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Date: Thu, 28 Jan 2010 11:49:14 -0600
From: Jason Wessel <jason.wessel@...driver.com>
To: Frederic Weisbecker <fweisbec@...il.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 kerneldebugger
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@...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.
>
>
>
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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