On Sat, Aug 19, 2017 at 11:12:46AM +1000, Dave Chinner wrote:

On Fri, Aug 18, 2017 at 05:58:16PM -0700, Darrick J. Wong wrote:

quoted

On Sat, Aug 19, 2017 at 10:33:00AM +1000, Dave Chinner wrote:

quoted

On Fri, Aug 18, 2017 at 11:45:11AM -0700, Darrick J. Wong wrote:

quoted

On Fri, Aug 18, 2017 at 10:06:07AM -0700, Darrick J. Wong wrote:

quoted

...which is totally worthless, unless we want to compile all the verifier
functions with __attribute__((optimize("O0"))), which is bogus.

<sigh> Back to the drawing board on that one.

Ok, there's /slightly/ less awful way to prevent gcc from optimizing the
verifier function to the point of imprecise pointer value, but it involves
writing to a volatile int:

/* stupidly prevent gcc from over-optimizing getting the instruction ptr */
extern volatile int xfs_lineno;
#define __this_address ({ __label__ __here; __here: xfs_lineno = __LINE__; &&__here; })

<grumble> Yucky, but it more or less works.

Can you declare the label as volatile, like you can an asm
statement to prevent the compiler from optimising out asm
statements?

Even so, given the yuckiness is very isolated and should only affect
the slow path code, I can live with this.

Hmmm.  I can't declare the label as volatile, but I /can/ inject
asm volatile("") and that seems to prevent gcc from moving code hunks
around:

#define __this_address	({ __label__ __here; __here: asm volatile(""); &&__here; })

That seems cleaner to me, and I /think/ the gcc manual says it won't
remove such statements, but it also says:

	Under certain circumstances, GCC may duplicate (or remove duplicates
	of) your assembly code when optimizing.

So I have no real idea whether this is going to be robust or not.
I'm not a gcc/asm expert at all (that stuff is mostly black magic
to me).

Same here.  I figure if we start getting complaints about totally wacko
function pointers in the dmesg/xfsrepair output, we can put the
set-a-volatile-int cobwebs back in.

--D

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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