Re: Re: [RFC][PATCH 2/5] mips/atomic: Fix loongson_llsc_mb() wreckage

[Linus Torvalds]

On Wed, Apr 24, 2019 at 9:59 PM <huangpei@xxxxxxxxxxx> wrote:
>
> In one wordï the bug only affect local cpuâs ll/sc operation, and affect MP system.
>
> If local_t is only ll/sc manipulated by current CPUï then no need fix it.

As to the whole MP vs UP issue:

Is this "guaranteed no problem on UP" true even in the presence of
DMA? I'm _hoping_ some MIPS chips are starting to be coherent with DMA
(but maybe DMA never participates in any coherency traffic that could
trigger the bug even if the DMA _were_ to be coherent?).

Also, as Peter mentioned, we do depend on ll/sc also reacting to
interrupts - again including on UP systems, of course. I assume that's
always handled correctly, and that an interrupt will set the right
state that the SC will not succeed?

Finally, it worries me a bit that the loongson_llsc_mb() things are
all at the C level, so who knows what compiler behavior you'll see
between the ll/sc inline asm, and the loongson_llsc_mb() generation.
Hopefully not a lot (and presumably mostly just stack reloads etc that
probably don't change any cache state), but I do wonder if the
loongson quirk should be *inside* the asm rather than separate.

                  Linus