mm: generalise COW SMC TLB flushing race comment

I'm not sure if I'm completely missing something here, but AFAIKS the
reference to the mysterious "COW SMC race" confuses the issue.  The
original changelog and mailing list thread didn't help me either.

This SMC race is where the problem was detected, but isn't the general
problem bigger and more obvious: that the new PTE could be picked up at
any time by any TLB while entries for the old PTE exist in other TLBs
before the TLB flush takes effect?

The case where the iTLB and dTLB of a CPU are pointing at different pages
is an interesting one but follows from the general problem.

The other (minor) thing with the comment I think it makes it a bit clearer
to say what the old code was doing (i.e., it avoids the race as opposed to
what?).

References: 4ce072f1

 ("mm: fix a race condition under SMC + COW")
Link: https://lkml.kernel.org/r/20201215121119.351650-1-npiggin@gmail.com
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Hugh Dickins <hughd@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>