[PATCH bpf-next] bpf: increment and use correct thread iterator

[Jonathan Lemon]

From: Jonathan Lemon <b...@fb.com>

If unable to obtain the file structure for the current task,
proceed to the next task number after the one returned from
task_seq_get_next(), instead of the next task number from the
original iterator.

Use thread_group_leader() instead of comparing tgid vs pid, which
might may be racy.

Only obtain the task reference count at the end of the RCU section
instead of repeatedly obtaining/releasing it when iterathing though
a thread group.

This patch fixes a recurring RCU stall seen from task_file_seq_next().

Fixes: a650da2ee52a ("bpf: Add task and task/file iterator targets")
Fixes: 67b6b863e6ab ("bpf: Avoid iterating duplicated files for task_file 
iterator")

Signed-off-by: Jonathan Lemon <jonathan.le...@gmail.com>
---
 kernel/bpf/task_iter.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
index 0458a40edf10..66a52fcf589a 100644
--- a/kernel/bpf/task_iter.c
+++ b/kernel/bpf/task_iter.c
@@ -33,17 +33,17 @@ static struct task_struct *task_seq_get_next(struct 
pid_namespace *ns,
        pid = find_ge_pid(*tid, ns);
        if (pid) {
                *tid = pid_nr_ns(pid, ns);
-               task = get_pid_task(pid, PIDTYPE_PID);
+               task = pid_task(pid, PIDTYPE_PID);
                if (!task) {
                        ++*tid;
                        goto retry;
-               } else if (skip_if_dup_files && task->tgid != task->pid &&
+               } else if (skip_if_dup_files && !thread_group_leader(task) &&
                           task->files == task->group_leader->files) {
-                       put_task_struct(task);
                        task = NULL;
                        ++*tid;
                        goto retry;
                }
+               get_task_struct(task);
        }
        rcu_read_unlock();
 
@@ -164,7 +164,7 @@ task_file_seq_get_next(struct bpf_iter_seq_task_file_info 
*info)
                curr_files = get_files_struct(curr_task);
                if (!curr_files) {
                        put_task_struct(curr_task);
-                       curr_tid = ++(info->tid);
+                       curr_tid = curr_tid + 1;
                        info->fd = 0;
                        goto again;
                }
-- 
2.24.1