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

Al Viro
2013-05-06T20:53:34Z

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).