printk/nmi: warn when some message has been lost in NMI context

We could not resize the temporary buffer in NMI context.  Let's warn if a
message is lost.

This is rather theoretical.  printk() should not be used in NMI.  The only
sensible use is when we want to print backtrace from all CPUs.  The
current buffer should be enough for this purpose.

[akpm@linux-foundation.org: whitespace fixlet]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jiri Kosina <jkosina@suse.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: David Miller <davem@davemloft.net>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 2de99faedfc1c78123f29ecfb9ce68d72e9eb42d..341bedccc065de5a8d18d143f8dc90555570a686 100644 (file)
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -34,6 +34,12 @@ 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)
+{
+       return atomic_xchg(&nmi_message_lost, 0);
+}
+
 #else /* CONFIG_PRINTK_NMI */
 
 static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
@@ -41,4 +47,9 @@ 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 */

diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 13b99ebff66e81158ea95a3afc630e94d55c6d56..0fb25c4cdeeae6792b659268ad480849a8931a39 100644 (file)
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -39,6 +39,7 @@
  */
 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
 static int printk_nmi_irq_ready;
+atomic_t nmi_message_lost;
 
 #define NMI_LOG_BUF_LEN (4096 - sizeof(atomic_t) - sizeof(struct irq_work))
 
@@ -64,8 +65,10 @@ static int vprintk_nmi(const char *fmt, va_list args)
 again:
        len = atomic_read(&s->len);
 
-       if (len >= sizeof(s->buffer))
+       if (len >= sizeof(s->buffer)) {
+               atomic_inc(&nmi_message_lost);
                return 0;
+       }
 
        /*
         * Make sure that all old data have been read before the buffer was

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 561fc43c6e0e462455851cb759d0bbbec3c31954..7ebcfea2490e909fadd501413bf5805e656789f4 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1670,6 +1670,7 @@ asmlinkage int vprintk_emit(int facility, int level,
        unsigned long flags;
        int this_cpu;
        int printed_len = 0;
+       int nmi_message_lost;
        bool in_sched = false;
        /* cpu currently holding logbuf_lock in this function */
        static unsigned int logbuf_cpu = UINT_MAX;
@@ -1720,6 +1721,15 @@ asmlinkage int vprintk_emit(int facility, int level,
                                         strlen(recursion_msg));
        }
 
+       nmi_message_lost = get_nmi_message_lost();
+       if (unlikely(nmi_message_lost)) {
+               text_len = scnprintf(textbuf, sizeof(textbuf),
+                                    "BAD LUCK: lost %d message(s) from NMI context!",
+                                    nmi_message_lost);
+               printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
+                                        NULL, 0, textbuf, text_len);
+       }
+
        /*
         * The printf needs to come first; we need the syslog
         * prefix which might be passed-in as a parameter.