tools/memory-model: Remove smp_read_barrier_depends() from informal doc

In practice, this difficulty is solved by inserting a special fence

between P1's two loads when the kernel is compiled for the Alpha

architecture.  In fact, as of version 4.15, the kernel automatically

adds this fence (called smp_read_barrier_depends() and defined as

nothing at all on non-Alpha builds) after every READ_ONCE() and atomic

load.  The effect of the fence is to cause the CPU not to execute any

po-later instructions until after the local cache has finished

processing all the stores it has already received.  Thus, if the code

was changed to:

adds this fence after every READ_ONCE() and atomic load on Alpha.  The

effect of the fence is to cause the CPU not to execute any po-later

instructions until after the local cache has finished processing all

the stores it has already received.  Thus, if the code was changed to:

	P1()

	{

directly.

The LKMM requires that smp_rmb(), acquire fences, and strong fences

share this property with smp_read_barrier_depends(): They do not allow

the CPU to execute any po-later instructions (or po-later loads in the

case of smp_rmb()) until all outstanding stores have been processed by

the local cache.  In the case of a strong fence, the CPU first has to

wait for all of its po-earlier stores to propagate to every other CPU

in the system; then it has to wait for the local cache to process all

the stores received as of that time -- not just the stores received

when the strong fence began.

share this property: They do not allow the CPU to execute any po-later

instructions (or po-later loads in the case of smp_rmb()) until all

outstanding stores have been processed by the local cache.  In the

case of a strong fence, the CPU first has to wait for all of its

po-earlier stores to propagate to every other CPU in the system; then

it has to wait for the local cache to process all the stores received

as of that time -- not just the stores received when the strong fence

began.

And of course, none of this matters for any architecture other than

Alpha.