[PATCH v5] proc: Allow pid_revalidate() during LOOKUP_RCU

[Stephen Brennan]

The pid_revalidate() function drops 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_lockref as each thread tries to
grab a reference to the /proc dentry (and drop it shortly thereafter).

Investigation indicates that it is not necessary to drop RCU in
pid_revalidate(), as no RCU data is modified and the function never
sleeps. So, remove the LOOKUP_RCU check.

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()
          d_revalidate()
            pid_revalidate() // returns -ECHILD
          unlazy_child()
            lockref_get_not_dead(&nd->path.dentry->d_lockref) <-- contention

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.

As a result this patch, several procfs methods which were only called in
ref-walk mode could now be called from RCU mode. To ensure that this patch
is safe, I audited all the inode get_link and permission() implementations,
as well as dentry d_revalidate() implementations, in fs/proc. These methods
are called in the following ways:

* get_link() receives a NULL dentry pointer when called in RCU mode.
* permission() receives MAY_NOT_BLOCK in the mode parameter when called
  from RCU.
* d_revalidate() receives LOOKUP_RCU in flags.

There were generally three groups I found. Group (1) are functions which
contain a check at the top of the function and return -ECHILD, and so
appear to be trivially RCU safe (although this is by dropping out of RCU
completely). Group (2) are functions which have no explicit check, but
on my audit, I was confident that there were no sleeping function calls,
and thus were RCU safe as is. However, I would appreciate any additional
review if possible. Group (3) are functions which might be be unsafe for some
reason or another.

Group (1):
 proc_ns_get_link()
 proc_pid_get_link()
 map_files_d_revalidate()
 proc_misc_d_revalidate()
 tid_fd_revalidate()

Group (2):
 proc_get_link()
 proc_self_get_link()
 proc_thread_self_get_link()
 proc_fd_permission()

Group (3):
 pid_revalidate()            -- addressed by my patch
 proc_pid_permission()       -- addressed by commits by Al
 proc_map_files_get_link()   -- calls capable() which could audit

I believe proc_map_files_get_link() is safe despite calling into the audit
framework, however I'm not confident and so I did not include it in Group 2.
proc_pid_permission() calls into the audit code, and is not safe with
LSM_AUDIT_DATA_DENTRY or LSM_AUDIT_DATA_INODE. Al's commits[1] address
these issues. This patch is tested and stable on the workload described
at the beginning of this cover letter, on a system with selinux enabled.

[1]: 23d8f5b684fc ("make dump_common_audit_data() safe to be called from
     RCU pathwalk") and 2 previous

Changes in v5:
- Al's commits are now in linux-next, resolving proc_pid_permission() issue.
- Add NULL check after d_inode_rcu(dentry), because inode may become NULL if
  we do not hold a reference.
Changes in v4:
- Simplify by unconditionally calling pid_update_inode() from pid_revalidate,
  and removing the LOOKUP_RCU check.
Changes in 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 in 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 | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ebea9501afb8..3e105bd05801 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1830,19 +1830,21 @@ static int pid_revalidate(struct dentry *dentry, unsigned int flags)
 {
 	struct inode *inode;
 	struct task_struct *task;
+	int ret = 0;
 
-	if (flags & LOOKUP_RCU)
-		return -ECHILD;
-
-	inode = d_inode(dentry);
-	task = get_proc_task(inode);
+	rcu_read_lock();
+	inode = d_inode_rcu(dentry);
+	if (!inode)
+		goto out;
+	task = pid_task(proc_pid(inode), PIDTYPE_PID);
 
 	if (task) {
 		pid_update_inode(task, inode);
-		put_task_struct(task);
-		return 1;
+		ret = 1;
 	}
-	return 0;
+out:
+	rcu_read_unlock();
+	return ret;
 }
 
 static inline bool proc_inode_is_dead(struct inode *inode)
-- 
2.27.0