lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Sat, 23 Apr 2016 11:40:29 +0800
From:	Boqun Feng <>
To:	Peter Zijlstra <>
Cc:	Will Deacon <>,,,,,,
Subject: Re: [RFC][PATCH 3/3] locking,arm64: Introduce cmpwait()

On Sat, Apr 23, 2016 at 04:37:36AM +0200, Peter Zijlstra wrote:
> On Sat, Apr 23, 2016 at 12:08:57AM +0800, Boqun Feng wrote:
> > How about replace smp_rmb() with a smp_acquire_barrier__after_cmpwait()?
> > This barrier is designed to provide an ACQUIRE ordering when combining a
> > cmpwait() .
> That's a horrible name for a barrier :-)

Indeed, and having another special barrier is a pain.

> > And cmpwait() only has minimal ordering guarantee, but if it is actually
> > an ACQUIRE, then the corresponding smp_acquire_barrier__after_cmpwait()
> > is just empty.
> > 
> > We might need this special barrier on ppc, because we can implement it
> > with "isync" given that cmpwait() has control dependency and ctrl+isync
> > is ACQUIRE on ppc.
> > 
> > Thoughts?
> Provide a PPC specific smp_cond_load_acquire() using ISYNC ?

That works, but I should do more investigation on that, I brought this
up because ISYNC may be better than smp_rmb() in two cases, IIUC:

1.	for old systems without LWSYNC, smp_rmb() is actually SYNC,
	which is heavier than ISYNC.

2.	for new systems, isync may be a little faster than lwsync,
	according to Michael Ellerman:

but I doubt these two cases will have any observed performance impact,
so the current version of smp_cond_load_acquire() in your patch is fine,
and we can switch to a specific one in the future we want to ;-)


Download attachment "signature.asc" of type "application/pgp-signature" (474 bytes)

Powered by blists - more mailing lists