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Subject[PATCH v2 2/2] kgdb: Fix kgdb_roundup_cpus() for arches who used smp_call_function()
Date
When I had lockdep turned on and dropped into kgdb I got a nice splat
on my system. Specifically it hit:
DEBUG_LOCKS_WARN_ON(current->hardirq_context)

Specifically it looked like this:
sysrq: SysRq : DEBUG
------------[ cut here ]------------
DEBUG_LOCKS_WARN_ON(current->hardirq_context)
WARNING: CPU: 0 PID: 0 at .../kernel/locking/lockdep.c:2875 lockdep_hardirqs_on+0xf0/0x160
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.19.0 #27
pstate: 604003c9 (nZCv DAIF +PAN -UAO)
pc : lockdep_hardirqs_on+0xf0/0x160
...
Call trace:
lockdep_hardirqs_on+0xf0/0x160
trace_hardirqs_on+0x188/0x1ac
kgdb_roundup_cpus+0x14/0x3c
kgdb_cpu_enter+0x53c/0x5cc
kgdb_handle_exception+0x180/0x1d4
kgdb_compiled_brk_fn+0x30/0x3c
brk_handler+0x134/0x178
do_debug_exception+0xfc/0x178
el1_dbg+0x18/0x78
kgdb_breakpoint+0x34/0x58
sysrq_handle_dbg+0x54/0x5c
__handle_sysrq+0x114/0x21c
handle_sysrq+0x30/0x3c
qcom_geni_serial_isr+0x2dc/0x30c
...
...
irq event stamp: ...45
hardirqs last enabled at (...44): [...] __do_softirq+0xd8/0x4e4
hardirqs last disabled at (...45): [...] el1_irq+0x74/0x130
softirqs last enabled at (...42): [...] _local_bh_enable+0x2c/0x34
softirqs last disabled at (...43): [...] irq_exit+0xa8/0x100
---[ end trace adf21f830c46e638 ]---

Looking closely at it, it seems like a really bad idea to be calling
local_irq_enable() in kgdb_roundup_cpus(). If nothing else that seems
like it could violate spinlock semantics and cause a deadlock.

Instead, let's use a private csd alongside
smp_call_function_single_async() to round up the other CPUs. Using
smp_call_function_single_async() doesn't require interrupts to be
enabled so we can remove the offending bit of code.

In order to avoid duplicating this across all the architectures that
use the default kgdb_roundup_cpus(), we'll add a "weak" implementation
to debug_core.c.

Looking at all the people who previously had copies of this code,
there were a few variants. I've attempted to keep the variants
working like they used to. Specifically:
* For arch/arc we passed NULL to kgdb_nmicallback() instead of
get_irq_regs().
* For arch/mips there was a bit of extra code around
kgdb_nmicallback()

Suggested-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v2:
- Removing irq flags separated from fixing lockdep splat.
- Don't use smp_call_function (Daniel).

arch/arc/kernel/kgdb.c | 10 ++--------
arch/arm/kernel/kgdb.c | 12 ------------
arch/arm64/kernel/kgdb.c | 12 ------------
arch/hexagon/kernel/kgdb.c | 27 ---------------------------
arch/mips/kernel/kgdb.c | 9 +--------
arch/powerpc/kernel/kgdb.c | 4 ++--
arch/sh/kernel/kgdb.c | 12 ------------
include/linux/kgdb.h | 15 +++++++++++++--
kernel/debug/debug_core.c | 36 ++++++++++++++++++++++++++++++++++++
9 files changed, 54 insertions(+), 83 deletions(-)

diff --git a/arch/arc/kernel/kgdb.c b/arch/arc/kernel/kgdb.c
index 0932851028e0..68d9fe4b5aa7 100644
--- a/arch/arc/kernel/kgdb.c
+++ b/arch/arc/kernel/kgdb.c
@@ -192,18 +192,12 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long ip)
instruction_pointer(regs) = ip;
}

-static void kgdb_call_nmi_hook(void *ignored)
+void kgdb_call_nmi_hook(void *ignored)
{
+ /* Default implementation passes get_irq_regs() but we don't */
kgdb_nmicallback(raw_smp_processor_id(), NULL);
}

-void kgdb_roundup_cpus(void)
-{
- local_irq_enable();
- smp_call_function(kgdb_call_nmi_hook, NULL, 0);
- local_irq_disable();
-}
-
struct kgdb_arch arch_kgdb_ops = {
/* breakpoint instruction: TRAP_S 0x3 */
#ifdef CONFIG_CPU_BIG_ENDIAN
diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c
index f21077b077be..d9a69e941463 100644
--- a/arch/arm/kernel/kgdb.c
+++ b/arch/arm/kernel/kgdb.c
@@ -170,18 +170,6 @@ static struct undef_hook kgdb_compiled_brkpt_hook = {
.fn = kgdb_compiled_brk_fn
};

-static void kgdb_call_nmi_hook(void *ignored)
-{
- kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
-}
-
-void kgdb_roundup_cpus(void)
-{
- local_irq_enable();
- smp_call_function(kgdb_call_nmi_hook, NULL, 0);
- local_irq_disable();
-}
-
static int __kgdb_notify(struct die_args *args, unsigned long cmd)
{
struct pt_regs *regs = args->regs;
diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
index 12c339ff6e75..da880247c734 100644
--- a/arch/arm64/kernel/kgdb.c
+++ b/arch/arm64/kernel/kgdb.c
@@ -284,18 +284,6 @@ static struct step_hook kgdb_step_hook = {
.fn = kgdb_step_brk_fn
};

-static void kgdb_call_nmi_hook(void *ignored)
-{
- kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
-}
-
-void kgdb_roundup_cpus(void)
-{
- local_irq_enable();
- smp_call_function(kgdb_call_nmi_hook, NULL, 0);
- local_irq_disable();
-}
-
static int __kgdb_notify(struct die_args *args, unsigned long cmd)
{
struct pt_regs *regs = args->regs;
diff --git a/arch/hexagon/kernel/kgdb.c b/arch/hexagon/kernel/kgdb.c
index 012e0e230ac2..b95d12038a4e 100644
--- a/arch/hexagon/kernel/kgdb.c
+++ b/arch/hexagon/kernel/kgdb.c
@@ -115,33 +115,6 @@ void kgdb_arch_set_pc(struct pt_regs *regs, unsigned long pc)
instruction_pointer(regs) = pc;
}

-#ifdef CONFIG_SMP
-
-/**
- * kgdb_roundup_cpus - Get other CPUs into a holding pattern
- *
- * On SMP systems, we need to get the attention of the other CPUs
- * and get them be in a known state. This should do what is needed
- * to get the other CPUs to call kgdb_wait(). Note that on some arches,
- * the NMI approach is not used for rounding up all the CPUs. For example,
- * in case of MIPS, smp_call_function() is used to roundup CPUs.
- *
- * On non-SMP systems, this is not called.
- */
-
-static void hexagon_kgdb_nmi_hook(void *ignored)
-{
- kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
-}
-
-void kgdb_roundup_cpus(void)
-{
- local_irq_enable();
- smp_call_function(hexagon_kgdb_nmi_hook, NULL, 0);
- local_irq_disable();
-}
-#endif
-

/* Not yet working */
void sleeping_thread_to_gdb_regs(unsigned long *gdb_regs,
diff --git a/arch/mips/kernel/kgdb.c b/arch/mips/kernel/kgdb.c
index 2b05effc17b4..42f057a6c215 100644
--- a/arch/mips/kernel/kgdb.c
+++ b/arch/mips/kernel/kgdb.c
@@ -207,7 +207,7 @@ void arch_kgdb_breakpoint(void)
".set\treorder");
}

-static void kgdb_call_nmi_hook(void *ignored)
+void kgdb_call_nmi_hook(void *ignored)
{
mm_segment_t old_fs;

@@ -219,13 +219,6 @@ static void kgdb_call_nmi_hook(void *ignored)
set_fs(old_fs);
}

-void kgdb_roundup_cpus(void)
-{
- local_irq_enable();
- smp_call_function(kgdb_call_nmi_hook, NULL, 0);
- local_irq_disable();
-}
-
static int compute_signal(int tt)
{
struct hard_trap_info *ht;
diff --git a/arch/powerpc/kernel/kgdb.c b/arch/powerpc/kernel/kgdb.c
index b0e804844be0..b4ce54d73337 100644
--- a/arch/powerpc/kernel/kgdb.c
+++ b/arch/powerpc/kernel/kgdb.c
@@ -117,7 +117,7 @@ int kgdb_skipexception(int exception, struct pt_regs *regs)
return kgdb_isremovedbreak(regs->nip);
}

-static int kgdb_call_nmi_hook(struct pt_regs *regs)
+static int kgdb_debugger_ipi(struct pt_regs *regs)
{
kgdb_nmicallback(raw_smp_processor_id(), regs);
return 0;
@@ -502,7 +502,7 @@ int kgdb_arch_init(void)
old__debugger_break_match = __debugger_break_match;
old__debugger_fault_handler = __debugger_fault_handler;

- __debugger_ipi = kgdb_call_nmi_hook;
+ __debugger_ipi = kgdb_debugger_ipi;
__debugger = kgdb_debugger;
__debugger_bpt = kgdb_handle_breakpoint;
__debugger_sstep = kgdb_singlestep;
diff --git a/arch/sh/kernel/kgdb.c b/arch/sh/kernel/kgdb.c
index cc57630f6bf2..14e012ad7c57 100644
--- a/arch/sh/kernel/kgdb.c
+++ b/arch/sh/kernel/kgdb.c
@@ -314,18 +314,6 @@ BUILD_TRAP_HANDLER(singlestep)
local_irq_restore(flags);
}

-static void kgdb_call_nmi_hook(void *ignored)
-{
- kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
-}
-
-void kgdb_roundup_cpus(void)
-{
- local_irq_enable();
- smp_call_function(kgdb_call_nmi_hook, NULL, 0);
- local_irq_disable();
-}
-
static int __kgdb_notify(struct die_args *args, unsigned long cmd)
{
int ret;
diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
index 05e5b2eb0d32..f4f866273ed2 100644
--- a/include/linux/kgdb.h
+++ b/include/linux/kgdb.h
@@ -176,14 +176,25 @@ kgdb_arch_handle_exception(int vector, int signo, int err_code,
char *remcom_out_buffer,
struct pt_regs *regs);

+/**
+ * kgdb_call_nmi_hook - Call kgdb_nmicallback() on the current CPU
+ *
+ * If you're using the default implementation of kgdb_roundup_cpus()
+ * this function will be called per CPU. If you don't implement
+ * kgdb_call_nmi_hook() a default will be used.
+ */
+
+extern void kgdb_call_nmi_hook(void *ignored);
+
+
/**
* kgdb_roundup_cpus - Get other CPUs into a holding pattern
*
* On SMP systems, we need to get the attention of the other CPUs
* and get them into a known state. This should do what is needed
* to get the other CPUs to call kgdb_wait(). Note that on some arches,
- * the NMI approach is not used for rounding up all the CPUs. For example,
- * in case of MIPS, smp_call_function() is used to roundup CPUs.
+ * the NMI approach is not used for rounding up all the CPUs. Normally
+ * those architectures can just not implement this and get the default.
*
* On non-SMP systems, this is not called.
*/
diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
index f3cadda45f07..9a3f952de6ed 100644
--- a/kernel/debug/debug_core.c
+++ b/kernel/debug/debug_core.c
@@ -55,6 +55,7 @@
#include <linux/mm.h>
#include <linux/vmacache.h>
#include <linux/rcupdate.h>
+#include <linux/irq.h>

#include <asm/cacheflush.h>
#include <asm/byteorder.h>
@@ -220,6 +221,39 @@ int __weak kgdb_skipexception(int exception, struct pt_regs *regs)
return 0;
}

+/*
+ * Default (weak) implementation for kgdb_roundup_cpus
+ */
+
+static DEFINE_PER_CPU(call_single_data_t, kgdb_roundup_csd);
+
+void __weak kgdb_call_nmi_hook(void *ignored)
+{
+ kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
+}
+
+void __weak kgdb_roundup_cpus(void)
+{
+ call_single_data_t *csd;
+ int cpu;
+
+ for_each_cpu(cpu, cpu_online_mask) {
+ csd = &per_cpu(kgdb_roundup_csd, cpu);
+ smp_call_function_single_async(cpu, csd);
+ }
+}
+
+static void kgdb_generic_roundup_init(void)
+{
+ call_single_data_t *csd;
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ csd = &per_cpu(kgdb_roundup_csd, cpu);
+ csd->func = kgdb_call_nmi_hook;
+ }
+}
+
/*
* Some architectures need cache flushes when we set/clear a
* breakpoint:
@@ -993,6 +1027,8 @@ int kgdb_register_io_module(struct kgdb_io *new_dbg_io_ops)
return -EBUSY;
}

+ kgdb_generic_roundup_init();
+
if (new_dbg_io_ops->init) {
err = new_dbg_io_ops->init();
if (err) {
--
2.19.1.568.g152ad8e336-goog
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