getrusage-use-sig-stats_lock-rather-than-lock_task_sighand.patch removed from -mm tree

The quilt patch titled

     Subject: getrusage: use sig->stats_lock rather than lock_task_sighand()

has been removed from the -mm tree. Its filename was
getrusage-use-sig-stats_lock-rather-than-lock_task_sighand.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------
From: Oleg Nesterov <[email protected]>
Subject: getrusage: use sig->stats_lock rather than lock_task_sighand()
Date: Mon, 22 Jan 2024 16:50:53 +0100

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/[email protected]/

- 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.

---

 kernel/sys.c |   16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

@@ -1788,7 +1788,9 @@ void getrusage(struct task_struct *p, in
 	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, in
 		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, in
 		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;

_

Patches currently in -mm which might be from [email protected] are

ptrace_attach-shift-sendsigstop-into-ptrace_set_stopped.patch