Re: [PATCH 3/3] perf,hw_breakpoint,kgdb: No mutex taken for kerneldebugger

[Jason Wessel]

Frederic Weisbecker wrote:
> 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@xxxxxxxxx>
>> CC: Ingo Molnar <mingo@xxxxxxx>
>> CC: K.Prasad <prasad@xxxxxxxxxxxxxxxxxx>
>> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> CC: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
>> Signed-off-by: Jason Wessel <jason.wessel@xxxxxxxxxxxxx>
>> ---
>>  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.
>
>
>   

This is a trick so to speak.   Either all the slot releases will return
0 or -1 depending on if the mutex is available, so it is not really
missed. 

Certainly I can change this to just quit immediately on 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.
>
>
>
>   

If it fails the debugger tried to remove it again later.   It seems to
me like it is a don't care corner case.   You get a printk if it ever
does happen (which it really shouldn't).


>   
>>  
>>  int register_perf_hw_breakpoint(struct perf_event *bp)
>>  {
>> -- 
>> 1.6.4.rc1
>>
>>     
>
>   

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