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mips/atomic: Fix loongson_llsc_mb() wreckage

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The comment describing the loongson_llsc_mb() reorder case doesn't
make any sense what so ever. Instruction re-ordering is not an SMP
artifact, but rather a CPU local phenomenon. Clarify the comment by
explaining that these issue cause a coherence fail.

For the branch speculation case; if futex_atomic_cmpxchg_inatomic()
needs one at the bne branch target, then surely the normal
__cmpxch_asm() implementation does too. We cannot rely on the
barriers from cmpxchg() because cmpxchg_local() is implemented with
the same macro, and branch prediction and speculation are, too, CPU
local.

Fixes: e02e07e312 ("MIPS: Loongson: Introduce and use loongson_llsc_mb()")
Cc: Huacai Chen <chenhc@lemote.com>
Cc: Huang Pei <huangpei@loongson.cn>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
This commit is contained in:
Peter Zijlstra 2019-06-13 15:43:19 +02:00 • committed by Paul Burton
commit 1c6c1ca318
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5 changed files with 32 additions and 16 deletions
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