namespaces: Use task_lock and not rcu to protect nsproxy

The synchronous syncrhonize_rcu in switch_task_namespaces makes setns
a sufficiently expensive system call that people have complained.

Upon inspect nsproxy no longer needs rcu protection for remote reads.
remote reads are rare.  So optimize for same process reads and write
by switching using rask_lock instead.

This yields a simpler to understand lock, and a faster setns system call.

In particular this fixes a performance regression observed
by Rafael David Tinoco <rafael.tinoco@canonical.com>.

This is effectively a revert of Pavel Emelyanov's commit
cf7b708c8d1d7a27736771bcf4c457b332b0f818 Make access to task's nsproxy lighter
from 2007.  The race this originialy fixed no longer exists as
do_notify_parent uses task_active_pid_ns(parent) instead of
parent->nsproxy.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

1 files changed, 4 insertions, 11 deletions

diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index 8e78110..ef42d0a 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -204,20 +204,13 @@ void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
 
 	might_sleep();
 
+	task_lock(p);
 	ns = p->nsproxy;
+	p->nsproxy = new;
+	task_unlock(p);
 
-	rcu_assign_pointer(p->nsproxy, new);
-
-	if (ns && atomic_dec_and_test(&ns->count)) {
-		/*
-		 * wait for others to get what they want from this nsproxy.
-		 *
-		 * cannot release this nsproxy via the call_rcu() since
-		 * put_mnt_ns() will want to sleep
-		 */
-		synchronize_rcu();
+	if (ns && atomic_dec_and_test(&ns->count))
 		free_nsproxy(ns);
-	}
 }
 
 void exit_task_namespaces(struct task_struct *p)