linux-kernel - Re: [PATCH] alpha: spinlock: don't perform memory access in locked critical section

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Message-ID: <20130506205329.GO25399@ZenIV.linux.org.uk>
Date:	Mon, 6 May 2013 21:53:30 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Matt Turner <mattst88@...il.com>
Cc:	Will Deacon <will.deacon@....com>, linux-alpha@...r.kernel.org,
	linux-kernel@...r.kernel.org, Richard Henderson <rth@...ddle.net>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>
Subject: Re: [PATCH] alpha: spinlock: don't perform memory access in locked
 critical section

On Mon, May 06, 2013 at 01:19:51PM -0700, Matt Turner wrote:

> I'm not sure of the interpretation that LDA counts as a memory access.
> 
> The manual says it's Ra <- Rbv + SEXT(disp).
> 
> It's not touching memory that I can see.

More to the point, the same manual gives explicit list of instructions
that shouldn't occur between LDx_L and STx_C, and LDA does not belong to any
of those.  I suspect that Will has misparsed the notations in there - LDx is
present in the list, but it's _not_ "all instructions with mnemonics starting
with LD", just the 4 "load integer from memory" ones.  FWIW, instructions
with that encoding (x01xxx<a:5><b:5><offs:16>) are grouped so:
	LDAx - LDA, LDAH; load address
	LDx -  LDL, LDQ, LDBU, LDWU; load memory data into integer register
	LDQ_U; load unaligned
	LDx_L - LDL_L, LDQ_L; load locked
	STx_C - STL_C, STQ_C; store conditional
	STx - STL, STQ, STB, STW; store
	STQ_U; store unaligned
They all have the same encoding, naturally enough (operation/register/address
representation), but that's it...  See section 4.2 in reference manual for
details; relevant note follows discussion of LDx_L and it spells the list
out.  LDx is present, LDAx isn't (and neither is LDA by itself).
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