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Message-Id: <20240527063749.391035-12-john.ogness@linutronix.de>
Date: Mon, 27 May 2024 08:43:30 +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,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH printk v6 11/30] nbcon: Add API to acquire context for non-printing operations
Provide functions nbcon_device_try_acquire() and
nbcon_device_release() which will try to acquire the nbcon
console ownership with NBCON_PRIO_NORMAL and mark it unsafe for
handover/takeover.
These functions are to be used together with the device-specific
locking when performing non-printing activities on the console
device. They will allow synchronization against the
atomic_write() callback which will be serialized, for higher
priority contexts, only by acquiring the console context
ownership.
Pitfalls:
The API requires to be called in a context with migration
disabled because it uses per-CPU variables internally.
The context is set unsafe for a takeover all the time. It
guarantees full serialization against any atomic_write() caller
except for the final flush in panic() which might try an unsafe
takeover.
Signed-off-by: John Ogness <john.ogness@...utronix.de>
Reviewed-by: Petr Mladek <pmladek@...e.com>
---
include/linux/console.h | 2 ++
include/linux/printk.h | 14 +++++++++++
kernel/printk/nbcon.c | 56 ++++++++++++++++++++++++++++++++++++++++-
3 files changed, 71 insertions(+), 1 deletion(-)
diff --git a/include/linux/console.h b/include/linux/console.h
index aea4f5aa4a45..dd3d4b5d3737 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -304,6 +304,7 @@ struct nbcon_write_context {
*
* @nbcon_state: State for nbcon consoles
* @nbcon_seq: Sequence number of the next record for nbcon to print
+ * @nbcon_device_ctxt: Context available for non-printing operations
* @pbufs: Pointer to nbcon private buffer
*/
struct console {
@@ -399,6 +400,7 @@ struct console {
atomic_t __private nbcon_state;
atomic_long_t __private nbcon_seq;
+ struct nbcon_context __private nbcon_device_ctxt;
struct printk_buffers *pbufs;
};
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 6e1f055afef6..31f72df29da5 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -9,6 +9,8 @@
#include <linux/ratelimit_types.h>
#include <linux/once_lite.h>
+struct console;
+
extern const char linux_banner[];
extern const char linux_proc_banner[];
@@ -193,6 +195,8 @@ void show_regs_print_info(const char *log_lvl);
extern asmlinkage void dump_stack_lvl(const char *log_lvl) __cold;
extern asmlinkage void dump_stack(void) __cold;
void printk_trigger_flush(void);
+extern bool nbcon_device_try_acquire(struct console *con);
+extern void nbcon_device_release(struct console *con);
#else
static inline __printf(1, 0)
int vprintk(const char *s, va_list args)
@@ -272,6 +276,16 @@ static inline void dump_stack(void)
static inline void printk_trigger_flush(void)
{
}
+
+static inline bool nbcon_device_try_acquire(struct console *con)
+{
+ return false;
+}
+
+static inline void nbcon_device_release(struct console *con)
+{
+}
+
#endif
bool this_cpu_in_panic(void);
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index dd7c3180b335..1c03508b90f2 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -5,7 +5,9 @@
#include <linux/kernel.h>
#include <linux/console.h>
#include <linux/delay.h>
+#include <linux/export.h>
#include <linux/slab.h>
+#include <linux/string.h>
#include "internal.h"
/*
* Printk console printing implementation for consoles which does not depend
@@ -528,6 +530,7 @@ static struct printk_buffers panic_nbcon_pbufs;
* nbcon_context_try_acquire - Try to acquire nbcon console
* @ctxt: The context of the caller
*
+ * Context: Under @ctxt->con->device_lock() or local_irq_save().
* Return: True if the console was acquired. False otherwise.
*
* If the caller allowed an unsafe hostile takeover, on success the
@@ -535,7 +538,6 @@ static struct printk_buffers panic_nbcon_pbufs;
* in an unsafe state. Otherwise, on success the caller may assume
* the console is not in an unsafe state.
*/
-__maybe_unused
static bool nbcon_context_try_acquire(struct nbcon_context *ctxt)
{
unsigned int cpu = smp_processor_id();
@@ -989,3 +991,55 @@ void nbcon_free(struct console *con)
con->pbufs = NULL;
}
+
+/**
+ * nbcon_device_try_acquire - Try to acquire nbcon console and enter unsafe
+ * section
+ * @con: The nbcon console to acquire
+ *
+ * Context: Under the locking mechanism implemented in
+ * @con->device_lock() including disabling migration.
+ * Return: True if the console was acquired. False otherwise.
+ *
+ * Console drivers will usually use their own internal synchronization
+ * mechasism to synchronize between console printing and non-printing
+ * activities (such as setting baud rates). However, nbcon console drivers
+ * supporting atomic consoles may also want to mark unsafe sections when
+ * performing non-printing activities in order to synchronize against their
+ * atomic_write() callback.
+ *
+ * This function acquires the nbcon console using priority NBCON_PRIO_NORMAL
+ * and marks it unsafe for handover/takeover.
+ */
+bool nbcon_device_try_acquire(struct console *con)
+{
+ struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_device_ctxt);
+
+ cant_migrate();
+
+ memset(ctxt, 0, sizeof(*ctxt));
+ ctxt->console = con;
+ ctxt->prio = NBCON_PRIO_NORMAL;
+
+ if (!nbcon_context_try_acquire(ctxt))
+ return false;
+
+ if (!nbcon_context_enter_unsafe(ctxt))
+ return false;
+
+ return true;
+}
+EXPORT_SYMBOL_GPL(nbcon_device_try_acquire);
+
+/**
+ * nbcon_device_release - Exit unsafe section and release the nbcon console
+ * @con: The nbcon console acquired in nbcon_device_try_acquire()
+ */
+void nbcon_device_release(struct console *con)
+{
+ struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_device_ctxt);
+
+ if (nbcon_context_exit_unsafe(ctxt))
+ nbcon_context_release(ctxt);
+}
+EXPORT_SYMBOL_GPL(nbcon_device_release);
--
2.39.2
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