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@xxxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
