[PATCH] arm64: kgdb: fix single stepping
AKASHI Takahiro
takahiro.akashi at linaro.org
Sun Nov 9 17:42:14 PST 2014
Vijay,
On 11/07/2014 10:10 PM, Vijay Kilari wrote:
> Hi Akashil,
>
>
>
> On Tue, Oct 28, 2014 at 5:48 AM, AKASHI Takahiro
> <takahiro.akashi at linaro.org> wrote:
>> Vijay,
>>
>> On 10/27/2014 09:45 PM, Vijay Kilari wrote:
>>>
>>> Hi Akashi,
>>>
>>> I could not reproduce this with my simulator.
>>> It would be good if you could post result of KGDB test suite.
>>>
>>> From sysfs you can launch using below command:
>>>
>>> echo V1F1000 > /sys/module/kgdbts/parameters/kgdbts
>>> or enable boot test.
>>
>>
>> V1F1000 doesn't reveal this issue even on my environment (FVP_VE_AEM8A),
>> but I can easily reproduce it with vanilla v3.15:
>> (gdb) b sys_sync
>> (gdb) c
>> At the target,
>> # sync
>> At gdb,
>> (gdb) info reg pc
>> (gdb) si
>> (gdb) info reg pc <= (a)
>> (gdb) si
>> (gdb) info reg pc <= (b)
>>
>> Here you can see that you are still at the same address as (a), that is
>> <sys_sync+4>
>> and you can never go forward with stepi.
>>
>> Please try these steps and let me know the result.
Have you tried these steps without my patch?
It will take only a few minutes for you to do so.
> Are you testing on hardware or simulator?
Fast Model (FVP VE and Base)
> On my simulator arm64 I see warnings in boot test
what kind of messages?
What I saw at boot test (KGDB_TESTS_ON_BOOT) with my patch applied to v3.18-rc3 was:
> kgdb: Registered I/O driver kgdbts.
> kgdbts:RUN plant and detach test
> kgdbts:RUN sw breakpoint test
> kgdbts:RUN bad memory access test
> kgdbts:RUN singlestep test 1000 iterations
> kgdbts:RUN singlestep [0/1000]
> kgdbts:RUN singlestep [100/1000]
> kgdbts:RUN singlestep [200/1000]
> kgdbts:RUN singlestep [300/1000]
> kgdbts:RUN singlestep [400/1000]
> kgdbts:RUN singlestep [500/1000]
> kgdbts:RUN singlestep [600/1000]
> kgdbts:RUN singlestep [700/1000]
> kgdbts:RUN singlestep [800/1000]
> kgdbts:RUN singlestep [900/1000]
> kgdbts:RUN do_fork for 100 breakpoints
NO warnings.
-Takahiro AKASHI
>>
>> -Takahiro AKASHI
>>
>>
>>
>>> Other than that it is OK.
>>>
>>>
>>> On Mon, Oct 27, 2014 at 4:04 PM, Will Deacon <will.deacon at arm.com> wrote:
>>>>
>>>> On Tue, Oct 21, 2014 at 06:07:30AM +0100, AKASHI Takahiro wrote:
>>>>>
>>>>> I tried to verify kgdb in vanilla kernel on fast model, but it seems
>>>>> that
>>>>> the single stepping with kgdb doesn't work correctly since its first
>>>>> appearance at v3.15.
>>>>>
>>>>> On v3.15, 'stepi' command after breaking the kernel at some breakpoint
>>>>> steps forward to the next instruction, but the succeeding 'stepi' never
>>>>> goes beyond that.
>>>>> On v3.16, 'stepi' moves forward and stops at the next instruction just
>>>>> after enable_dbg in el1_dbg, and never goes beyond that. This variance
>>>>> of
>>>>> behavior seems to come in with the following patch in v3.16:
>>>>>
>>>>> commit 2a2830703a23 ("arm64: debug: avoid accessing mdscr_el1 on
>>>>> fault
>>>>> paths where possible")
>>>>>
>>>>> This patch
>>>>> (1) moves kgdb_disable_single_step() from 'c' command handling to single
>>>>> step handler.
>>>>> This makes sure that single stepping gets effective at every 's'
>>>>> command.
>>>>> Please note that, under the current implementation, single step bit
>>>>> in
>>>>> spsr, which is cleared by the first single stepping, will not be
>>>>> set
>>>>> again for the consecutive 's' commands because single step bit in
>>>>> mdscr
>>>>> is still kept on (that is, kernel_active_single_step() in
>>>>> kgdb_arch_handle_exception() is true).
>>>>> (2) re-implements kgdb_roundup_cpus() because the current implementation
>>>>> enabled interrupts naively. See below.
>>>>> (3) removes 'enable_dbg' in el1_dbg.
>>>>> Single step bit in mdscr is turned on in do_handle_exception()->
>>>>> kgdb_handle_expection() before returning to debugged context, and
>>>>> if
>>>>> debug exception is enabled in el1_dbg, we will see unexpected
>>>>> single-
>>>>> stepping in el1_dbg.
>>>>> Since v3.18, the following patch does the same:
>>>>> commit 1059c6bf8534 ("arm64: debug: don't re-enable debug
>>>>> exceptions
>>>>> on return from el1_dbg)
>>>>> (4) masks interrupts while single-stepping one instruction.
>>>>> If an interrupt is caught during processing a single-stepping,
>>>>> debug
>>>>> exception is unintentionally enabled by el1_irq's 'enable_dbg'
>>>>> before
>>>>> returning to debugged context.
>>>>> Thus, like in (2), we will see unexpected single-stepping in
>>>>> el1_irq.
>>>>>
>>>>> Basically (1) and (2) are for v3.15, (3) and (4) for v3.1[67].
>>>>
>>>>
>>>> (3) was CC'd for stable, so I don't think you need to mention that here.
>>>>
>>>> I'd like an ack from a KGDB person before I take this via the arm64 tree.
>>>>
>>>> Will
>>>>
>>>>>
>>>>> * issue fixed by (2):
>>>>> Without (2), we would see another problem if a breakpoint is set at
>>>>> interrupt-sensible places, like gic_handle_irq():
>>>>>
>>>>> KGDB: re-enter error: breakpoint removed ffffffc000081258
>>>>> ------------[ cut here ]------------
>>>>> WARNING: CPU: 0 PID: 650 at kernel/debug/debug_core.c:435
>>>>>
>>>>> kgdb_handle_exception+0x1dc/0x1f4()
>>>>> Modules linked in:
>>>>> CPU: 0 PID: 650 Comm: sh Not tainted 3.17.0-rc2+ #177
>>>>> Call trace:
>>>>> [<ffffffc000087fac>] dump_backtrace+0x0/0x130
>>>>> [<ffffffc0000880ec>] show_stack+0x10/0x1c
>>>>> [<ffffffc0004d683c>] dump_stack+0x74/0xb8
>>>>> [<ffffffc0000ab824>] warn_slowpath_common+0x8c/0xb4
>>>>> [<ffffffc0000ab90c>] warn_slowpath_null+0x14/0x20
>>>>> [<ffffffc000121bfc>] kgdb_handle_exception+0x1d8/0x1f4
>>>>> [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28
>>>>> [<ffffffc0000821c8>] brk_handler+0x9c/0xe8
>>>>> [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac
>>>>> Exception stack(0xffffffc07e027650 to 0xffffffc07e027770)
>>>>> ...
>>>>> [<ffffffc000083cac>] el1_dbg+0x14/0x68
>>>>> [<ffffffc00012178c>] kgdb_cpu_enter+0x464/0x5c0
>>>>> [<ffffffc000121bb4>] kgdb_handle_exception+0x190/0x1f4
>>>>> [<ffffffc000092ffc>] kgdb_brk_fn+0x18/0x28
>>>>> [<ffffffc0000821c8>] brk_handler+0x9c/0xe8
>>>>> [<ffffffc0000811e8>] do_debug_exception+0x3c/0xac
>>>>> Exception stack(0xffffffc07e027ac0 to 0xffffffc07e027be0)
>>>>> ...
>>>>> [<ffffffc000083cac>] el1_dbg+0x14/0x68
>>>>> [<ffffffc00032e4b4>] __handle_sysrq+0x11c/0x190
>>>>> [<ffffffc00032e93c>] write_sysrq_trigger+0x4c/0x60
>>>>> [<ffffffc0001e7d58>] proc_reg_write+0x54/0x84
>>>>> [<ffffffc000192fa4>] vfs_write+0x98/0x1c8
>>>>> [<ffffffc0001939b0>] SyS_write+0x40/0xa0
>>>>>
>>>>> Once some interrupt occurs, a breakpoint at gic_handle_irq() triggers
>>>>> kgdb.
>>>>> Kgdb then calls kgdb_roundup_cpus() to sync with other cpus.
>>>>> Current kgdb_roundup_cpus() unmasks interrupts temporarily to
>>>>> use smp_call_function().
>>>>> This eventually allows another interrupt to occur and likely results in
>>>>> hitting a breakpoint at gic_handle_irq() again since debug exception is
>>>>> always enabled in el1_irq.
>>>>>
>>>>> We can avoid this issue by specifying "nokgdbroundup" in kernel
>>>>> parameter,
>>>>> but this will also leave other cpus be in unknown state in terms of
>>>>> kgdb,
>>>>> and may result in interfering with kgdb activity.
>>>>>
>>>>> Signed-off-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
>>>>> ---
>>>>> arch/arm64/kernel/kgdb.c | 60
>>>>> +++++++++++++++++++++++++++++++++++-----------
>>>>> 1 file changed, 46 insertions(+), 14 deletions(-)
>>>>>
>>>>> diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
>>>>> index a0d10c5..81b5910 100644
>>>>> --- a/arch/arm64/kernel/kgdb.c
>>>>> +++ b/arch/arm64/kernel/kgdb.c
>>>>> @@ -19,9 +19,13 @@
>>>>> * along with this program. If not, see
>>>>> <http://www.gnu.org/licenses/>.
>>>>> */
>>>>>
>>>>> +#include <linux/cpumask.h>
>>>>> #include <linux/irq.h>
>>>>> +#include <linux/irq_work.h>
>>>>> #include <linux/kdebug.h>
>>>>> #include <linux/kgdb.h>
>>>>> +#include <linux/percpu.h>
>>>>> +#include <asm/ptrace.h>
>>>>> #include <asm/traps.h>
>>>>>
>>>>> struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
>>>>> @@ -95,6 +99,9 @@ struct dbg_reg_def_t dbg_reg_def[DBG_MAX_REG_NUM] = {
>>>>> { "fpcr", 4, -1 },
>>>>> };
>>>>>
>>>>> +static DEFINE_PER_CPU(unsigned int, kgdb_pstate);
>>>>> +static DEFINE_PER_CPU(struct irq_work, kgdb_irq_work);
>>>>> +
>>>>> char *dbg_get_reg(int regno, void *mem, struct pt_regs *regs)
>>>>> {
>>>>> if (regno >= DBG_MAX_REG_NUM || regno < 0)
>>>>> @@ -176,18 +183,14 @@ int kgdb_arch_handle_exception(int
>>>>> exception_vector, int signo,
>>>>> * over and over again.
>>>>> */
>>>>> kgdb_arch_update_addr(linux_regs, remcom_in_buffer);
>>>>> - atomic_set(&kgdb_cpu_doing_single_step, -1);
>>>>> - kgdb_single_step = 0;
>>>>> -
>>>>> - /*
>>>>> - * Received continue command, disable single step
>>>>> - */
>>>>> - if (kernel_active_single_step())
>>>>> - kernel_disable_single_step();
>>>>>
>>>>> err = 0;
>>>>> break;
>>>>> case 's':
>>>>> + /* mask interrupts while single stepping */
>>>>> + __this_cpu_write(kgdb_pstate, linux_regs->pstate);
>>>>> + linux_regs->pstate |= PSR_I_BIT;
>>>>> +
>>>>> /*
>>>>> * Update step address value with address passed
>>>>> * with step packet.
>>>>> @@ -198,8 +201,6 @@ int kgdb_arch_handle_exception(int exception_vector,
>>>>> int signo,
>>>>> */
>>>>> kgdb_arch_update_addr(linux_regs, remcom_in_buffer);
>>>>> atomic_set(&kgdb_cpu_doing_single_step,
>>>>> raw_smp_processor_id());
>>>>> - kgdb_single_step = 1;
>>>>> -
>>>>> /*
>>>>> * Enable single step handling
>>>>> */
>>>>> @@ -229,6 +230,18 @@ static int kgdb_compiled_brk_fn(struct pt_regs
>>>>> *regs, unsigned int esr)
>>>>>
>>>>> static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned int esr)
>>>>> {
>>>>> + unsigned int pstate;
>>>>> +
>>>>> + kernel_disable_single_step();
>>>>> + atomic_set(&kgdb_cpu_doing_single_step, -1);
>>>>> +
>>>>> + /* restore interrupt mask status */
>>>>> + pstate = __this_cpu_read(kgdb_pstate);
>>>>> + if (pstate & PSR_I_BIT)
>>>>> + regs->pstate |= PSR_I_BIT;
>>>>> + else
>>>>> + regs->pstate &= ~PSR_I_BIT;
>>>>> +
>>>>> kgdb_handle_exception(1, SIGTRAP, 0, regs);
>>>>> return 0;
>>>>> }
>>>>> @@ -249,16 +262,27 @@ static struct step_hook kgdb_step_hook = {
>>>>> .fn = kgdb_step_brk_fn
>>>>> };
>>>>>
>>>>> -static void kgdb_call_nmi_hook(void *ignored)
>>>>> +static void kgdb_roundup_hook(struct irq_work *work)
>>>>> {
>>>>> kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
>>>>> }
>>>>>
>>>>> void kgdb_roundup_cpus(unsigned long flags)
>>>>> {
>>>>> - local_irq_enable();
>>>>> - smp_call_function(kgdb_call_nmi_hook, NULL, 0);
>>>>> - local_irq_disable();
>>>>> + int cpu;
>>>>> + struct cpumask mask;
>>>>> + struct irq_work *work;
>>>>> +
>>>>> + mask = *cpu_online_mask;
>>>>> + cpumask_clear_cpu(smp_processor_id(), &mask);
>>>>> + cpu = cpumask_first(&mask);
>>>>> + if (cpu >= nr_cpu_ids)
>>>>> + return;
>>>>> +
>>>>> + for_each_cpu(cpu, &mask) {
>>>>> + work = per_cpu_ptr(&kgdb_irq_work, cpu);
>>>>> + irq_work_queue_on(work, cpu);
>>>>> + }
>>>>> }
>>>>>
>>>>> static int __kgdb_notify(struct die_args *args, unsigned long cmd)
>>>>> @@ -299,6 +323,8 @@ static struct notifier_block kgdb_notifier = {
>>>>> int kgdb_arch_init(void)
>>>>> {
>>>>> int ret = register_die_notifier(&kgdb_notifier);
>>>>> + int cpu;
>>>>> + struct irq_work *work;
>>>>>
>>>>> if (ret != 0)
>>>>> return ret;
>>>>> @@ -306,6 +332,12 @@ int kgdb_arch_init(void)
>>>>> register_break_hook(&kgdb_brkpt_hook);
>>>>> register_break_hook(&kgdb_compiled_brkpt_hook);
>>>>> register_step_hook(&kgdb_step_hook);
>>>>> +
>>>>> + for_each_possible_cpu(cpu) {
>>>>> + work = per_cpu_ptr(&kgdb_irq_work, cpu);
>>>>> + init_irq_work(work, kgdb_roundup_hook);
>>>>> + }
>>>>> +
>>>>> return 0;
>>>>> }
>>>>>
>>>>> --
>>>>> 1.7.9.5
>>>>>
>>>
>>> Acked-by: Vijaya Kumar K <Vijaya.Kumar at caviumnetworks.com>
>>>
>>> Regards
>>> Vijay
>>>
>>
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