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authorOleg Nesterov <oleg@redhat.com>2014-09-28 23:44:18 +0200
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-10-29 10:07:18 -0700
commit392809b25833548ccfc55e61b76c8451a5073216 (patch)
tree9cffa1e96d8ecc7208fc3a3c9fc7ef8725d2aa55 /kernel/signal.c
parenteca1a08986f622c11b75b3b44d561a1f901c9cec (diff)
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signal: Document the RCU protection of ->sighand
__cleanup_sighand() frees sighand without RCU grace period. This is correct but this looks "obviously buggy" and constantly confuses the readers, add the comments to explain how this works. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Reviewed-by: Steven Rostedt <rostedt@goodmis.org> Reviewed-by: Rik van Riel <riel@redhat.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
Diffstat (limited to 'kernel/signal.c')
-rw-r--r--kernel/signal.c12
1 files changed, 11 insertions, 1 deletions
diff --git a/kernel/signal.c b/kernel/signal.c
index 54820984a872..19e35135fc60 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1275,7 +1275,17 @@ struct sighand_struct *__lock_task_sighand(struct task_struct *tsk,
local_irq_restore(*flags);
break;
}
-
+ /*
+ * This sighand can be already freed and even reused, but
+ * we rely on SLAB_DESTROY_BY_RCU and sighand_ctor() which
+ * initializes ->siglock: this slab can't go away, it has
+ * the same object type, ->siglock can't be reinitialized.
+ *
+ * We need to ensure that tsk->sighand is still the same
+ * after we take the lock, we can race with de_thread() or
+ * __exit_signal(). In the latter case the next iteration
+ * must see ->sighand == NULL.
+ */
spin_lock(&sighand->siglock);
if (likely(sighand == tsk->sighand)) {
rcu_read_unlock();