[PATCH v13 04/17] preempt: Introduce __preempt_count_{sub, add}_return()

Lyude Paul
2025-10-13T15:54:10Z

From: Boqun Feng <boqun.feng@gmail.com>

In order to use preempt_count() to tracking the interrupt disable
nesting level, __preempt_count_{add,sub}_return() are introduced, as
their name suggest, these primitives return the new value of the
preempt_count() after changing it. The following example shows the usage
of it in local_interrupt_disable():

// increase the HARDIRQ_DISABLE bit
new_count = __preempt_count_add_return(HARDIRQ_DISABLE_OFFSET);

// if it's the first-time increment, then disable the interrupt
// at hardware level.
if (new_count & HARDIRQ_DISABLE_MASK == HARDIRQ_DISABLE_OFFSET) {
local_irq_save(flags);
raw_cpu_write(local_interrupt_disable_state.flags, flags);
}

Having these primitives will avoid a read of preempt_count() after
changing preempt_count() on certain architectures.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

---
V10:
* Add commit message I forgot
* Rebase against latest pcpu_hot changes
V11:
* Remove CONFIG_PROFILE_ALL_BRANCHES workaround from
__preempt_count_add_return()

arch/arm64/include/asm/preempt.h | 18 ++++++++++++++++++
arch/s390/include/asm/preempt.h | 10 ++++++++++
arch/x86/include/asm/preempt.h | 10 ++++++++++
include/asm-generic/preempt.h | 14 ++++++++++++++
4 files changed, 52 insertions(+)

diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
index 932ea4b620428..0dd8221d1bef7 100644
--- a/arch/arm64/include/asm/preempt.h
+++ b/arch/arm64/include/asm/preempt.h
@@ -55,6 +55,24 @@ static inline void __preempt_count_sub(int val)
WRITE_ONCE(current_thread_info()->preempt.count, pc);
}

+static inline int __preempt_count_add_return(int val)
+{
+ u32 pc = READ_ONCE(current_thread_info()->preempt.count);
+ pc += val;
+ WRITE_ONCE(current_thread_info()->preempt.count, pc);
+
+ return pc;
+}
+
+static inline int __preempt_count_sub_return(int val)
+{
+ u32 pc = READ_ONCE(current_thread_info()->preempt.count);
+ pc -= val;
+ WRITE_ONCE(current_thread_info()->preempt.count, pc);
+
+ return pc;
+}
+
static inline bool __preempt_count_dec_and_test(void)
{
struct thread_info *ti = current_thread_info();
diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h
index 6ccd033acfe52..5ae366e26c57d 100644
--- a/arch/s390/include/asm/preempt.h
+++ b/arch/s390/include/asm/preempt.h
@@ -98,6 +98,16 @@ static __always_inline bool should_resched(int preempt_offset)
return unlikely(READ_ONCE(get_lowcore()->preempt_count) == preempt_offset);
}

+static __always_inline int __preempt_count_add_return(int val)
+{
+ return val + __atomic_add(val, &get_lowcore()->preempt_count);
+}
+
+static __always_inline int __preempt_count_sub_return(int val)
+{
+ return __preempt_count_add_return(-val);
+}
+
#define init_task_preempt_count(p) do { } while (0)
/* Deferred to CPU bringup time */
#define init_idle_preempt_count(p, cpu) do { } while (0)
diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
index 578441db09f0b..1220656f3370b 100644
--- a/arch/x86/include/asm/preempt.h
+++ b/arch/x86/include/asm/preempt.h
@@ -85,6 +85,16 @@ static __always_inline void __preempt_count_sub(int val)
raw_cpu_add_4(__preempt_count, -val);
}

+static __always_inline int __preempt_count_add_return(int val)
+{
+ return raw_cpu_add_return_4(__preempt_count, val);
+}
+
+static __always_inline int __preempt_count_sub_return(int val)
+{
+ return raw_cpu_add_return_4(__preempt_count, -val);
+}
+
/*
* Because we keep PREEMPT_NEED_RESCHED set when we do _not_ need to reschedule
* a decrement which hits zero means we have no preempt_count and should
diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
index 51f8f3881523a..c8683c046615d 100644
--- a/include/asm-generic/preempt.h
+++ b/include/asm-generic/preempt.h
@@ -59,6 +59,20 @@ static __always_inline void __preempt_count_sub(int val)
*preempt_count_ptr() -= val;
}

+static __always_inline int __preempt_count_add_return(int val)
+{
+ *preempt_count_ptr() += val;
+
+ return *preempt_count_ptr();
+}
+
+static __always_inline int __preempt_count_sub_return(int val)
+{
+ *preempt_count_ptr() -= val;
+
+ return *preempt_count_ptr();
+}
+
static __always_inline bool __preempt_count_dec_and_test(void)
{
/*
--
2.51.0