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Subject[PATCH 1/2] printk/panic: Access the main printk log in panic() only when safe
Date
Kernel tries hard to store and show printk messages when panicking. Even
logbuf_lock gets re-initialized when only one CPU is running after
smp_send_stop().

Unfortunately, smp_send_stop() might fail on architectures that do not
use NMI as a fallback. Then printk log buffer might stay locked and
a deadlock is almost inevitable.

Now, printk_safe_flush_on_panic() is safe because it accesses the main
log buffer only when the lock is available. But kmgs_dump() and
console_unlock() callers might still cause a deadlock.

A better approach is to move printk lock busting into a separate function.
And call all dependent operations only when the lock is not blocked.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
include/linux/printk.h | 6 ++++++
kernel/panic.c | 49 +++++++++++++++++++++++++++------------------
kernel/printk/printk_safe.c | 35 ++++++++++++++++++++------------
3 files changed, 58 insertions(+), 32 deletions(-)

diff --git a/include/linux/printk.h b/include/linux/printk.h
index cefd374c47b1..4d15a0eda9c6 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -202,6 +202,7 @@ __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 asmlinkage void dump_stack(void) __cold;
+extern int printk_bust_lock_safe(void);
extern void printk_safe_init(void);
extern void printk_safe_flush(void);
extern void printk_safe_flush_on_panic(void);
@@ -269,6 +270,11 @@ static inline void dump_stack(void)
{
}

+static inline int printk_bust_lock_safe(void)
+{
+ return 0;
+}
+
static inline void printk_safe_init(void)
{
}
diff --git a/kernel/panic.c b/kernel/panic.c
index 4d9f55bf7d38..aa50cdb75022 100644
--- a/kernel/panic.c
+++ b/kernel/panic.c
@@ -155,6 +155,26 @@ static void panic_print_sys_info(void)
ftrace_dump(DUMP_ALL);
}

+/* Try hard to make the messages visible on the console. */
+void panic_console_dump(void)
+{
+#ifdef CONFIG_VT
+ unblank_screen();
+#endif
+ console_unblank();
+
+ /*
+ * We may have ended up stopping the CPU holding the lock (in
+ * smp_send_stop()) while still having some valuable data in the console
+ * buffer. Try to acquire the lock then release it regardless of the
+ * result. The release will also print the buffers out. Locks debug
+ * should be disabled to avoid reporting bad unlock balance when
+ * panic() is not being callled from OOPS.
+ */
+ debug_locks_off();
+ console_flush_on_panic(CONSOLE_FLUSH_PENDING);
+}
+
/**
* panic - halt the system
* @fmt: The text string to print
@@ -169,6 +189,7 @@ void panic(const char *fmt, ...)
va_list args;
long i, i_next = 0, len;
int state = 0;
+ int printk_blocked;
int old_cpu, this_cpu;
bool _crash_kexec_post_notifiers = crash_kexec_post_notifiers;

@@ -253,8 +274,11 @@ void panic(const char *fmt, ...)
atomic_notifier_call_chain(&panic_notifier_list, 0, buf);

/* Call flush even twice. It tries harder with a single online CPU */
- printk_safe_flush_on_panic();
- kmsg_dump(KMSG_DUMP_PANIC);
+ printk_blocked = printk_bust_lock_safe();
+ if (!printk_blocked) {
+ printk_safe_flush_on_panic();
+ kmsg_dump(KMSG_DUMP_PANIC);
+ }

/*
* If you doubt kdump always works fine in any situation,
@@ -268,23 +292,10 @@ void panic(const char *fmt, ...)
if (_crash_kexec_post_notifiers)
__crash_kexec(NULL);

-#ifdef CONFIG_VT
- unblank_screen();
-#endif
- console_unblank();
-
- /*
- * We may have ended up stopping the CPU holding the lock (in
- * smp_send_stop()) while still having some valuable data in the console
- * buffer. Try to acquire the lock then release it regardless of the
- * result. The release will also print the buffers out. Locks debug
- * should be disabled to avoid reporting bad unlock balance when
- * panic() is not being callled from OOPS.
- */
- debug_locks_off();
- console_flush_on_panic(CONSOLE_FLUSH_PENDING);
-
- panic_print_sys_info();
+ if (!printk_blocked) {
+ panic_console_dump();
+ panic_print_sys_info();
+ }

if (!panic_blink)
panic_blink = no_blink;
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index b4045e782743..71d4b763f811 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -253,6 +253,26 @@ void printk_safe_flush(void)
}
}

+/**
+ * printk_try_bust_lock - make printk log accessible when safe
+ *
+ * Return 0 when the log is accessible. Return -EWOULDBLOCK when
+ * it is not safe and likely to cause a deadlock.
+ */
+int printk_bust_lock_safe(void)
+{
+ if (!raw_spin_is_locked(&logbuf_lock))
+ return 0;
+
+ if (num_online_cpus() == 1) {
+ debug_locks_off();
+ raw_spin_lock_init(&logbuf_lock);
+ return 0;
+ }
+
+ return -EWOULDBLOCK;
+}
+
/**
* printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
* goes down.
@@ -265,19 +285,8 @@ void printk_safe_flush(void)
*/
void printk_safe_flush_on_panic(void)
{
- /*
- * Make sure that we could access the main ring buffer.
- * Do not risk a double release when more CPUs are up.
- */
- if (raw_spin_is_locked(&logbuf_lock)) {
- if (num_online_cpus() > 1)
- return;
-
- debug_locks_off();
- raw_spin_lock_init(&logbuf_lock);
- }
-
- printk_safe_flush();
+ if (printk_bust_lock_safe() == 0)
+ printk_safe_flush();
}

#ifdef CONFIG_PRINTK_NMI
--
2.16.4
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