4 files changed, 45 insertions, 9 deletions

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 1db044f808b7..2a7d04049af4 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -18,12 +18,14 @@
 
 #ifdef CONFIG_PRINTK
 
-#define PRINTK_SAFE_CONTEXT_MASK        0x7fffffff
-#define PRINTK_NMI_CONTEXT_MASK 0x80000000
+#define PRINTK_SAFE_CONTEXT_MASK         0x3fffffff
+#define PRINTK_NMI_DEFERRED_CONTEXT_MASK 0x40000000
+#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_deferred(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);
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 779479ac9f57..e5278e7d1922 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2722,16 +2722,13 @@ void wake_up_klogd(void)
         preempt_enable();
 }
 
-int printk_deferred(const char *fmt, ...)
+int vprintk_deferred(const char *fmt, va_list args)
 {
-        va_list args;
         int r;
 
-        preempt_disable();
-        va_start(args, fmt);
         r = vprintk_emit(0, LOGLEVEL_SCHED, NULL, 0, fmt, args);
-        va_end(args);
 
+        preempt_disable();
         __this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);
         irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
         preempt_enable();
@@ -2739,6 +2736,18 @@ int printk_deferred(const char *fmt, ...)
         return r;
 }
 
+int printk_deferred(const char *fmt, ...)
+{
+        va_list args;
+        int r;
+
+        va_start(args, fmt);
+        r = vprintk_deferred(fmt, args);
+        va_end(args);
+
+        return r;
+}
+
 /*
  * printk rate limiting, lifted from the networking subsystem.
  *
diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 033e50a7d706..03a42a539b20 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -308,12 +308,24 @@ static int vprintk_nmi(const char *fmt, va_list args)
 
 void printk_nmi_enter(void)
 {
-        this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
+        /*
+         * The size of the extra per-CPU buffer is limited. Use it only when
+         * the main one is locked. If this CPU is not in the safe context,
+         * the lock must be taken on another CPU and we could wait for it.
+         */
+        if ((this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK) &&
+            raw_spin_is_locked(&logbuf_lock)) {
+                this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
+        } else {
+                this_cpu_or(printk_context, PRINTK_NMI_DEFERRED_CONTEXT_MASK);
+        }
 }
 
 void printk_nmi_exit(void)
 {
-        this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
+        this_cpu_and(printk_context,
+                     ~(PRINTK_NMI_CONTEXT_MASK |
+                       PRINTK_NMI_DEFERRED_CONTEXT_MASK));
 }
 
 #else
@@ -351,12 +363,22 @@ void __printk_safe_exit(void)
 
 __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 {
+        /* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
         if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
                 return vprintk_nmi(fmt, args);
 
+        /* Use extra buffer to prevent a recursion deadlock in safe mode. */
         if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
                 return vprintk_safe(fmt, args);
 
+        /*
+         * Use the main logbuf when logbuf_lock is available in NMI.
+         * But avoid calling console drivers that might have their own locks.
+         */
+        if (this_cpu_read(printk_context) & PRINTK_NMI_DEFERRED_CONTEXT_MASK)
+                return vprintk_deferred(fmt, args);
+
+        /* No obstacles. */
         return vprintk_default(fmt, args);
 }
 
diff --git a/lib/nmi_backtrace.c b/lib/nmi_backtrace.c
index 4e8a30d1c22f..0bc0a3535a8a 100644
--- a/lib/nmi_backtrace.c
+++ b/lib/nmi_backtrace.c
@@ -86,9 +86,11 @@ void nmi_trigger_cpumask_backtrace(const cpumask_t *mask,
 
 bool nmi_cpu_backtrace(struct pt_regs *regs)
 {
+        static arch_spinlock_t lock = __ARCH_SPIN_LOCK_UNLOCKED;
         int cpu = smp_processor_id();
 
         if (cpumask_test_cpu(cpu, to_cpumask(backtrace_mask))) {
+                arch_spin_lock(&lock);
                 if (regs && cpu_in_idle(instruction_pointer(regs))) {
                         pr_warn("NMI backtrace for cpu %d skipped: idling at pc %#lx\n",
                                 cpu, instruction_pointer(regs));
@@ -99,6 +101,7 @@ bool nmi_cpu_backtrace(struct pt_regs *regs)
                         else
                                 dump_stack();
                 }
+                arch_spin_unlock(&lock);
                 cpumask_clear_cpu(cpu, to_cpumask(backtrace_mask));
                 return true;
         }