From 392809b25833548ccfc55e61b76c8451a5073216 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <oleg@redhat.com>
Date: Sun, 28 Sep 2014 23:44:18 +0200
Subject: 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>
---
kernel/fork.c | 5 ++++-
kernel/signal.c | 12 +++++++++++-
2 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/kernel/fork.c b/kernel/fork.c
index 9b7d746d6d62..9ca84189cfc2 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1022,11 +1022,14 @@ void __cleanup_sighand(struct sighand_struct *sighand)
{
if (atomic_dec_and_test(&sighand->count)) {
signalfd_cleanup(sighand);
+ /*
+ * sighand_cachep is SLAB_DESTROY_BY_RCU so we can free it
+ * without an RCU grace period, see __lock_task_sighand().
+ */
kmem_cache_free(sighand_cachep, sighand);
}
}
-
/*
* Initialize POSIX timer handling for a thread group.
*/
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();
--
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