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    SubjectRe: [Kgdb-bugreport] 2.6.23-rc3-mm1: kgdb build failure on powerpc
    On Wed, 22 Aug 2007 17:44:12 -0500
    Jason Wessel <jason.wessel@windriver.com> 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 <jason.wessel@windriver.com>
    >
    > ---
    > 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())
    ;


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