[PATCH v2] proc: Allow pid_revalidate() during LOOKUP_RCU

[Stephen Brennan]

The pid_revalidate() function requires dropping from RCU into REF lookup
mode. When many threads are resolving paths within /proc in parallel,
this can result in heavy spinlock contention as each thread tries to
grab a reference to the /proc dentry lock (and drop it shortly
thereafter).

Allow the pid_revalidate() function to execute under LOOKUP_RCU. When
updates must be made to the inode due to the owning task performing
setuid(), drop out of RCU and into REF mode. Remove the call to
security_task_to_inode(), since we can rely on the call from
proc_pid_make_inode().

Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---
I'd like to use this patch as an RFC on this approach for reducing spinlock
contention during many parallel path lookups in the /proc filesystem. The
contention can be triggered by, for example, running ~100 parallel instances of
"TZ=/etc/localtime ps -fe >/dev/null" on a 100CPU machine. The %sys utilization
in such a case reaches around 90%, and profiles show two code paths with high
utilization:

    walk_component
      lookup_fast
        unlazy_child
          legitimize_root
            __legitimize_path
              lockref_get_not_dead

    terminate_walk
      dput
        dput

By applying this patch, %sys utilization falls to around 60% under the same
workload. Although this particular workload is a bit contrived, we have seen
some monitoring scripts which produced high %sys time due to this contention.

Changes from v2:
- Remove get_pid_task_rcu_user() and get_proc_task_rcu(), since they were
  unnecessary.
- Remove the call to security_task_to_inode().

 fs/proc/base.c | 47 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 33 insertions(+), 14 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ebea9501afb8..833d55a59e20 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1813,12 +1813,28 @@ int pid_getattr(const struct path *path, struct kstat *stat,
 /*
  * Set <pid>/... inode ownership (can change due to setuid(), etc.)
  */
-void pid_update_inode(struct task_struct *task, struct inode *inode)
+static int do_pid_update_inode(struct task_struct *task, struct inode *inode,
+			       unsigned int flags)
 {
-	task_dump_owner(task, inode->i_mode, &inode->i_uid, &inode->i_gid);
+	kuid_t uid;
+	kgid_t gid;
+
+	task_dump_owner(task, inode->i_mode, &uid, &gid);
+	if (uid_eq(uid, inode->i_uid) && gid_eq(gid, inode->i_gid) &&
+			!(inode->i_mode & (S_ISUID | S_ISGID)))
+		return 1;
+	if (flags & LOOKUP_RCU)
+		return -ECHILD;
 
+	inode->i_uid = uid;
+	inode->i_gid = gid;
 	inode->i_mode &= ~(S_ISUID | S_ISGID);
-	security_task_to_inode(task, inode);
+	return 1;
+}
+
+void pid_update_inode(struct task_struct *task, struct inode *inode)
+{
+	do_pid_update_inode(task, inode, 0);
 }
 
 /*
@@ -1830,19 +1846,22 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
 {
 	struct inode *inode;
 	struct task_struct *task;
+	int rv = 0;
 
-	if (flags & LOOKUP_RCU)
-		return -ECHILD;
-
-	inode = d_inode(dentry);
-	task = get_proc_task(inode);
-
-	if (task) {
-		pid_update_inode(task, inode);
-		put_task_struct(task);
-		return 1;
+	if (flags & LOOKUP_RCU) {
+		inode = d_inode_rcu(dentry);
+		task = pid_task(proc_pid(inode), PIDTYPE_PID);
+		if (task)
+			rv = do_pid_update_inode(task, inode, flags);
+	} else {
+		inode = d_inode(dentry);
+		task = get_proc_task(inode);
+		if (task) {
+			rv = do_pid_update_inode(task, inode, flags);
+			put_task_struct(task);
+		}
 	}
-	return 0;
+	return rv;
 }
 
 static inline bool proc_inode_is_dead(struct inode *inode)
-- 
2.25.1