* [RFC PATCH] ARM, ARM64, LOONGARCH: Delay HW BP notification to task_work()
@ 2026-07-13 14:49 Sebastian Andrzej Siewior
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From: Sebastian Andrzej Siewior @ 2026-07-13 14:49 UTC (permalink / raw)
To: Luis Claudio R. Goncalves
Cc: Mark Rutland, Ian Rogers, Alexander Shishkin, Catalin Marinas,
linux-rt-devel, linux-kernel, WANG Xuerui, Will Deacon,
Huacai Chen, Russell King, Peter Zijlstra, Ingo Molnar,
Waiman Long, Clark Williams, James Clark, Steven Rostedt,
Arnaldo Carvalho de Melo, loongarch, Namhyung Kim, Adrian Hunter,
linux-arm-kernel, Oleg Nesterov, linux-perf-users, Jiri Olsa
Waiman, Luis, Ada reported that HW breakpoints on ARM64 trigger
"sleeping while atomic" warnings on PREEMPT_RT. The hardware event is
delivered with disabled interrupts and perf intrastrucure expects
disabled interrupts while the overflow callback is invoked.
The callback then sends a SIGTRAP signal for which it acquires
sighand_struct::siglock, a spinlock_t which becomes a sleeping lock and
must not be acquired in atomic context.
Delay the event callback until the return to userland.
Add perf_arch_hwbp_notify(), a generic perf callback which delayes the
actual callback invocation to task_work_add() callback. This callback
invokes the architecture defines callback arch_hwbp_send_sig().
This requires struct callback_head and the functions require
ARCH_NEED_PERF_HW_NOTIF to be defined.
This was reported against ARM64. ARM and LongARCH follow the same
pattern are also converted.
Reported-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Reported-by: Waiman Long <longman@redhat.com>
Closes: https://lore.kernel.org/all/aho0eqjMESuHxECr@redhat.com/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
This is approximately the fifth iteration at which point I think I can
share what I have.
Could this be tested, please? I tested it on arm64 for arm64 based on
Luis' test. ARM compiles my HW lacks support for this I think (gdb
aborts early). I hope the best for LoongARCH.
Are the perf bits okay or is it too gross?
arch/arm/include/asm/hw_breakpoint.h | 1 +
arch/arm/kernel/ptrace.c | 6 ++----
arch/arm64/include/asm/hw_breakpoint.h | 1 +
arch/arm64/kernel/ptrace.c | 6 ++----
arch/loongarch/include/asm/hw_breakpoint.h | 1 +
arch/loongarch/kernel/ptrace.c | 6 ++----
include/linux/hw_breakpoint.h | 3 +++
include/linux/perf_event.h | 3 +++
kernel/events/core.c | 24 ++++++++++++++++++++++
9 files changed, 39 insertions(+), 12 deletions(-)
diff --git a/arch/arm/include/asm/hw_breakpoint.h b/arch/arm/include/asm/hw_breakpoint.h
index e7f9961c53b2d..5b3a373348ff5 100644
--- a/arch/arm/include/asm/hw_breakpoint.h
+++ b/arch/arm/include/asm/hw_breakpoint.h
@@ -7,6 +7,7 @@
struct task_struct;
#ifdef CONFIG_HAVE_HW_BREAKPOINT
+#define ARCH_NEED_PERF_HW_NOTIF
struct arch_hw_breakpoint_ctrl {
u32 __reserved : 9,
diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index 7951b2c06fec6..1650f12a04702 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -347,9 +347,7 @@ static long ptrace_hbp_idx_to_num(int idx)
/*
* Handle hitting a HW-breakpoint.
*/
-static void ptrace_hbptriggered(struct perf_event *bp,
- struct perf_sample_data *data,
- struct pt_regs *regs)
+void arch_hwbp_send_sig(struct perf_event *bp)
{
struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
long num;
@@ -424,7 +422,7 @@ static struct perf_event *ptrace_hbp_create(struct task_struct *tsk, int type)
attr.bp_type = type;
attr.disabled = 1;
- return register_user_hw_breakpoint(&attr, ptrace_hbptriggered, NULL,
+ return register_user_hw_breakpoint(&attr, perf_arch_hwbp_notify, NULL,
tsk);
}
diff --git a/arch/arm64/include/asm/hw_breakpoint.h b/arch/arm64/include/asm/hw_breakpoint.h
index bd81cf17744af..58befb79c885c 100644
--- a/arch/arm64/include/asm/hw_breakpoint.h
+++ b/arch/arm64/include/asm/hw_breakpoint.h
@@ -124,6 +124,7 @@ extern void hw_breakpoint_pmu_read(struct perf_event *bp);
extern int hw_breakpoint_slots(int type);
#ifdef CONFIG_HAVE_HW_BREAKPOINT
+#define ARCH_NEED_PERF_HW_NOTIF
extern void hw_breakpoint_thread_switch(struct task_struct *next);
extern void ptrace_hw_copy_thread(struct task_struct *task);
#else
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 4d08598e2891d..df6122779ada6 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -168,9 +168,7 @@ void ptrace_disable(struct task_struct *child)
/*
* Handle hitting a HW-breakpoint.
*/
-static void ptrace_hbptriggered(struct perf_event *bp,
- struct perf_sample_data *data,
- struct pt_regs *regs)
+void arch_hwbp_send_sig(struct perf_event *bp)
{
struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
const char *desc = "Hardware breakpoint trap (ptrace)";
@@ -312,7 +310,7 @@ static struct perf_event *ptrace_hbp_create(unsigned int note_type,
attr.bp_type = type;
attr.disabled = 1;
- bp = register_user_hw_breakpoint(&attr, ptrace_hbptriggered, NULL, tsk);
+ bp = register_user_hw_breakpoint(&attr, perf_arch_hwbp_notify, NULL, tsk);
if (IS_ERR(bp))
return bp;
diff --git a/arch/loongarch/include/asm/hw_breakpoint.h b/arch/loongarch/include/asm/hw_breakpoint.h
index 5faa97a87a9e2..2b51922eb5492 100644
--- a/arch/loongarch/include/asm/hw_breakpoint.h
+++ b/arch/loongarch/include/asm/hw_breakpoint.h
@@ -120,6 +120,7 @@ void breakpoint_handler(struct pt_regs *regs);
void watchpoint_handler(struct pt_regs *regs);
#ifdef CONFIG_HAVE_HW_BREAKPOINT
+#define ARCH_NEED_PERF_HW_NOTIF
extern void ptrace_hw_copy_thread(struct task_struct *task);
extern void hw_breakpoint_thread_switch(struct task_struct *next);
#else
diff --git a/arch/loongarch/kernel/ptrace.c b/arch/loongarch/kernel/ptrace.c
index be38430f7e280..1d5e05a70b8d3 100644
--- a/arch/loongarch/kernel/ptrace.c
+++ b/arch/loongarch/kernel/ptrace.c
@@ -384,9 +384,7 @@ static int lbt_set(struct task_struct *target,
/*
* Handle hitting a HW-breakpoint.
*/
-static void ptrace_hbptriggered(struct perf_event *bp,
- struct perf_sample_data *data,
- struct pt_regs *regs)
+void arch_hwbp_send_sig(struct perf_event *bp)
{
int i;
struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
@@ -479,7 +477,7 @@ static struct perf_event *ptrace_hbp_create(unsigned int note_type,
attr.bp_type = type;
attr.disabled = 1;
- bp = register_user_hw_breakpoint(&attr, ptrace_hbptriggered, NULL, tsk);
+ bp = register_user_hw_breakpoint(&attr, perf_arch_hwbp_notify, NULL, tsk);
if (IS_ERR(bp))
return bp;
diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
index db199d653dd1a..bb4c1064a103d 100644
--- a/include/linux/hw_breakpoint.h
+++ b/include/linux/hw_breakpoint.h
@@ -85,6 +85,9 @@ extern int register_perf_hw_breakpoint(struct perf_event *bp);
extern void unregister_hw_breakpoint(struct perf_event *bp);
extern void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events);
extern bool hw_breakpoint_is_used(void);
+extern void arch_hwbp_send_sig(struct perf_event *bp);
+extern void perf_arch_hwbp_notify(struct perf_event *bp, struct perf_sample_data *data,
+ struct pt_regs *regs);
extern int dbg_reserve_bp_slot(struct perf_event *bp);
extern int dbg_release_bp_slot(struct perf_event *bp);
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 48d851fbd8ea5..bc7fae88a2742 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -215,6 +215,9 @@ struct hw_perf_event {
/* Last sync'ed generation of filters */
unsigned long addr_filters_gen;
+#ifdef ARCH_NEED_PERF_HW_NOTIF
+ struct callback_head arch_hw_notif;
+#endif
/*
* hw_perf_event::state flags; used to track the PERF_EF_* state.
diff --git a/kernel/events/core.c b/kernel/events/core.c
index d7f3e2c2ecb1e..1f70cdb52f068 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13336,6 +13336,26 @@ static void account_event(struct perf_event *event)
account_pmu_sb_event(event);
}
+#ifdef ARCH_NEED_PERF_HW_NOTIF
+static void perf_arch_hwbp_send_sig(struct callback_head *head)
+{
+ struct perf_event *bp;
+
+ bp = container_of(head, struct perf_event, hw.arch_hw_notif);
+ arch_hwbp_send_sig(bp);
+ put_event(bp);
+}
+
+void perf_arch_hwbp_notify(struct perf_event *bp, struct perf_sample_data *data,
+ struct pt_regs *regs)
+{
+ if (WARN_ON_ONCE(!atomic_long_inc_not_zero(&bp->refcount)))
+ return;
+ if (WARN_ON_ONCE(task_work_add(current, &bp->hw.arch_hw_notif, TWA_RESUME)))
+ put_event(bp);
+}
+#endif
+
/*
* Allocate and initialize an event structure
*/
@@ -13441,6 +13461,10 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
}
if (overflow_handler) {
+#ifdef ARCH_NEED_PERF_HW_NOTIF
+ if (overflow_handler == perf_arch_hwbp_notify)
+ init_task_work(&event->hw.arch_hw_notif, perf_arch_hwbp_send_sig);
+#endif
event->overflow_handler = overflow_handler;
event->overflow_handler_context = context;
} else if (is_write_backward(event)){
--
2.53.0
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