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Subject[PATCHv6 3/7] printk: introduce per-cpu safe_print seq buffer
Date
This patch extends the idea of NMI per-cpu buffers to regions
that may cause recursive printk() calls and possible deadlocks.
Namely, printk() can't handle printk calls from schedule code
or printk() calls from lock debugging code (spin_dump() for instance);
because those may be called with `sem->lock' already taken or any
other `critical' locks (p->pi_lock, etc.). An example of deadlock
can be

vprintk_emit()
console_unlock()
up() << raw_spin_lock_irqsave(&sem->lock, flags);
wake_up_process()
try_to_wake_up()
ttwu_queue()
ttwu_activate()
activate_task()
enqueue_task()
enqueue_task_fair()
cfs_rq_of()
task_of()
WARN_ON_ONCE(!entity_is_task(se))
vprintk_emit()
console_trylock()
down_trylock()
raw_spin_lock_irqsave(&sem->lock, flags)
^^^^ deadlock

and some other cases.

Just like in NMI implementation, the solution uses a per-cpu
`printk_func' pointer to 'redirect' printk() calls to a 'safe'
callback, that store messages in a per-cpu buffer and flushes
them back to logbuf buffer later.

Usage example:

printk()
printk_safe_enter(flags)
//
// any printk() call from here will endup in vprintk_safe(),
// that stores messages in a special per-CPU buffer.
//
printk_safe_exit(flags)

The 'redirection' mechanism, though, has been reworked, as suggested
by Petr Mladek. Instead of using a per-cpu @print_func callback we now
keep a per-cpu printk-context variable and call either default or nmi
vprintk function depending on its value. printk_nmi_entrer/exit and
printk_safe_enter/exit, thus, just set/celar corresponding bits in
printk-context functions.

The patch only adds printk_safe support, we don't use it yet.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
include/linux/printk.h | 21 +++++--
kernel/printk/Makefile | 2 +-
kernel/printk/internal.h | 56 +++++++++++--------
kernel/printk/printk.c | 3 -
kernel/printk/printk_safe.c | 130 +++++++++++++++++++++++++++++++++++---------
5 files changed, 153 insertions(+), 59 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index 67df41eca51c..f7d291cf61d9 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -147,17 +147,11 @@ void early_printk(const char *s, ...) { }
#endif

#ifdef CONFIG_PRINTK_NMI
-extern void printk_safe_init(void);
extern void printk_safe_nmi_enter(void);
extern void printk_safe_nmi_exit(void);
-extern void printk_safe_flush(void);
-extern void printk_safe_flush_on_panic(void);
#else
-static inline void printk_safe_init(void) { }
static inline void printk_safe_nmi_enter(void) { }
static inline void printk_safe_nmi_exit(void) { }
-static inline void printk_safe_flush(void) { }
-static inline void printk_safe_flush_on_panic(void) { }
#endif /* PRINTK_NMI */

#ifdef CONFIG_PRINTK
@@ -209,6 +203,9 @@ void __init setup_log_buf(int early);
__printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...);
void dump_stack_print_info(const char *log_lvl);
void show_regs_print_info(const char *log_lvl);
+extern void printk_safe_init(void);
+extern void printk_safe_flush(void);
+extern void printk_safe_flush_on_panic(void);
#else
static inline __printf(1, 0)
int vprintk(const char *s, va_list args)
@@ -268,6 +265,18 @@ static inline void dump_stack_print_info(const char *log_lvl)
static inline void show_regs_print_info(const char *log_lvl)
{
}
+
+static inline void printk_safe_init(void)
+{
+}
+
+static inline void printk_safe_flush(void)
+{
+}
+
+static inline void printk_safe_flush_on_panic(void)
+{
+}
#endif

extern asmlinkage void dump_stack(void) __cold;
diff --git a/kernel/printk/Makefile b/kernel/printk/Makefile
index 607928119f26..4a2ffc39eb95 100644
--- a/kernel/printk/Makefile
+++ b/kernel/printk/Makefile
@@ -1,3 +1,3 @@
obj-y = printk.o
-obj-$(CONFIG_PRINTK_NMI) += printk_safe.o
+obj-$(CONFIG_PRINTK) += printk_safe.o
obj-$(CONFIG_A11Y_BRAILLE_CONSOLE) += braille.o
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 7fd2838fa417..97cee4f482f4 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -16,26 +16,8 @@
*/
#include <linux/percpu.h>

-typedef __printf(1, 0) int (*printk_func_t)(const char *fmt, va_list args);
-
-int __printf(1, 0) vprintk_default(const char *fmt, va_list args);
-
#ifdef CONFIG_PRINTK_NMI

-extern raw_spinlock_t logbuf_lock;
-
-/*
- * printk() could not take logbuf_lock in NMI context. Instead,
- * it temporary stores the strings into a per-CPU buffer.
- * The alternative implementation is chosen transparently
- * via per-CPU variable.
- */
-DECLARE_PER_CPU(printk_func_t, printk_func);
-static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
-{
- return this_cpu_read(printk_func)(fmt, args);
-}
-
extern atomic_t nmi_message_lost;
static inline int get_nmi_message_lost(void)
{
@@ -44,14 +26,42 @@ static inline int get_nmi_message_lost(void)

#else /* CONFIG_PRINTK_NMI */

-static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
-{
- return vprintk_default(fmt, args);
-}
-
static inline int get_nmi_message_lost(void)
{
return 0;
}

#endif /* CONFIG_PRINTK_NMI */
+
+#ifdef CONFIG_PRINTK
+
+#define PRINTK_SAFE_CONTEXT_MASK 0x7fffffff
+#define PRINTK_NMI_CONTEXT_MASK 0x80000000
+
+extern raw_spinlock_t logbuf_lock;
+
+__printf(1, 0) int vprintk_default(const char *fmt, va_list args);
+__printf(1, 0) int vprintk_func(const char *fmt, va_list args);
+void __printk_safe_enter(void);
+void __printk_safe_exit(void);
+
+#define printk_safe_enter(flags) \
+ do { \
+ local_irq_save(flags); \
+ __printk_safe_enter(); \
+ } while (0)
+
+#define printk_safe_exit(flags) \
+ do { \
+ __printk_safe_exit(); \
+ local_irq_restore(flags); \
+ } while (0)
+
+#else
+
+__printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; }
+
+#define printk_safe_enter(f) ((void)(f))
+#define printk_safe_exit(f) ((void)(f))
+
+#endif /* CONFIG_PRINTK */
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 265e815730f9..335b0fcb538e 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1902,9 +1902,6 @@ static size_t msg_print_text(const struct printk_log *msg,
bool syslog, char *buf, size_t size) { return 0; }
static bool suppress_message_printing(int level) { return false; }

-/* Still needs to be defined for users */
-DEFINE_PER_CPU(printk_func_t, printk_func);
-
#endif /* CONFIG_PRINTK */

#ifdef CONFIG_EARLY_PRINTK
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 09e5552f5b58..a041927a127b 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -1,5 +1,5 @@
/*
- * printk_safe.c - Safe printk in NMI context
+ * printk_safe.c - Safe printk for printk-deadlock-prone contexts
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
@@ -32,15 +32,14 @@
* is later flushed into the main ring buffer via IRQ work.
*
* The alternative implementation is chosen transparently
- * via @printk_func per-CPU variable.
+ * by examinig current printk() context mask stored in @printk_context
+ * per-CPU variable.
*
* The implementation allows to flush the strings also from another CPU.
* There are situations when we want to make sure that all buffers
* were handled or when IRQs are blocked.
*/
-DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
static int printk_safe_irq_ready;
-atomic_t nmi_message_lost;

#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \
sizeof(atomic_t) - sizeof(struct irq_work))
@@ -50,18 +49,29 @@ struct printk_safe_seq_buf {
struct irq_work work; /* IRQ work that flushes the buffer */
unsigned char buffer[SAFE_LOG_BUF_LEN];
};
+
+static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
+static DEFINE_PER_CPU(int, printk_context);
+
+#ifdef CONFIG_PRINTK_NMI
static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
+atomic_t nmi_message_lost;
+#endif

/*
- * Safe printk() for NMI context. It uses a per-CPU buffer to
- * store the message. NMIs are not nested, so there is always only
- * one writer running. But the buffer might get flushed from another
- * CPU, so we need to be careful.
+ * Add a message to per-CPU context-dependent buffer. NMI and printk-safe
+ * have dedicated buffers, because otherwise printk-safe preempted by
+ * NMI-printk would have overwritten the NMI messages.
+ *
+ * The messages are fushed from irq work (of from panic()), possibly,
+ * from other CPU, concurrently with printk_safe_log_store(). Should this
+ * happen, printk_safe_log_store() will notice the buffer->len mismatch
+ * and repeat the write.
*/
-static int vprintk_safe_nmi(const char *fmt, va_list args)
+static int printk_safe_log_store(struct printk_safe_seq_buf *s,
+ const char *fmt, va_list args)
{
- struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
- int add = 0;
+ int add;
size_t len;

again:
@@ -74,8 +84,8 @@ static int vprintk_safe_nmi(const char *fmt, va_list args)
}

/*
- * Make sure that all old data have been read before the buffer was
- * reseted. This is not needed when we just append data.
+ * Make sure that all old data have been read before the buffer
+ * was reset. This is not needed when we just append data.
*/
if (!len)
smp_rmb();
@@ -161,7 +171,7 @@ static int printk_safe_flush_buffer(const char *start, size_t len)
}

/*
- * Flush data from the associated per_CPU buffer. The function
+ * Flush data from the associated per-CPU buffer. The function
* can be called either via IRQ work or independently.
*/
static void __printk_safe_flush(struct irq_work *work)
@@ -231,8 +241,12 @@ void printk_safe_flush(void)
{
int cpu;

- for_each_possible_cpu(cpu)
+ for_each_possible_cpu(cpu) {
+#ifdef CONFIG_PRINTK_NMI
__printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
+#endif
+ __printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
+ }
}

/**
@@ -262,14 +276,88 @@ void printk_safe_flush_on_panic(void)
printk_safe_flush();
}

+#ifdef CONFIG_PRINTK_NMI
+/*
+ * Safe printk() for NMI context. It uses a per-CPU buffer to
+ * store the message. NMIs are not nested, so there is always only
+ * one writer running. But the buffer might get flushed from another
+ * CPU, so we need to be careful.
+ */
+static int vprintk_safe_nmi(const char *fmt, va_list args)
+{
+ struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
+
+ return printk_safe_log_store(s, fmt, args);
+}
+
+void printk_safe_nmi_enter(void)
+{
+ this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
+}
+
+void printk_safe_nmi_exit(void)
+{
+ this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
+}
+
+#else
+
+static int vprintk_safe_nmi(const char *fmt, va_list args)
+{
+ return 0;
+}
+
+#endif /* CONFIG_PRINTK_NMI */
+
+/*
+ * Lock-less printk(), to avoid deadlocks should the printk() recurse
+ * into itself. It uses a per-CPU buffer to store the message, just like
+ * NMI.
+ */
+static int vprintk_safe(const char *fmt, va_list args)
+{
+ struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
+
+ return printk_safe_log_store(s, fmt, args);
+}
+
+/* Can be preempted by NMI. */
+void __printk_safe_enter(void)
+{
+ this_cpu_inc(printk_context);
+}
+
+/* Can be preempted by NMI. */
+void __printk_safe_exit(void)
+{
+ this_cpu_dec(printk_context);
+}
+
+__printf(1, 0) int vprintk_func(const char *fmt, va_list args)
+{
+ if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
+ return vprintk_safe_nmi(fmt, args);
+
+ if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
+ return vprintk_safe(fmt, args);
+
+ return vprintk_default(fmt, args);
+}
+
void __init printk_safe_init(void)
{
int cpu;

for_each_possible_cpu(cpu) {
- struct printk_safe_seq_buf *s = &per_cpu(nmi_print_seq, cpu);
+ struct printk_safe_seq_buf *s;
+
+ s = &per_cpu(safe_print_seq, cpu);
+ init_irq_work(&s->work, __printk_safe_flush);

+#ifdef CONFIG_PRINTK_NMI
+ s = &per_cpu(nmi_print_seq, cpu);
init_irq_work(&s->work, __printk_safe_flush);
+#endif
}

/* Make sure that IRQ works are initialized before enabling. */
@@ -279,13 +367,3 @@ void __init printk_safe_init(void)
/* Flush pending messages that did not have scheduled IRQ works. */
printk_safe_flush();
}
-
-void printk_safe_nmi_enter(void)
-{
- this_cpu_write(printk_func, vprintk_safe_nmi);
-}
-
-void printk_safe_nmi_exit(void)
-{
- this_cpu_write(printk_func, vprintk_default);
-}
--
2.11.0
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