trace_hardirqs_on/off vs. extra stack frames

[Benjamin Herrenschmidt]

Hi Steven !

I'm trying to untangle something, and I need your help :-)

In commit 3cb5f1a3e58c0bd70d47d9907cc5c65192281dee, you added a summy
stack frame around the assembly calls to trace_hardirqs_on/off on the
ground that when using the latency tracer (irqsoff), you might poke at
CALLER_ADDR1 and that could blow up if there's only one frame at hand.

However, I can't see where it would be doing that. lockdep.c only uses
CALLER_ADDR0 and irqsoff uses the values passed by it. In fact, that
was already the case when the above commit was merged.

I tried on a 32-bit kernel to remove the dummy stack frame with no
issue so far .... (though I do get stupid values reported with or
without a stack frame, but I think that's normal, looking into it).

The reason I'm asking is that we have other code path, on return
from interrupts for example, at least on 32-bits where we call the
tracing without the extra stack frame, and I yet to see it crash.

I wonder if the commit and bug fix above relates to some older code
that no longer existed even at the point where the commit was merged...

Any idea ?

Cheers,
Ben.