[PATCH v2 2/2] getrusage: use sig->stats_lock rather than lock_task_sighand()
From: Oleg Nesterov
Date: Mon Jan 22 2024 - 12:07:17 EST
lock_task_sighand() can trigger a hard lockup. If NR_CPUS threads call
getrusage() at the same time and the process has NR_THREADS, spin_lock_irq
will spin with irqs disabled O(NR_CPUS * NR_THREADS) time.
Change getrusage() to use sig->stats_lock, it was specifically designed
for this type of use. This way it runs lockless in the likely case.
TODO:
- Change do_task_stat() to use sig->stats_lock too, then we can
remove spin_lock_irq(siglock) in wait_task_zombie().
- Turn sig->stats_lock into seqcount_rwlock_t, this way the
readers in the slow mode won't exclude each other. See
https://lore.kernel.org/all/20230913154907.GA26210@xxxxxxxxxx/
- stats_lock has to disable irqs because ->siglock can be taken
in irq context, it would be very nice to change __exit_signal()
to avoid the siglock->stats_lock dependency.
Reported-and-tested-by: Dylan Hatch <dylanbhatch@xxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
kernel/sys.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)
diff --git a/kernel/sys.c b/kernel/sys.c
index 70ad06ad852e..f8e543f1e38a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1788,7 +1788,9 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
unsigned long maxrss;
struct mm_struct *mm;
struct signal_struct *sig = p->signal;
+ unsigned int seq = 0;
+retry:
memset(r, 0, sizeof(*r));
utime = stime = 0;
maxrss = 0;
@@ -1800,8 +1802,7 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
goto out_thread;
}
- if (!lock_task_sighand(p, &flags))
- return;
+ flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq);
switch (who) {
case RUSAGE_BOTH:
@@ -1829,14 +1830,23 @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
r->ru_oublock += sig->oublock;
if (maxrss < sig->maxrss)
maxrss = sig->maxrss;
+
+ rcu_read_lock();
__for_each_thread(sig, t)
accumulate_thread_rusage(t, r);
+ rcu_read_unlock();
+
break;
default:
BUG();
}
- unlock_task_sighand(p, &flags);
+
+ if (need_seqretry(&sig->stats_lock, seq)) {
+ seq = 1;
+ goto retry;
+ }
+ done_seqretry_irqrestore(&sig->stats_lock, seq, flags);
if (who == RUSAGE_CHILDREN)
goto out_children;
--
2.25.1.362.g51ebf55