Several threads in a process may do vfork() simultaneously. Then, all

parent threads sleep on the parent' struct proc until corresponding
child releases the vmspace. Each sleep is interlocked with proc mutex of
the child, that triggers assertion in the sleepq_add(). The assertion
requires that at any time, all simultaneous sleepers for the channel use
the same interlock.

Silent the assertion by using conditional variable allocated in the
child. Broadcast the variable event on exec() and exit().

Since struct proc * sleep wait channel is overloaded for several
unrelated events, I was unable to remove wakeups from the places where
cv_broadcast() is added, except exec().

Reported and tested by:	ganbold
Suggested and reviewed by:	jhb
MFC after:	2 week

sys/kern/kern_exec.c

@@ -609,7 +609,7 @@ interpret:
 	p->p_flag |= P_EXEC;
 	if (p->p_pptr && (p->p_flag & P_PPWAIT)) {
 		p->p_flag &= ~P_PPWAIT;
-		wakeup(p->p_pptr);
+		cv_broadcast(&p->p_pwait);
 	}
 
 	/*

sys/kern/kern_exit.c

@@ -543,6 +543,7 @@ exit1(struct thread *td, int rv)
 	 * proc lock.
 	 */
 	wakeup(p->p_pptr);
+	cv_broadcast(&p->p_pwait);
 	sched_exit(p->p_pptr, td);
 	PROC_SLOCK(p);
 	p->p_state = PRS_ZOMBIE;
@@ -774,6 +775,7 @@ loop:
 				PROC_UNLOCK(p);
 				tdsignal(t, NULL, SIGCHLD, p->p_ksi);
 				wakeup(t);
+				cv_broadcast(&p->p_pwait);
 				PROC_UNLOCK(t);
 				sx_xunlock(&proctree_lock);
 				return (0);

sys/kern/kern_fork.c

@@ -754,7 +754,7 @@ again:
 	 */
 	PROC_LOCK(p2);
 	while (p2->p_flag & P_PPWAIT)
-		msleep(p1, &p2->p_mtx, PWAIT, "ppwait", 0);
+		cv_wait(&p2->p_pwait, &p2->p_mtx);
 	PROC_UNLOCK(p2);
 
 	/*

sys/kern/kern_proc.c

@@ -231,6 +231,7 @@ proc_init(void *mem, int size, int flags)
 	bzero(&p->p_mtx, sizeof(struct mtx));
 	mtx_init(&p->p_mtx, "process lock", NULL, MTX_DEF | MTX_DUPOK);
 	mtx_init(&p->p_slock, "process slock", NULL, MTX_SPIN | MTX_RECURSE);
+	cv_init(&p->p_pwait, "ppwait");
 	TAILQ_INIT(&p->p_threads);	     /* all threads in proc */
 	EVENTHANDLER_INVOKE(process_init, p);
 	p->p_stats = pstats_alloc();

sys/sys/proc.h

@@ -40,6 +40,7 @@
 
 #include <sys/callout.h>		/* For struct callout. */
 #include <sys/event.h>			/* For struct klist. */
+#include <sys/condvar.h>
 #ifndef _KERNEL
 #include <sys/filedesc.h>
 #endif
@@ -540,6 +541,7 @@ struct proc {
 	STAILQ_HEAD(, ktr_request)	p_ktr;	/* (o) KTR event queue. */
 	LIST_HEAD(, mqueue_notifier)	p_mqnotifier; /* (c) mqueue notifiers.*/
 	struct kdtrace_proc	*p_dtrace; /* (*) DTrace-specific data. */
+	struct cv	p_pwait;	/* (*) wait cv for exit/exec */
 };
 
 #define	p_session	p_pgrp->pg_session