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Subject[RFC PATCH v1 02/25] printk-rb: add prb locking functions
Date
Add processor-reentrant spin locking functions. These allow
restricting the number of possible contexts to 2, which can simplify
implementing code that also supports NMI interruptions.

prb_lock();

/*
* This code is synchronized with all contexts
* except an NMI on the same processor.
*/

prb_unlock();

In order to support printk's emergency messages, a
processor-reentrant spin lock will be used to control raw access to
the emergency console. However, it must be the same
processor-reentrant spin lock as the one used by the ring buffer,
otherwise a deadlock can occur:

CPU1: printk lock -> emergency -> serial lock
CPU2: serial lock -> printk lock

By making the processor-reentrant implemtation available externally,
printk can use the same atomic_t for the ring buffer as for the
emergency console and thus avoid the above deadlock.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
include/linux/printk_ringbuffer.h | 24 ++++++++++++
lib/Makefile | 2 +-
lib/printk_ringbuffer.c | 77 +++++++++++++++++++++++++++++++++++++++
3 files changed, 102 insertions(+), 1 deletion(-)
create mode 100644 include/linux/printk_ringbuffer.h
create mode 100644 lib/printk_ringbuffer.c

diff --git a/include/linux/printk_ringbuffer.h b/include/linux/printk_ringbuffer.h
new file mode 100644
index 000000000000..75f5708ea902
--- /dev/null
+++ b/include/linux/printk_ringbuffer.h
@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _LINUX_PRINTK_RINGBUFFER_H
+#define _LINUX_PRINTK_RINGBUFFER_H
+
+#include <linux/atomic.h>
+#include <linux/percpu.h>
+
+struct prb_cpulock {
+ atomic_t owner;
+ unsigned long __percpu *irqflags;
+};
+
+#define DECLARE_STATIC_PRINTKRB_CPULOCK(name) \
+static DEFINE_PER_CPU(unsigned long, _##name##_percpu_irqflags); \
+static struct prb_cpulock name = { \
+ .owner = ATOMIC_INIT(-1), \
+ .irqflags = &_##name##_percpu_irqflags, \
+}
+
+/* utility functions */
+void prb_lock(struct prb_cpulock *cpu_lock, unsigned int *cpu_store);
+void prb_unlock(struct prb_cpulock *cpu_lock, unsigned int cpu_store);
+
+#endif /*_LINUX_PRINTK_RINGBUFFER_H */
diff --git a/lib/Makefile b/lib/Makefile
index e1b59da71418..77a20bfd232e 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -19,7 +19,7 @@ KCOV_INSTRUMENT_dynamic_debug.o := n

lib-y := ctype.o string.o vsprintf.o cmdline.o \
rbtree.o radix-tree.o timerqueue.o xarray.o \
- idr.o int_sqrt.o extable.o \
+ idr.o int_sqrt.o extable.o printk_ringbuffer.o \
sha1.o chacha.o irq_regs.o argv_split.o \
flex_proportions.o ratelimit.o show_mem.o \
is_single_threaded.o plist.o decompress.o kobject_uevent.o \
diff --git a/lib/printk_ringbuffer.c b/lib/printk_ringbuffer.c
new file mode 100644
index 000000000000..28958b0cf774
--- /dev/null
+++ b/lib/printk_ringbuffer.c
@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/smp.h>
+#include <linux/printk_ringbuffer.h>
+
+static bool __prb_trylock(struct prb_cpulock *cpu_lock,
+ unsigned int *cpu_store)
+{
+ unsigned long *flags;
+ unsigned int cpu;
+
+ cpu = get_cpu();
+
+ *cpu_store = atomic_read(&cpu_lock->owner);
+ /* memory barrier to ensure the current lock owner is visible */
+ smp_rmb();
+ if (*cpu_store == -1) {
+ flags = per_cpu_ptr(cpu_lock->irqflags, cpu);
+ local_irq_save(*flags);
+ if (atomic_try_cmpxchg_acquire(&cpu_lock->owner,
+ cpu_store, cpu)) {
+ return true;
+ }
+ local_irq_restore(*flags);
+ } else if (*cpu_store == cpu) {
+ return true;
+ }
+
+ put_cpu();
+ return false;
+}
+
+/*
+ * prb_lock: Perform a processor-reentrant spin lock.
+ * @cpu_lock: A pointer to the lock object.
+ * @cpu_store: A "flags" pointer to store lock status information.
+ *
+ * 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. If lock is locked by another
+ * processor, this function spins until the calling processor becomes the
+ * owner.
+ *
+ * It is safe to call this function from any context and state.
+ */
+void prb_lock(struct prb_cpulock *cpu_lock, unsigned int *cpu_store)
+{
+ for (;;) {
+ if (__prb_trylock(cpu_lock, cpu_store))
+ break;
+ cpu_relax();
+ }
+}
+
+/*
+ * prb_unlock: Perform a processor-reentrant spin unlock.
+ * @cpu_lock: A pointer to the lock object.
+ * @cpu_store: A "flags" object storing lock status information.
+ *
+ * Release the lock. The calling processor must be the owner of the lock.
+ *
+ * It is safe to call this function from any context and state.
+ */
+void prb_unlock(struct prb_cpulock *cpu_lock, unsigned int cpu_store)
+{
+ unsigned long *flags;
+ unsigned int cpu;
+
+ cpu = atomic_read(&cpu_lock->owner);
+ atomic_set_release(&cpu_lock->owner, cpu_store);
+
+ if (cpu_store == -1) {
+ flags = per_cpu_ptr(cpu_lock->irqflags, cpu);
+ local_irq_restore(*flags);
+ }
+
+ put_cpu();
+}
--
2.11.0
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