Re: [RFC GIT PULL] softirq: Consolidation and stack overrun fix

[Benjamin Herrenschmidt]

On Mon, 2013-09-23 at 13:59 -0400, Chris Metcalf wrote:
> We just came up against this on tilegx with a customer bug report in a
> PREEMPT environment.  On tile, %tp is a GPR that points to the percpu area.
> The following seems to be the right abstraction -- though I'd also argue
> that letting barrier() clobber not just memory, but %tp, might be a better
> solution, but it's not clear what the best way is to do per-architecture
> overrides of per-compiler definitions like barrier().  See also the ARM v7
> code, which has to do something similar, though their percpu pointer is
> not a GPR, which changes the tradeoffs somewhat.

Hrm, if I read correctly what you did is that you read "tp" into another
register *and* also mark that action as clobbering the top int on the stack ?

I don't quite get what the stack clobber brings you here and how it works
around the fact that gcc might still cache that copy of tp into another
register accross preempt_enable/disable...

It's hard to tell with gcc ... the best I've had so far as an option was
something that would mark my per-cpu register (r13) *itself* as clobbered...

Cheers,
Ben.


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