[PATCH 8/8] proc: take a lock-free exec_update_seq fast path in do_io_accounting()

[Christian Brauner]

/proc/<pid>/io took exec_update_lock for read to check
ptrace_may_access() before sampling the task's (or thread group's) IO
accounting. Convert it to exec_update_read_begin_or_lock(): snapshot the
accounting into a local under the speculative section (the whole-process
variant keeps its inner rcu + stats_lock), then emit after validation;
fall back to exec_update_lock on a racing exec()/TSYNC.

Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
---
 fs/proc/base.c | 31 +++++++++++++++++--------------
 1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 83b851b7f9d9..3706c5167df0 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3033,20 +3033,18 @@ static const struct file_operations proc_coredump_filter_operations = {
 #ifdef CONFIG_TASK_IO_ACCOUNTING
 static int do_io_accounting(struct task_struct *task, struct seq_file *m, int whole)
 {
+	struct signal_struct *sig = task->signal;
 	struct task_io_accounting acct;
+	unsigned int seq = 0;
+	bool allowed = false;
 	int result;
 
-	result = down_read_killable(&task->signal->exec_update_lock);
+retry:
+	result = exec_update_read_begin_or_killable(sig, &seq);
 	if (result)
 		return result;
-
-	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
-		result = -EACCES;
-		goto out_unlock;
-	}
-
-	if (whole) {
-		struct signal_struct *sig = task->signal;
+	allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
+	if (allowed && whole) {
 		struct task_struct *t;
 
 		guard(rcu)();
@@ -3056,9 +3054,17 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
 				task_io_accounting_add(&acct, &t->ioac);
 
 		}
-	} else {
+	} else if (allowed) {
 		acct = task->ioac;
 	}
+	if (exec_update_read_needs_retry(sig, seq)) {
+		seq = 1;
+		goto retry;
+	}
+	exec_update_read_done(sig, seq);
+
+	if (!allowed)
+		return -EACCES;
 
 	seq_printf(m,
 		   "rchar: %llu\n"
@@ -3075,11 +3081,8 @@ static int do_io_accounting(struct task_struct *task, struct seq_file *m, int wh
 		   (unsigned long long)acct.read_bytes,
 		   (unsigned long long)acct.write_bytes,
 		   (unsigned long long)acct.cancelled_write_bytes);
-	result = 0;
 
-out_unlock:
-	up_read(&task->signal->exec_update_lock);
-	return result;
+	return 0;
 }
 
 static int proc_tid_io_accounting(struct seq_file *m, struct pid_namespace *ns,
-- 
2.47.3


--j3ezp33mpunnwnqz--