Diff below avoids a lock ordering issue between the SCHED_LOCK() and
printf(9)'s mutex. This problem has been reported multiple times in
drm(4) code where debug printfs are placed between sleep_setup() and
sleep_finish().
The problem is easily fixed. logwakeup() must not be called while
holding the mutex. This function can end up in wakeup_n() which will
grab the SCHED_LOCK().
While here ensure that the mutex is always when TOCONS is specified.
This was not the case in the error path of log(9) and addlog(9).
Ok?
Index: kern/subr_prf.c
===================================================================
RCS file: /cvs/src/sys/kern/subr_prf.c,v
retrieving revision 1.94
diff -u -p -r1.94 subr_prf.c
--- kern/subr_prf.c 20 Mar 2018 15:45:32 -0000 1.94
+++ kern/subr_prf.c 9 Apr 2018 11:35:11 -0000
@@ -89,8 +89,7 @@
int kprintf(const char *, int, void *, char *, va_list);
void kputchar(int, int, struct tty *);
-struct mutex kprintf_mutex =
- MUTEX_INITIALIZER_FLAGS(IPL_HIGH, "kprintf", MTX_NOWITNESS);
+struct mutex kprintf_mutex = MUTEX_INITIALIZER(IPL_HIGH);
/*
* globals
@@ -264,7 +263,9 @@ log(int level, const char *fmt, ...)
splx(s);
if (!log_open) {
va_start(ap, fmt);
+ mtx_enter(&kprintf_mutex);
kprintf(fmt, TOCONS, NULL, NULL, ap);
+ mtx_leave(&kprintf_mutex);
va_end(ap);
}
logwakeup(); /* wake up anyone waiting for log msgs */
@@ -304,7 +305,9 @@ addlog(const char *fmt, ...)
splx(s);
if (!log_open) {
va_start(ap, fmt);
+ mtx_enter(&kprintf_mutex);
kprintf(fmt, TOCONS, NULL, NULL, ap);
+ mtx_leave(&kprintf_mutex);
va_end(ap);
}
logwakeup();
@@ -502,15 +505,15 @@ printf(const char *fmt, ...)
va_list ap;
int retval;
- mtx_enter(&kprintf_mutex);
va_start(ap, fmt);
+ mtx_enter(&kprintf_mutex);
retval = kprintf(fmt, TOCONS | TOLOG, NULL, NULL, ap);
+ mtx_leave(&kprintf_mutex);
va_end(ap);
if (!panicstr)
logwakeup();
- mtx_leave(&kprintf_mutex);
return(retval);
}
@@ -526,12 +529,11 @@ vprintf(const char *fmt, va_list ap)
int retval;
mtx_enter(&kprintf_mutex);
-
retval = kprintf(fmt, TOCONS | TOLOG, NULL, NULL, ap);
+ mtx_leave(&kprintf_mutex);
if (!panicstr)
logwakeup();
- mtx_leave(&kprintf_mutex);
return (retval);
}
@@ -686,6 +688,9 @@ kprintf(const char *fmt0, int oflags, vo
char *xdigs = NULL; /* digits for [xX] conversion */
char buf[KPRINTF_BUFSIZE]; /* space for %c, %[diouxX] */
char *tailp = NULL; /* tail pointer for snprintf */
+
+ if (oflags & TOCONS)
+ MUTEX_ASSERT_LOCKED(&kprintf_mutex);
if ((oflags & TOBUFONLY) && (vp != NULL))
tailp = *(char **)vp;
