Subject: Re: [Kgdb-bugreport] 2.6.23-rc3-mm1: kgdb build failure on powerpc

From: Andrew Morton <[email-masked]>
Date: Wed, 22 Aug 2007 16:53:18 -0700

On Wed, 22 Aug 2007 17:44:12 -0500
Jason Wessel <[email-masked]> wrote:

> Perhaps there is a cleaner way to do the same thing and avoid the 
> cmpxchg all together.  I used the attached patch to eliminate the 
> cmpxchg operation.
> 
> 
> Jason.
> 
> 
> [kgdb_enter_atomic.patch  text/plain (2.0KB)]
> Signed-off-by: Jason Wessel <[email-masked]>
> 
> ---
>  kernel/kgdb.c |   18 ++++++++++++++++--
>  1 file changed, 16 insertions(+), 2 deletions(-)
> 
> --- a/kernel/kgdb.c
> +++ b/kernel/kgdb.c
> @@ -121,6 +121,7 @@ struct task_struct *kgdb_usethread, *kgd
>  
>  int debugger_step;
>  atomic_t debugger_active;
> +static atomic_t kgdb_sync = ATOMIC_INIT(-1);
>  
>  /* Our I/O buffers. */
>  static char remcom_in_buffer[BUFMAX];
> @@ -638,8 +639,14 @@ static void kgdb_wait(struct pt_regs *re
>  	kgdb_info[processor].task = current;
>  	atomic_set(&procindebug[processor], 1);
>  
> +	/* The master processor must be active to enter here, but this is
> +	 * gaurd in case the master processor had not been selected if
> +	 * this was an entry via nmi.
> +	 */
> +	while (!atomic_read(&debugger_active));

eek.  We're in the process of hunting down and eliminating exactly this
construct.  There have been cases where the compiler cached the
atomic_read() result in a register, turning the above into an infinite
loop.

Plus we should never add power-burners like that into the kernel anyway. 
That loop should have a cpu_relax() in it.  Which will also fix the
compiler problem described above.

Thirdly, please always add a newline when coding statements like that:

	while (expr())
		;