[RFC][PATCHv2 6/7] printk: report printk recursion from alt_printk flush

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

If we end up executing vprintk_alt() then we have a printk
recursion. Set alt_printk_ctx `ALT_PRINTK_RECURSION_MASK' bit
in vprintk_alt() to indicate that recutsion and report the
"BUG: recent printk recursion!" problem later from
__alt_printk_flush().

Example:

 BUG: recent printk recursion!
 ------------[ cut here ]------------
 WARNING: CPU: 3 PID: 366 at kernel/printk/printk.c:1803 vprintk_emit+0x139/0x38c
 CPU: 3 PID: 366 Comm: bash
 Call Trace:
  [<ffffffff811be508>] dump_stack+0x4d/0x63
  [<ffffffff81039932>] __warn+0xb8/0xd3
  [<ffffffff810399b3>] warn_slowpath_null+0x18/0x1a
  [<ffffffff8106bfdb>] vprintk_emit+0x139/0x38c
  [<ffffffff8106c390>] vprintk_default+0x18/0x1a
  [<ffffffff8106d1e6>] vprintk_func+0x65/0x67
  [<ffffffff810ab27f>] printk+0x3e/0x46
[..]
  [<ffffffff8145fb60>] entry_SYSCALL_64_fastpath+0x13/0x94
  ---[ end trace ]---

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
 kernel/printk/alt_printk.c | 9 +++++++++
 kernel/printk/internal.h   | 1 +
 2 files changed, 10 insertions(+)

diff --git a/kernel/printk/alt_printk.c b/kernel/printk/alt_printk.c
index db0bfc8..0010089 100644
--- a/kernel/printk/alt_printk.c
+++ b/kernel/printk/alt_printk.c
@@ -150,6 +150,13 @@ static void __alt_printk_flush(struct irq_work *work)
 more:
 	len = atomic_read(&s->len);
 
+	if (this_cpu_read(alt_printk_ctx) & ALT_PRINTK_RECURSION_MASK) {
+		const char *msg = "BUG: recent printk recursion!\n";
+
+		this_cpu_and(alt_printk_ctx, ~ALT_PRINTK_RECURSION_MASK);
+		alt_printk_flush_line(msg, strlen(msg));
+	}
+
 	/*
 	 * This is just a paranoid check that nobody has manipulated
 	 * the buffer an unexpected way. If we printed something then
@@ -290,6 +297,8 @@ static int vprintk_alt(const char *fmt, va_list args)
 {
 	struct alt_printk_seq_buf *s = this_cpu_ptr(&alt_print_seq);
 
+	/* There is only one way to get here -- a printk recursion. */
+	this_cpu_or(alt_printk_ctx, ALT_PRINTK_RECURSION_MASK);
 	return alt_printk_log_store(s, fmt, args);
 }
 
diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 6338f5b..403b6ed 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -37,6 +37,7 @@ static inline int get_nmi_message_lost(void)
 
 #define ALT_PRINTK_CONTEXT_MASK		0x07ffffff
 #define ALT_PRINTK_NMI_CONTEXT_MASK	0x08000000
+#define ALT_PRINTK_RECURSION_MASK	0x10000000
 
 extern raw_spinlock_t logbuf_lock;
 
-- 
2.10.0.372.g6fe1b14