svn commit: r368680 - stable/12/sys/kern

Author: kevans
Date: Tue Dec 15 21:52:31 2020
New Revision: 368680
URL: https://svnweb.freebsd.org/changeset/base/368680

Log:
  MFC r368461: kern: cpuset: resolve race between cpuset_lookup/cpuset_rel
  
  The race plays out like so between threads A and B:
  
  1. A ref's cpuset 10
  2. B does a lookup of cpuset 10, grabs the cpuset lock and searches
     cpuset_ids
  3. A rel's cpuset 10 and observes the last ref, waits on the cpuset lock
     while B is still searching and not yet ref'd
  4. B ref's cpuset 10 and drops the cpuset lock
  5. A proceeds to free the cpuset out from underneath B
  
  Resolve the race by only releasing the last reference under the cpuset lock.
  Thread A now picks up the spinlock and observes that the cpuset has been
  revived, returning immediately for B to deal with later.

Modified:
  stable/12/sys/kern/kern_cpuset.c
Directory Properties:
  stable/12/   (props changed)

Modified: stable/12/sys/kern/kern_cpuset.c
==============================================================================
--- stable/12/sys/kern/kern_cpuset.c	Tue Dec 15 21:51:45 2020	(r368679)
+++ stable/12/sys/kern/kern_cpuset.c	Tue Dec 15 21:52:31 2020	(r368680)
@@ -207,9 +207,13 @@ cpuset_rel(struct cpuset *set)
 {
 	cpusetid_t id;
 
-	if (refcount_release(&set->cs_ref) == 0)
+	if (refcount_release_if_not_last(&set->cs_ref))
 		return;
 	mtx_lock_spin(&cpuset_lock);
+	if (!refcount_release(&set->cs_ref)) {
+		mtx_unlock_spin(&cpuset_lock);
+		return;
+	}
 	LIST_REMOVE(set, cs_siblings);
 	id = set->cs_id;
 	if (id != CPUSET_INVALID)
@@ -229,9 +233,13 @@ static void
 cpuset_rel_defer(struct setlist *head, struct cpuset *set)
 {
 
-	if (refcount_release(&set->cs_ref) == 0)
+	if (refcount_release_if_not_last(&set->cs_ref))
 		return;
 	mtx_lock_spin(&cpuset_lock);
+	if (!refcount_release(&set->cs_ref)) {
+		mtx_unlock_spin(&cpuset_lock);
+		return;
+	}
 	LIST_REMOVE(set, cs_siblings);
 	if (set->cs_id != CPUSET_INVALID)
 		LIST_REMOVE(set, cs_link);
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