do_io_accounting: use sig->stats_lock

Rather than lock_task_sighand(), sig->stats_lock was specifically designed
for this type of use.

This way the "if (whole)" branch runs lockless in the likely case.

Link: https://lkml.kernel.org/r/20231023153405.GA4639@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 7779efda9fe22b3ca216ae5f1c45a32c26977fb6..ca4f0ce64df652385aa31a234c27626355ce35d2 100644 (file)
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2973,7 +2973,6 @@ static const struct file_operations proc_coredump_filter_operations = {
 static int do_io_accounting(struct task_struct *task, struct seq_file *m, int whole)
 {
        struct task_io_accounting acct;
-       unsigned long flags;
        int result;
 
        result = down_read_killable(&task->signal->exec_update_lock);
@@ -2985,15 +2984,24 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
                goto out_unlock;
        }
 
-       if (whole && lock_task_sighand(task, &flags)) {
+       if (whole) {
                struct signal_struct *sig = task->signal;
                struct task_struct *t;
+               unsigned int seq = 1;
+               unsigned long flags;
+
+               rcu_read_lock();
+               do {
+                       seq++; /* 2 on the 1st/lockless path, otherwise odd */
+                       flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
 
-               acct = sig->ioac;
-               __for_each_thread(sig, t)
-                       task_io_accounting_add(&acct, &t->ioac);
+                       acct = sig->ioac;
+                       __for_each_thread(sig, t)
+                               task_io_accounting_add(&acct, &t->ioac);
 
-               unlock_task_sighand(task, &flags);
+               } while (need_seqretry(&sig->stats_lock, seq));
+               done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
+               rcu_read_unlock();
        } else {
                acct = task->ioac;
        }