[PATCH v3 1/2] 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 on d_locrkef as each thread
tries to grab a reference to the /proc dentry (and drop it shortly
thereafter).

Allow the pid_revalidate() function to execute under LOOKUP_RCU. When
updates must be made to the inode, drop out of RCU and into REF mode.

Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
---

When running running ~100 parallel instances of "TZ=/etc/localtime ps -fe
>/dev/null" on a 100CPU machine, the %sys utilization reaches 90%, and perf
shows the following code path as being responsible for heavy contention on
the d_lockref spinlock:

      walk_component()
        lookup_fast()
          unlazy_child()
            lockref_get_not_dead(&nd->path.dentry->d_lockref)

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 similarly high %sys time due to this
contention.

Changes from v3:
- Rather than call pid_update_inode() with flags, create
  proc_inode_needs_update() to determine whether the call can be skipped.
- Restore the call to the security hook (see next patch).
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 | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b3422cda2a91..4b246e9bd5df 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1968,6 +1968,20 @@ void pid_update_inode(struct task_struct *task, struct inode *inode)
 	security_task_to_inode(task, inode);
 }
 
+/* See if we can avoid the above call. Assumes RCU lock held */
+static bool pid_inode_needs_update(struct task_struct *task, struct inode *inode)
+{
+	kuid_t uid;
+	kgid_t gid;
+
+	if (inode->i_mode & (S_ISUID | S_ISGID))
+		return true;
+	task_dump_owner(task, inode->i_mode, &uid, &gid);
+	if (!uid_eq(uid, inode->i_uid) || !gid_eq(gid, inode->i_gid))
+		return true;
+	return false;
+}
+
 /*
  * Rewrite the inode's ownerships here because the owning task may have
  * performed a setuid(), etc.
@@ -1977,19 +1991,20 @@ 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);
-
+	rcu_read_lock();
+	inode = d_inode_rcu(dentry);
+	task = pid_task(proc_pid(inode), PIDTYPE_PID);
 	if (task) {
-		pid_update_inode(task, inode);
-		put_task_struct(task);
-		return 1;
+		rv = 1;
+		if ((flags & LOOKUP_RCU) && pid_inode_needs_update(task, inode))
+			rv = -ECHILD;
+		else if (!(flags & LOOKUP_RCU))
+			pid_update_inode(task, inode);
 	}
-	return 0;
+	rcu_read_unlock();
+	return rv;
 }
 
 static inline bool proc_inode_is_dead(struct inode *inode)
-- 
2.25.1