Add a mechanism for associating a mutex with a callout when the

callout is first initialised, using a new function callout_init_mtx().
The callout system will acquire this mutex before calling the callout
function and release it on return.

In addition, the callout system uses the mutex to avoid most of the
complications and race conditions inherent in asynchronous timer
facilities, so mutex-protected callouts have much simpler semantics.
As long as the mutex is held when invoking callout_stop() or
callout_reset(), then these functions will guarantee that the callout
will be stopped, even if softclock() had already begun to process
the callout.

Existing Giant-locked callouts will automatically pick up the new
race-free semantics. This should close a number of race conditions
in the USB code and probably other areas of the kernel too.

There should be no change in behaviour for "MP-safe" callouts; these
still need to use the techniques mentioned in timeout(9) to avoid
race conditions.

1 files changed, 5 insertions, 0 deletions

diff --git a/sys/sys/callout.h b/sys/sys/callout.h
index 50dedbf..cec9079 100644
--- a/sys/sys/callout.h
+++ b/sys/sys/callout.h
@@ -40,6 +40,8 @@
 
 #include <sys/queue.h>
 
+struct mtx;
+
 SLIST_HEAD(callout_list, callout);
 TAILQ_HEAD(callout_tailq, callout);
 
@@ -51,6 +53,7 @@ struct callout {
 	int	c_time;				/* ticks to the event */
 	void	*c_arg;				/* function argument */
 	void	(*c_func)(void *);	/* function to call */
+	struct mtx *c_mtx;			/* mutex to lock */
 	int	c_flags;			/* state of this entry */
 };
 
@@ -58,6 +61,7 @@ struct callout {
 #define	CALLOUT_ACTIVE		0x0002 /* callout is currently active */
 #define	CALLOUT_PENDING		0x0004 /* callout is waiting for timeout */
 #define	CALLOUT_MPSAFE		0x0008 /* callout handler is mp safe */
+#define	CALLOUT_RETURNUNLOCKED	0x0010 /* handler returns with mtx unlocked */
 
 struct callout_handle {
 	struct callout *callout;
@@ -75,6 +79,7 @@ extern struct mtx callout_lock;
 #define	callout_deactivate(c)	((c)->c_flags &= ~CALLOUT_ACTIVE)
 #define	callout_drain(c)	_callout_stop_safe(c, 1)
 void	callout_init(struct callout *, int);
+void	callout_init_mtx(struct callout *, struct mtx *, int);
 #define	callout_pending(c)	((c)->c_flags & CALLOUT_PENDING)
 void	callout_reset(struct callout *, int, void (*)(void *), void *);
 #define	callout_stop(c)		_callout_stop_safe(c, 0)