Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Linus Torvalds
2018-07-17T18:44:38Z

On Tue, Jul 17, 2018 at 11:31 AM Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
>
> The isync provides ordering roughly similar to lwsync, but nowhere near
> as strong as sync, and it is sync that would be needed to cause lock
> acquisition to provide full ordering.

That's only true when looking at isync in isolation.

Read the part I quoted. The AIX documentation implies that the
*sequence* of load-compare-conditional branch-isync is a memory
barrier, even if isync on its own is now.

So I'm just saying that

(a) isync-on-lock is supposed to be much cheaper than sync-on-lock

(b) the AIX documentation at least implies that isync-on-lock (when
used together the the whole locking sequence) is actually a memory
barrier

Now, admittedly the powerpc barrier instructions are unfathomable
crazy stuff, so who knows. But:

(a) lwsync is a memory barrier for all the "easy" cases (ie
load->store, load->load, and store->load).

(b) lwsync is *not* a memory barrier for the store->load case.

(c) isync *is* (when in that *sequence*) a memory barrier for a
store->load case (and has to be: loads inside a spinlocked region MUST
NOT be done earlier than stores outside of it!).

So a unlock/lock sequence where the unlock is using lwsync, and the
lock is using isync, should in fact be a full memory barrier (which is
the semantics we're looking for).

So doing performance testing on sync/lwsync (for lock/unlock
respectively) seems the wrong thing to do. Please test the
isync/lwsync case instead.

Hmm? What am I missing?

Linus