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:

quoted

On 25 September 2015 at 13:33, Rafael J. Wysocki [off-list ref] wrote:

quoted

You're going to change that into bool in the next patch, right?

Yeah.

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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?

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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.

James