On Wed, Jan 27, 2010 at 04:25:24PM -0600, Jason Wessel wrote:
> The kernel debugger cannot use any mutex_lock() calls because it can
> start the kernel running from an invalid context.
>
> The possibility for a breakpoint reservation to be concurrently
> processed at the time that kgdb interrupts the system is improbable.
> As a safety check against this condition the kernel debugger will
> prohibit updating the hardware breakpoint reservations and an error
> will be returned to the end user.
>
> Any time the kernel debugger reserves a hardware breakpoint it will be
> a system wide reservation.
>
> CC: Frederic Weisbecker <fweis...@gmail.com>
> CC: Ingo Molnar <mi...@elte.hu>
> CC: K.Prasad <pra...@linux.vnet.ibm.com>
> CC: Peter Zijlstra <pet...@infradead.org>
> CC: Alan Stern <st...@rowland.harvard.edu>
> Signed-off-by: Jason Wessel <jason.wes...@windriver.com>
> ---
> arch/x86/kernel/kgdb.c | 49 +++++++++++++++++++++++++++++++++++++-
> include/linux/hw_breakpoint.h | 2 +
> kernel/hw_breakpoint.c | 52
> +++++++++++++++++++++++++++++++++--------
> 3 files changed, 92 insertions(+), 11 deletions(-)
>
> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
> index 9f47cd3..7c3e929 100644
> --- a/arch/x86/kernel/kgdb.c
> +++ b/arch/x86/kernel/kgdb.c
> @@ -239,6 +239,45 @@ static void kgdb_correct_hw_break(void)
> hw_breakpoint_restore();
> }
>
> +static int hw_break_reserve_slot(int breakno)
> +{
> + int cpu;
> + int cnt = 0;
> + struct perf_event **pevent;
> +
> + for_each_online_cpu(cpu) {
> + cnt++;
> + pevent = per_cpu_ptr(breakinfo[breakno].pev, cpu);
> + if (dbg_reserve_bp_slot(*pevent))
> + goto fail;
> + }
> +
> + return 0;
> +
> +fail:
> + for_each_online_cpu(cpu) {
> + cnt--;
> + if (!cnt)
> + break;
> + pevent = per_cpu_ptr(breakinfo[breakno].pev, cpu);
> + dbg_release_bp_slot(*pevent);
> + }
> + return -1;
> +}
> +
> +static int hw_break_release_slot(int breakno)
> +{
> + struct perf_event **pevent;
> + int ret;
> + int cpu;
> +
> + for_each_online_cpu(cpu) {
> + pevent = per_cpu_ptr(breakinfo[breakno].pev, cpu);
> + ret = dbg_release_bp_slot(*pevent);
So, you are missing some return errors there. Actually, a slot
release shouldn't return an error.
> +/*
> + * Allow the kernel debugger to reserve breakpoint slots without
> + * taking a lock using the dbg_* variant of for the reserve and
> + * release breakpoint slots.
> + */
> +int dbg_reserve_bp_slot(struct perf_event *bp)
> +{
> + if (mutex_is_locked(&nr_bp_mutex))
> + return -1;
> +
> + return __reserve_bp_slot(bp);
> +}
> +
> +int dbg_release_bp_slot(struct perf_event *bp)
> +{
> + if (mutex_is_locked(&nr_bp_mutex))
> + return -1;
> +
> + __release_bp_slot(bp);
> +
> + return 0;
> +}
Ok, best effort fits well for reserve, but is certainly not
suitable for release. We can't leave a fake occupied slot like
this. If it fails, we should do this asynchronously, using the
usual release_bp_slot, may be toward the workqueues.
>
> int register_perf_hw_breakpoint(struct perf_event *bp)
> {
> --
> 1.6.4.rc1
>
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