent store
> integrity, even in the face of SMP, so it's x==1 and y==2.  If you're
> thinking of old alpha SMP system where the lowest store width is 32 bits
> and thus you have to do RMW to update a byte, this was usually fixed by
> padding (assuming the structure is not packed).  However, it was such a
> problem that even the later alpha chips had byte extensions.

OK, thanks!

Rafael

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[PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' Viresh Kumar <viresh.kumar@linaro.org> - 2015-09-25 18:50 +0200
  [PATCH V4 2/2] debugfs: Pass bool pointer to debugfs_create_bool() Viresh Kumar <viresh.kumar@linaro.org> - 2015-09-25 18:50 +0200
  Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' Johannes Berg <johannes@sipsolutions.net> - 2015-09-25 19:50 +0200
    Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' Viresh Kumar <viresh.kumar@linaro.org> - 2015-09-25 20:50 +0200
      Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' Johannes Berg <johannes@sipsolutions.net> - 2015-09-25 20:50 +0200
        Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' Viresh Kumar <viresh.kumar@linaro.org> - 2015-09-25 21:00 +0200
          Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-09-25 22:00 +0200
            Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-09-25 22:10 +0200
              Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' Viresh Kumar <viresh.kumar@linaro.org> - 2015-09-25 22:30 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-09-25 22:40 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' Viresh Kumar <viresh.kumar@linaro.org> - 2015-09-25 23:50 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rafael@kernel.org> - 2015-09-26 00:30 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' Viresh Kumar <viresh.kumar@linaro.org> - 2015-09-26 20:50 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' Arnd Bergmann <arnd@arndb.de> - 2015-09-26 21:40 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-09-27 15:50 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' Arnd Bergmann <arnd@arndb.de> - 2015-09-28 10:30 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-09-28 14:40 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' James Bottomley <James.Bottomley@HansenPartnership.com> - 2015-09-26 22:00 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-09-27 15:50 +0200
                RE: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' David Laight <David.Laight@ACULAB.COM> - 2015-09-28 11:00 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' James Bottomley <James.Bottomley@HansenPartnership.com> - 2015-09-28 16:30 +0200
                RE: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' David Laight <David.Laight@ACULAB.COM> - 2015-09-28 17:00 +0200
                Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' James Bottomley <James.Bottomley@HansenPartnership.com> - 2015-09-28 17:20 +0200
                RE: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of  'global_lock' David Laight <David.Laight@ACULAB.COM> - 2015-09-28 17:40 +0200
  Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-09-25 22:00 +0200
    Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2015-09-25 22:00 +0200
  Re: [PATCH V4 1/2] ACPI / EC: Fix broken 64bit big-endian users of 'global_lock' Viresh Kumar <viresh.kumar@linaro.org> - 2015-09-27 16:40 +0200

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