Re: Thread flags modified without set_thread_flag() (non atomically)

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 1 Mar 2007 10:34:51 +0100 Haavard Skinnemoen <hskinnemoen@atmel.com> wrote:

> > On Mon, 26 Feb 2007 12:10:37 -0800 Mathieu Desnoyers <compudj@google.com> wrote:
> 
> > > avr32/kernel/ptrace.c:          ti->flags |= _TIF_BREAKPOINT;
> > 
> > No, I don't immediately see anything in the flush_old_exec() code path
> > which tells us that nobody else can look up this thread_info (or be holding
> > a ref to it) in this context.
> > 
> > 
> > > avr32/kernel/ptrace.c:                  ti->flags |= TIF_SINGLE_STEP;
> > 
> > heh.  Haarvard, you got a bug.
> 
> Heh, yeah. That would indeed explain some strange gdb behaviour. It
> will only trigger when single-stepping into an exception or interrupt
> handler so thanks for pointing it out; I would have had a hard time
> figuring it out on my own...

yup, tricky.

If there's a lesson here, it is "don't provide #defines in the header for
both versions".

The block code does a similar thing:

#define REQ_RW          (1 << __REQ_RW)
#define REQ_FAILFAST    (1 << __REQ_FAILFAST)
#define REQ_SORTED      (1 << __REQ_SORTED)
#define REQ_SOFTBARRIER (1 << __REQ_SOFTBARRIER)

and I've caught Jens using the wrong identifier at least twice in the past.

It's better I think to just provide #defines for the bit offsets and
open-code the shifting if needed.  Like PG_foo and BH_Foo.

> I don't think either of those need to be atomic though, since both of
> them happen in monitor mode with interrupts disabled.

That's true until you implement SMP ;)