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Message-Id: <20210615174947.32057-2-john.ogness@linutronix.de>
Date: Tue, 15 Jun 2021 19:55:46 +0206
From: John Ogness <john.ogness@...utronix.de>
To: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>,
Steven Rostedt <rostedt@...dmis.org>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel@...r.kernel.org,
Stephen Rothwell <sfr@...b.auug.org.au>,
Andrew Morton <akpm@...ux-foundation.org>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Stephen Boyd <swboyd@...omium.org>,
Alexander Potapenko <glider@...gle.com>,
Peter Zijlstra <peterz@...radead.org>
Subject: [PATCH next v3 1/2] dump_stack: move cpu lock to printk.c
dump_stack() implements its own cpu-reentrant spinning lock to
best-effort serialize stack traces in the printk log. However,
there are other functions (such as show_regs()) that can also
benefit from this serialization.
Move the cpu-reentrant spinning lock (cpu lock) into new helper
functions printk_cpu_lock_irqsave()/printk_cpu_unlock_irqrestore()
so that it is available for others as well. For !CONFIG_SMP the
cpu lock is a NOP.
Note that having multiple cpu locks in the system can easily
lead to deadlock. Code needing a cpu lock should use the
printk cpu lock, since the printk cpu lock could be acquired
from any code and any context.
Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
include/linux/printk.h | 41 ++++++++++++++++++++++++++
kernel/printk/printk.c | 67 ++++++++++++++++++++++++++++++++++++++++++
lib/dump_stack.c | 38 ++----------------------
3 files changed, 110 insertions(+), 36 deletions(-)
diff --git a/include/linux/printk.h b/include/linux/printk.h
index f589b8b60806..d796183f26c9 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -287,6 +287,47 @@ static inline void printk_safe_flush_on_panic(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);
+
+/**
+ * printk_cpu_lock_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().
+ *
+ * If the lock is owned by another CPU, spin until it becomes available.
+ * Interrupts are restored while spinning.
+ */
+#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(); \
+ }
+
+/**
+ * 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().
+ */
+#define printk_cpu_unlock_irqrestore(flags) \
+ do { \
+ __printk_cpu_unlock(); \
+ local_irq_restore(flags); \
+ } while (0) \
+
+#else
+
+#define printk_cpu_lock_irqsave(flags) ((void)flags)
+#define printk_cpu_unlock_irqrestore(flags) ((void)flags)
+
+#endif /* CONFIG_SMP */
+
extern int kptr_restrict;
/**
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 114e9963f903..5369d8f33299 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3532,3 +3532,70 @@ void kmsg_dump_rewind(struct kmsg_dump_iter *iter)
EXPORT_SYMBOL_GPL(kmsg_dump_rewind);
#endif
+
+#ifdef CONFIG_SMP
+static atomic_t printk_cpulock_owner = ATOMIC_INIT(-1);
+static bool printk_cpulock_nested;
+
+/**
+ * __printk_wait_on_cpu_lock() - Busy wait until the printk cpu-reentrant
+ * spinning lock is not owned by any CPU.
+ */
+void __printk_wait_on_cpu_lock(void)
+{
+ do {
+ cpu_relax();
+ } while (atomic_read(&printk_cpulock_owner) != -1);
+}
+EXPORT_SYMBOL(__printk_wait_on_cpu_lock);
+
+/**
+ * __printk_cpu_trylock() - 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
+ * lock, this function succeeds immediately.
+ *
+ * Context: Any context. Expects interrupts to be disabled.
+ * Return: 1 on success, otherwise 0.
+ */
+int __printk_cpu_trylock(void)
+{
+ int cpu;
+ int old;
+
+ cpu = smp_processor_id();
+
+ old = atomic_cmpxchg(&printk_cpulock_owner, -1, cpu);
+ if (old == -1) {
+ /* This CPU is now the owner. */
+ return 1;
+ } else if (old == cpu) {
+ /* This CPU is already the owner. */
+ printk_cpulock_nested = true;
+ return 1;
+ }
+
+ return 0;
+}
+EXPORT_SYMBOL(__printk_cpu_trylock);
+
+/**
+ * __printk_cpu_unlock() - 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)
+{
+ if (printk_cpulock_nested) {
+ printk_cpulock_nested = false;
+ return;
+ }
+
+ atomic_set_release(&printk_cpulock_owner, -1);
+}
+EXPORT_SYMBOL(__printk_cpu_unlock);
+#endif /* CONFIG_SMP */
diff --git a/lib/dump_stack.c b/lib/dump_stack.c
index 6e7ca3d67710..cd3387bb34e5 100644
--- a/lib/dump_stack.c
+++ b/lib/dump_stack.c
@@ -93,52 +93,18 @@ static void __dump_stack(const char *log_lvl)
*
* Architectures can override this implementation by implementing its own.
*/
-#ifdef CONFIG_SMP
-static atomic_t dump_lock = ATOMIC_INIT(-1);
-
asmlinkage __visible void dump_stack_lvl(const char *log_lvl)
{
unsigned long flags;
- int was_locked;
- int old;
- int cpu;
/*
* Permit this cpu to perform nested stack dumps while serialising
* against other CPUs
*/
-retry:
- local_irq_save(flags);
- cpu = smp_processor_id();
- old = atomic_cmpxchg(&dump_lock, -1, cpu);
- if (old == -1) {
- was_locked = 0;
- } else if (old == cpu) {
- was_locked = 1;
- } else {
- local_irq_restore(flags);
- /*
- * Wait for the lock to release before jumping to
- * atomic_cmpxchg() in order to mitigate the thundering herd
- * problem.
- */
- do { cpu_relax(); } while (atomic_read(&dump_lock) != -1);
- goto retry;
- }
-
- __dump_stack(log_lvl);
-
- if (!was_locked)
- atomic_set(&dump_lock, -1);
-
- local_irq_restore(flags);
-}
-#else
-asmlinkage __visible void dump_stack_lvl(const char *log_lvl)
-{
+ printk_cpu_lock_irqsave(flags);
__dump_stack(log_lvl);
+ printk_cpu_unlock_irqrestore(flags);
}
-#endif
EXPORT_SYMBOL(dump_stack_lvl);
asmlinkage __visible void dump_stack(void)
--
2.20.1
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