[PATCH] arm64: kgdb: fix single stepping
Vijay Kilari
vijay.kilari at gmail.com
Mon Oct 27 05:45:17 PDT 2014
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.
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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