The patch titled
Subject: document while_each_thread(), change first_tid() to use for_each_thread()
has been added to the -mm mm-nonmm-unstable branch. Its filename is
document-while_each_thread-change-first_tid-to-use-for_each_thread.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/document-while_each_thread-change-first_tid-to-use-for_each_thread.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Oleg Nesterov <oleg@xxxxxxxxxx>
Subject: document while_each_thread(), change first_tid() to use for_each_thread()
Date: Wed, 23 Aug 2023 19:08:06 +0200
Add the comment to explain that while_each_thread(g,t) is not rcu-safe
unless g is stable (e.g. current). Even if g is a group leader and thus
can't exit before t, t or another sub-thread can exec and remove g from
the thread_group list.
The only lockless user of while_each_thread() is first_tid() and it is
fine in that it can't loop forever, yet for_each_thread() looks better and
I am going to change while_each_thread/next_thread.
Link: https://lkml.kernel.org/r/20230823170806.GA11724@xxxxxxxxxx
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
fs/proc/base.c | 5 ++---
include/linux/sched/signal.h | 4 ++++
2 files changed, 6 insertions(+), 3 deletions(-)
--- a/fs/proc/base.c~document-while_each_thread-change-first_tid-to-use-for_each_thread
+++ a/fs/proc/base.c
@@ -3814,11 +3814,10 @@ static struct task_struct *first_tid(str
/* If we haven't found our starting place yet start
* with the leader and walk nr threads forward.
*/
- pos = task = task->group_leader;
- do {
+ for_each_thread(task, pos) {
if (!nr--)
goto found;
- } while_each_thread(task, pos);
+ };
fail:
pos = NULL;
goto out;
--- a/include/linux/sched/signal.h~document-while_each_thread-change-first_tid-to-use-for_each_thread
+++ a/include/linux/sched/signal.h
@@ -648,6 +648,10 @@ extern void flush_itimer_signals(void);
extern bool current_is_single_threaded(void);
+/*
+ * Without tasklist/siglock it is only rcu-safe if g can't exit/exec,
+ * otherwise next_thread(t) will never reach g after list_del_rcu(g).
+ */
#define while_each_thread(g, t) \
while ((t = next_thread(t)) != g)
_
Patches currently in -mm which might be from oleg@xxxxxxxxxx are
document-while_each_thread-change-first_tid-to-use-for_each_thread.patch
