Re: [RFC] parent in ->d_compare() arguments

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Jul 30, 2016 at 01:44:48PM -0700, Linus Torvalds wrote:

> No. Let's not. "smp_rmb()" is completely free on x86 (ok, so it's a
> instruction scheduling barrer - close enough), so trying to optimize
> away rmb's and replacing them with double compares sounds entirely
> misdesigned.
> 
> Yes, yes, there are other architectures where rmb is much more
> expensive. But quite frankly, in most cases those architectures have
> broken synchronization to begin with ("synchronization is unusual, so
> let's not optimize it"). They'll fix it eventually.
> 
> Instead, what we should look at, is to make raw_seqcount_begin() use a
> smp_load_acquire() on architectures where that is cheaper than the
> rmb.
> 
> But again, I don't see the point of double-testing "parent" when a
> load-acquire or load+rmb _should_ be cheap (and absolutely is on x86).

Umm...  Even on x86, a lot of hash chain elements will have ->d_parent
mismatch.  Suppose rmb was a no-op; current code does
	fetch ->d_seq
	fetch ->d_parent
	compare with register
	branch taken to the end of body
while this would avoid the first fetch.  On the entries with the same
->d_parent we'd do
	fetch ->d_parent
	compare with register
	branch not taken
	fetch ->d_seq
	fetch ->d_parent
	compare with register
	branch (expectedly) not taken
which is the same as the mainline in terms of actual memory accesses and extra
3 insns.  I suspect that the win on the entries with ->d_parent mismatch can
outweight that, but that needs profiling to verify.