Re: + kthread-add-a-missing-memory-barrier-to-kthread_stop.patch added to -mm tree

On 23/02/2008, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

On Sat, 23 Feb 2008, Dmitry Adamushko wrote:
>
> it's not a LOAD that escapes *out* of the region. It's a MODIFY that gets *in*:


Not with the smp_wmb(). That's the whole point.

Ie the patch I'm suggesting is sufficient is appended, and the point is
that any write before the critical region will be ordered wrt the critical
region because of the write barrier before the spinlock (which itself is a
write).

Yeah, good point!

(heh... I wouldn't dare to say this 'obvious thing' only to Anton
Blanchard who is "the only person who always 'have a point' by
definition" :-))

This is also why I mentioned that if you have a really odd architecure
that considers spinlocks to be "outside" the normal cache coherency
domain, that would be broken, but I cannot think of a single valid case of
that, and I consider it insane.

Yeah, some potential implementations come into my mind but, I guess,
they are as far away from real hardware as science-fiction from
science :-/

So how should we proceed with this issue?

let's use your patch and declare try_to_wake_up() a 'full' mb for the case:

MODIFY
try_to_wake_up
LOAD or MODIFY (that take place either after or inside try_to_wake_up())

so we'll fix (lots of) potentially problematic cases with a single shot.

and

LOAD
try_to_wake_up()
LOAD or MODIFY

is probably not that common so we don't care.


--
Best regards,
Dmitry Adamushko
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