[PATCH printk v2 01/12] printk: rename cpulock functions

John Ogness
2022-04-06T00:09:50Z

Since the printk cpulock is CPU-reentrant and since it is used
in all contexts, its usage must be carefully considered and
most likely will require programming locklessly. To avoid
mistaking the printk cpulock as a typical lock, rename it to
cpu_sync. The main functions then become:

printk_cpu_sync_get_irqsave(flags);
printk_cpu_sync_put_irqrestore(flags);

Add extra notes of caution in the function description to help
developers understand the requirements for correct usage.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
include/linux/printk.h | 54 +++++++++++++++++++-------------
kernel/printk/printk.c | 71 +++++++++++++++++++++---------------------
lib/dump_stack.c | 4 +--
lib/nmi_backtrace.c | 4 +--
4 files changed, 73 insertions(+), 60 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 1522df223c0f..859323a52985 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -277,43 +277,55 @@ static inline void printk_trigger_flush(void)
#endif

#ifdef CONFIG_SMP
-extern int __printk_cpu_trylock(void);
-extern void __printk_wait_on_cpu_lock(void);
-extern void __printk_cpu_unlock(void);
+extern int __printk_cpu_sync_try_get(void);
+extern void __printk_cpu_sync_wait(void);
+extern void __printk_cpu_sync_put(void);

/**
- * printk_cpu_lock_irqsave() - Acquire the printk cpu-reentrant spinning
- * lock and disable interrupts.
+ * printk_cpu_sync_get_irqsave() - Acquire the printk cpu-reentrant spinning
+ * lock and disable interrupts.
* @flags: Stack-allocated storage for saving local interrupt state,
- * to be passed to printk_cpu_unlock_irqrestore().
+ * to be passed to printk_cpu_sync_put_irqrestore().
*
* If the lock is owned by another CPU, spin until it becomes available.
* Interrupts are restored while spinning.
+ *
+ * CAUTION: This function must be used carefully. It does not behave like a
+ * typical lock. Here are important things to watch out for...
+ *
+ * * This function is reentrant on the same CPU. Therefore the calling
+ * code must not assume exclusive access to data if code accessing the
+ * data can run reentrant or within NMI context on the same CPU.
+ *
+ * * If there exists usage of this function from NMI context, it becomes
+ * unsafe to perform any type of locking or spinning to wait for other
+ * CPUs after calling this function from any context. This includes
+ * using spinlocks or any other busy-waiting synchronization methods.
*/
-#define printk_cpu_lock_irqsave(flags) \
- for (;;) { \
- local_irq_save(flags); \
- if (__printk_cpu_trylock()) \
- break; \
- local_irq_restore(flags); \
- __printk_wait_on_cpu_lock(); \
+#define printk_cpu_sync_get_irqsave(flags) \
+ for (;;) { \
+ local_irq_save(flags); \
+ if (__printk_cpu_sync_try_get()) \
+ break; \
+ local_irq_restore(flags); \
+ __printk_cpu_sync_wait(); \
}

/**
- * printk_cpu_unlock_irqrestore() - Release the printk cpu-reentrant spinning
- * lock and restore interrupts.
- * @flags: Caller's saved interrupt state, from printk_cpu_lock_irqsave().
+ * printk_cpu_sync_put_irqrestore() - Release the printk cpu-reentrant spinning
+ * lock and restore interrupts.
+ * @flags: Caller's saved interrupt state, from printk_cpu_sync_get_irqsave().
*/
-#define printk_cpu_unlock_irqrestore(flags) \
+#define printk_cpu_sync_put_irqrestore(flags) \
do { \
- __printk_cpu_unlock(); \
+ __printk_cpu_sync_put(); \
local_irq_restore(flags); \
- } while (0) \
+ } while (0)

#else

-#define printk_cpu_lock_irqsave(flags) ((void)flags)
-#define printk_cpu_unlock_irqrestore(flags) ((void)flags)
+#define printk_cpu_sync_get_irqsave(flags) ((void)flags)
+#define printk_cpu_sync_put_irqrestore(flags) ((void)flags)

#endif /* CONFIG_SMP */

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index da03c15ecc89..13a1eebe72af 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3667,26 +3667,26 @@ EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
#endif

#ifdef CONFIG_SMP
-static atomic_t printk_cpulock_owner = ATOMIC_INIT(-1);
-static atomic_t printk_cpulock_nested = ATOMIC_INIT(0);
+static atomic_t printk_cpu_sync_owner = ATOMIC_INIT(-1);
+static atomic_t printk_cpu_sync_nested = ATOMIC_INIT(0);

/**
- * __printk_wait_on_cpu_lock() - Busy wait until the printk cpu-reentrant
- * spinning lock is not owned by any CPU.
+ * __printk_cpu_sync_wait() - Busy wait until the printk cpu-reentrant
+ * spinning lock is not owned by any CPU.
*
* Context: Any context.
*/
-void __printk_wait_on_cpu_lock(void)
+void __printk_cpu_sync_wait(void)
{
do {
cpu_relax();
- } while (atomic_read(&printk_cpulock_owner) != -1);
+ } while (atomic_read(&printk_cpu_sync_owner) != -1);
}
-EXPORT_SYMBOL(__printk_wait_on_cpu_lock);
+EXPORT_SYMBOL(__printk_cpu_sync_wait);

/**
- * __printk_cpu_trylock() - Try to acquire the printk cpu-reentrant
- * spinning lock.
+ * __printk_cpu_sync_try_get() - Try to acquire the printk cpu-reentrant
+ * spinning lock.
*
* If no processor has the lock, the calling processor takes the lock and
* becomes the owner. If the calling processor is already the owner of the
@@ -3695,7 +3695,7 @@ EXPORT_SYMBOL(__printk_wait_on_cpu_lock);
* Context: Any context. Expects interrupts to be disabled.
* Return: 1 on success, otherwise 0.
*/
-int __printk_cpu_trylock(void)
+int __printk_cpu_sync_try_get(void)
{
int cpu;
int old;
@@ -3705,79 +3705,80 @@ int __printk_cpu_trylock(void)
/*
* Guarantee loads and stores from this CPU when it is the lock owner
* are _not_ visible to the previous lock owner. This pairs with
- * __printk_cpu_unlock:B.
+ * __printk_cpu_sync_put:B.
*
* Memory barrier involvement:
*
- * If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B, then
- * __printk_cpu_unlock:A can never read from __printk_cpu_trylock:B.
+ * If __printk_cpu_sync_try_get:A reads from __printk_cpu_sync_put:B,
+ * then __printk_cpu_sync_put:A can never read from
+ * __printk_cpu_sync_try_get:B.
*
* Relies on:
*
- * RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B
+ * RELEASE from __printk_cpu_sync_put:A to __printk_cpu_sync_put:B
* of the previous CPU
* matching
- * ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B
- * of this CPU
+ * ACQUIRE from __printk_cpu_sync_try_get:A to
+ * __printk_cpu_sync_try_get:B of this CPU
*/
- old = atomic_cmpxchg_acquire(&printk_cpulock_owner, -1,
- cpu); /* LMM(__printk_cpu_trylock:A) */
+ old = atomic_cmpxchg_acquire(&printk_cpu_sync_owner, -1,
+ cpu); /* LMM(__printk_cpu_sync_try_get:A) */
if (old == -1) {
/*
* This CPU is now the owner and begins loading/storing
- * data: LMM(__printk_cpu_trylock:B)
+ * data: LMM(__printk_cpu_sync_try_get:B)
*/
return 1;

} else if (old == cpu) {
/* This CPU is already the owner. */
- atomic_inc(&printk_cpulock_nested);
+ atomic_inc(&printk_cpu_sync_nested);
return 1;
}

return 0;
}
-EXPORT_SYMBOL(__printk_cpu_trylock);
+EXPORT_SYMBOL(__printk_cpu_sync_try_get);

/**
- * __printk_cpu_unlock() - Release the printk cpu-reentrant spinning lock.
+ * __printk_cpu_sync_put() - Release the printk cpu-reentrant spinning lock.
*
* The calling processor must be the owner of the lock.
*
* Context: Any context. Expects interrupts to be disabled.
*/
-void __printk_cpu_unlock(void)
+void __printk_cpu_sync_put(void)
{
- if (atomic_read(&printk_cpulock_nested)) {
- atomic_dec(&printk_cpulock_nested);
+ if (atomic_read(&printk_cpu_sync_nested)) {
+ atomic_dec(&printk_cpu_sync_nested);
return;
}

/*
* This CPU is finished loading/storing data:
- * LMM(__printk_cpu_unlock:A)
+ * LMM(__printk_cpu_sync_put:A)
*/

/*
* Guarantee loads and stores from this CPU when it was the
* lock owner are visible to the next lock owner. This pairs
- * with __printk_cpu_trylock:A.
+ * with __printk_cpu_sync_try_get:A.
*
* Memory barrier involvement:
*
- * If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B,
- * then __printk_cpu_trylock:B reads from __printk_cpu_unlock:A.
+ * If __printk_cpu_sync_try_get:A reads from __printk_cpu_sync_put:B,
+ * then __printk_cpu_sync_try_get:B reads from __printk_cpu_sync_put:A.
*
* Relies on:
*
- * RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B
+ * RELEASE from __printk_cpu_sync_put:A to __printk_cpu_sync_put:B
* of this CPU
* matching
- * ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B
- * of the next CPU
+ * ACQUIRE from __printk_cpu_sync_try_get:A to
+ * __printk_cpu_sync_try_get:B of the next CPU
*/
- atomic_set_release(&printk_cpulock_owner,
- -1); /* LMM(__printk_cpu_unlock:B) */
+ atomic_set_release(&printk_cpu_sync_owner,
+ -1); /* LMM(__printk_cpu_sync_put:B) */
}
-EXPORT_SYMBOL(__printk_cpu_unlock);
+EXPORT_SYMBOL(__printk_cpu_sync_put);
#endif /* CONFIG_SMP */
diff --git a/lib/dump_stack.c b/lib/dump_stack.c
index 6b7f1bf6715d..83471e81501a 100644
--- a/lib/dump_stack.c
+++ b/lib/dump_stack.c
@@ -102,9 +102,9 @@ asmlinkage __visible void dump_stack_lvl(const char *log_lvl)
* Permit this cpu to perform nested stack dumps while serialising
* against other CPUs
*/
- printk_cpu_lock_irqsave(flags);
+ printk_cpu_sync_get_irqsave(flags);
__dump_stack(log_lvl);
- printk_cpu_unlock_irqrestore(flags);
+ printk_cpu_sync_put_irqrestore(flags);
}
EXPORT_SYMBOL(dump_stack_lvl);

diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
index 199ab201d501..d01aec6ae15c 100644
--- a/lib/nmi_backtrace.c
+++ b/lib/nmi_backtrace.c
@@ -99,7 +99,7 @@ bool nmi_cpu_backtrace(struct pt_regs *regs)
* Allow nested NMI backtraces while serializing
* against other CPUs.
*/
- printk_cpu_lock_irqsave(flags);
+ printk_cpu_sync_get_irqsave(flags);
if (!READ_ONCE(backtrace_idle) && regs && cpu_in_idle(instruction_pointer(regs))) {
pr_warn("NMI backtrace for cpu %d skipped: idling at %pS\n",
cpu, (void *)instruction_pointer(regs));
@@ -110,7 +110,7 @@ bool nmi_cpu_backtrace(struct pt_regs *regs)
else
dump_stack();
}
- printk_cpu_unlock_irqrestore(flags);
+ printk_cpu_sync_put_irqrestore(flags);
cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
return true;
}
--
2.30.2