linux-kernel - Re: [PATCH 3/3] perf,hw_breakpoint,kgdb: No mutex taken for kernel debugger

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Date:	Thu, 28 Jan 2010 18:33:10 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jason Wessel <jason.wessel@...driver.com>
Cc:	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	mingo@...e.hu, "K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH 3/3] perf,hw_breakpoint,kgdb: No mutex taken for kernel
	debugger

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 <fweisbec@...il.com>
> CC: Ingo Molnar <mingo@...e.hu>
> CC: K.Prasad <prasad@...ux.vnet.ibm.com>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Alan Stern <stern@...land.harvard.edu>
> Signed-off-by: Jason Wessel <jason.wessel@...driver.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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