linux-kernel - Re: [RFC][PATCH] mips: Fix arch_spin

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Message-ID: <alpine.DEB.2.00.1601271224550.5958@tp.orcam.me.uk>
Date:	Wed, 27 Jan 2016 12:41:29 +0000
From:	"Maciej W. Rozycki" <macro@...tec.com>
To:	Will Deacon <will.deacon@....com>
CC:	David Daney <ddaney@...iumnetworks.com>,
	Måns Rullgård <mans@...sr.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ralf Baechle <ralf@...ux-mips.org>,
	<linux-kernel@...r.kernel.org>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	<boqun.feng@...il.com>
Subject: Re: [RFC][PATCH] mips: Fix arch_spin_unlock()

On Wed, 27 Jan 2016, Will Deacon wrote:

> >  Overall I think it should be safe after all to use SYNC_RELEASE and other 
> > modern lightweight barriers uncondtionally under the assumption that 
> > architecture was meant to remain backward compatible.  Even though it 
> > might be possible someone would implement unusual semantics for the then 
> > undefined `stype' values, I highly doubt it as it would be extra effort 
> > and hardware logic space for no gain.  We could try and reach architecture 
> > overseers to double-check whether the `stype' encodings, somewhat 
> > irregularly distributed, were indeed defined in a manner so as not to 
> > clash with values implementers chose to use before rev. 2.61 of the 
> > architecture specification.
> 
> Do you know whether a SYNC 18 (RELEASE) followed in program order by a
> SYNC 17 (ACQUIRE) creates a full barrier (i.e. something like SYNC 16)?

 By my reading of architecture specifications it does.  Specifically 
SYNC_RELEASE (18) applies to older loads and stores, and newer stores, and 
SYNC_ACQUIRE (17) applies to older loads, and newer loads and stores.  So 
the two combined ought to be the equivalent to SYNC_MB (16), which applies 
to both older and newer loads and stores.  Of course care has to be taken 
about what happens between SYNC_RELEASE and SYNC_ACQUIRE.  This is still 
more lightweight than classic SYNC (0).  See the architecture documents, 
e.g. the MIPS32 one[1] for details.

References:

[1] "MIPS Architecture For Programmers, Volume II-A: The MIPS32 
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00086,
    Revision 5.04, December 11, 2013, Table 4.7 "Encodings of the 
    Bits[10:6] of the SYNC instruction; the SType Field", p. 305

 HTH,

  Maciej