Generalised support for copy-on-write structures shared by threads.

Thread credentials are maintained as follows: each thread has a pointer to
creds and a reference on them. The pointer is compared with proc's creds on
userspace<->kernel boundary and updated if needed.

This patch introduces a counter which can be compared instead, so that more
structures can use this scheme without adding more comparisons on the boundary.

1 files changed, 40 insertions, 3 deletions

diff --git a/sys/kern/kern_thread.c b/sys/kern/kern_thread.c
index dce760c..2e97553 100644
--- a/sys/kern/kern_thread.c
+++ b/sys/kern/kern_thread.c
@@ -327,8 +327,7 @@ thread_reap(void)
 		mtx_unlock_spin(&zombie_lock);
 		while (td_first) {
 			td_next = TAILQ_NEXT(td_first, td_slpq);
-			if (td_first->td_ucred)
-				crfree(td_first->td_ucred);
+			thread_cow_free(td_first);
 			thread_free(td_first);
 			td_first = td_next;
 		}
@@ -384,6 +383,44 @@ thread_free(struct thread *td)
 	uma_zfree(thread_zone, td);
 }
 
+void
+thread_cow_get_proc(struct thread *newtd, struct proc *p)
+{
+
+	PROC_LOCK_ASSERT(p, MA_OWNED);
+	newtd->td_ucred = crhold(p->p_ucred);
+	newtd->td_cowgen = p->p_cowgen;
+}
+
+void
+thread_cow_get(struct thread *newtd, struct thread *td)
+{
+
+	newtd->td_ucred = crhold(td->td_ucred);
+	newtd->td_cowgen = td->td_cowgen;
+}
+
+void
+thread_cow_free(struct thread *td)
+{
+
+	if (td->td_ucred)
+		crfree(td->td_ucred);
+}
+
+void
+thread_cow_update(struct thread *td)
+{
+	struct proc *p;
+
+	p = td->td_proc;
+	PROC_LOCK(p);
+	if (td->td_ucred != p->p_ucred)
+		cred_update_thread(td);
+	td->td_cowgen = p->p_cowgen;
+	PROC_UNLOCK(p);
+}
+
 /*
  * Discard the current thread and exit from its context.
  * Always called with scheduler locked.
@@ -521,7 +558,7 @@ thread_wait(struct proc *p)
 	cpuset_rel(td->td_cpuset);
 	td->td_cpuset = NULL;
 	cpu_thread_clean(td);
-	crfree(td->td_ucred);
+	thread_cow_free(td);
 	thread_reap();	/* check for zombie threads etc. */
 }