[PATCH 6.1 67/71] fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats

Sasha Levin
2024-03-13T16:41:19Z

From: Oleg Nesterov <oleg@redhat.com>

[ Upstream commit 7601df8031fd67310af891897ef6cc0df4209305 ]

lock_task_sighand() can trigger a hard lockup. If NR_CPUS threads call
do_task_stat() at the same time and the process has NR_THREADS, it will
spin with irqs disabled O(NR_CPUS * NR_THREADS) time.

Change do_task_stat() to use sig->stats_lock to gather the statistics
outside of ->siglock protected section, in the likely case this code will
run lockless.

Link: https://lkml.kernel.org/r/20240123153357.GA21857@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Dylan Hatch <dylanbhatch@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/proc/array.c | 58 +++++++++++++++++++++++++++----------------------
1 file changed, 32 insertions(+), 26 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index bcb645627991e..d210b2f8b7ed5 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -467,13 +467,13 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
int permitted;
struct mm_struct *mm;
unsigned long long start_time;
- unsigned long cmin_flt = 0, cmaj_flt = 0;
- unsigned long min_flt = 0, maj_flt = 0;
- u64 cutime, cstime, utime, stime;
- u64 cgtime, gtime;
+ unsigned long cmin_flt, cmaj_flt, min_flt, maj_flt;
+ u64 cutime, cstime, cgtime, utime, stime, gtime;
unsigned long rsslim = 0;
unsigned long flags;
int exit_code = task->exit_code;
+ struct signal_struct *sig = task->signal;
+ unsigned int seq = 1;

state = *get_task_state(task);
vsize = eip = esp = 0;
@@ -501,12 +501,8 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,

sigemptyset(&sigign);
sigemptyset(&sigcatch);
- cutime = cstime = 0;
- cgtime = gtime = 0;

if (lock_task_sighand(task, &flags)) {
- struct signal_struct *sig = task->signal;
-
if (sig->tty) {
struct pid *pgrp = tty_get_pgrp(sig->tty);
tty_pgrp = pid_nr_ns(pgrp, ns);
@@ -517,27 +513,9 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
num_threads = get_nr_threads(task);
collect_sigign_sigcatch(task, &sigign, &sigcatch);

- cmin_flt = sig->cmin_flt;
- cmaj_flt = sig->cmaj_flt;
- cutime = sig->cutime;
- cstime = sig->cstime;
- cgtime = sig->cgtime;
rsslim = READ_ONCE(sig->rlim[RLIMIT_RSS].rlim_cur);

- /* add up live thread stats at the group level */
if (whole) {
- struct task_struct *t;
-
- __for_each_thread(sig, t) {
- min_flt += t->min_flt;
- maj_flt += t->maj_flt;
- gtime += task_gtime(t);
- }
-
- min_flt += sig->min_flt;
- maj_flt += sig->maj_flt;
- gtime += sig->gtime;
-
if (sig->flags & (SIGNAL_GROUP_EXIT | SIGNAL_STOP_STOPPED))
exit_code = sig->group_exit_code;
}
@@ -552,6 +530,34 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
if (permitted && (!whole || num_threads < 2))
wchan = !task_is_running(task);

+ do {
+ seq++; /* 2 on the 1st/lockless path, otherwise odd */
+ flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
+
+ cmin_flt = sig->cmin_flt;
+ cmaj_flt = sig->cmaj_flt;
+ cutime = sig->cutime;
+ cstime = sig->cstime;
+ cgtime = sig->cgtime;
+
+ if (whole) {
+ struct task_struct *t;
+
+ min_flt = sig->min_flt;
+ maj_flt = sig->maj_flt;
+ gtime = sig->gtime;
+
+ rcu_read_lock();
+ __for_each_thread(sig, t) {
+ min_flt += t->min_flt;
+ maj_flt += t->maj_flt;
+ gtime += task_gtime(t);
+ }
+ rcu_read_unlock();
+ }
+ } while (need_seqretry(&sig->stats_lock, seq));
+ done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
+
if (whole) {
thread_group_cputime_adjusted(task, &utime, &stime);
} else {
--
2.43.0