From 9837947b07a67c89a1ae88ca883df5e0b142b618 Mon Sep 17 00:00:00 2001
From: Konstantin Belousov <kib@FreeBSD.org>
Date: Thu, 21 Jul 2016 16:34:56 +0000
Subject: [PATCH] Provide counter_warning(9) KPI which allows to issue limited
 number of warnings for some kernel events, mostly intended for the use of
 obsoleted or otherwise undersired interfaces.

This is an abstracted and race-expelled code from compat pty driver.

Requested and reviewed by:	jhb
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D7270
---
 sys/dev/pty/pty.c   | 13 ++++---------
 sys/kern/subr_prf.c | 19 +++++++++++++++++++
 sys/sys/systm.h     |  2 ++
 3 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/sys/dev/pty/pty.c b/sys/dev/pty/pty.c
index 5036cb292349..ad34e1188d5d 100644
--- a/sys/dev/pty/pty.c
+++ b/sys/dev/pty/pty.c
@@ -52,10 +52,10 @@ __FBSDID("$FreeBSD$");
  * binary emulation.
  */
 
-static unsigned int pty_warningcnt = 1;
+static unsigned pty_warningcnt = 1;
 SYSCTL_UINT(_kern, OID_AUTO, tty_pty_warningcnt, CTLFLAG_RW,
-	&pty_warningcnt, 0,
-	"Warnings that will be triggered upon legacy PTY allocation");
+    &pty_warningcnt, 0,
+    "Warnings that will be triggered upon legacy PTY allocation");
 
 static int
 ptydev_fdopen(struct cdev *dev, int fflags, struct thread *td, struct file *fp)
@@ -77,12 +77,7 @@ ptydev_fdopen(struct cdev *dev, int fflags, struct thread *td, struct file *fp)
 	}
 
 	/* Raise a warning when a legacy PTY has been allocated. */
-	if (pty_warningcnt > 0) {
-		pty_warningcnt--;
-		log(LOG_INFO, "pid %d (%s) is using legacy pty devices%s\n",
-		    td->td_proc->p_pid, td->td_name,
-		    pty_warningcnt ? "" : " - not logging anymore");
-	}
+	counted_warning(&pty_warningcnt, "is using legacy pty devices");
 
 	return (0);
 }
diff --git a/sys/kern/subr_prf.c b/sys/kern/subr_prf.c
index 5bcf39b9400b..5f9796e7d876 100644
--- a/sys/kern/subr_prf.c
+++ b/sys/kern/subr_prf.c
@@ -1196,3 +1196,22 @@ sbuf_hexdump(struct sbuf *sb, const void *ptr, int length, const char *hdr,
 	}
 }
 
+void
+counted_warning(unsigned *counter, const char *msg)
+{
+	struct thread *td;
+	unsigned c;
+
+	for (;;) {
+		c = *counter;
+		if (c == 0)
+			break;
+		if (atomic_cmpset_int(counter, c, c - 1)) {
+			td = curthread;
+			log(LOG_INFO, "pid %d (%s) %s%s\n",
+			    td->td_proc->p_pid, td->td_name, msg,
+			    c > 1 ? "" : " - not logging anymore");
+			break;
+		}
+	}
+}
diff --git a/sys/sys/systm.h b/sys/sys/systm.h
index dab9ee2f4393..caffd7baafdf 100644
--- a/sys/sys/systm.h
+++ b/sys/sys/systm.h
@@ -447,4 +447,6 @@ void	intr_prof_stack_use(struct thread *td, struct trapframe *frame);
 
 extern void (*softdep_ast_cleanup)(void);
 
+void counted_warning(unsigned *counter, const char *msg);
+
 #endif /* !_SYS_SYSTM_H_ */