FAILED: patch "[PATCH] getrusage: use sig->stats_lock rather than" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-4.19.y
git checkout FETCH_HEAD
git cherry-pick -x f7ec1cd5cc7ef3ad964b677ba82b8b77f1c93009
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024021941-showman-diaper-6105@gregkh' --subject-prefix 'PATCH 4.19.y' HEAD^..

Possible dependencies:

f7ec1cd5cc7e ("getrusage: use sig->stats_lock rather than lock_task_sighand()") daa694e41375 ("getrusage: move thread_group_cputime_adjusted() outside of lock_task_sighand()") 13b7bc60b535 ("getrusage: use __for_each_thread()")
c7ac8231ace9 ("getrusage: add the "signal_struct *sig" local variable") bdd565f817a7 ("y2038: rusage: use __kernel_old_timeval")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f7ec1cd5cc7ef3ad964b677ba82b8b77f1c93009 Mon Sep 17 00:00:00 2001
From: Oleg Nesterov <[email protected]>
Date: Mon, 22 Jan 2024 16:50:53 +0100
Subject: [PATCH] getrusage: use sig->stats_lock rather than
lock_task_sighand()

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.

index 70ad06ad852e..f8e543f1e38a 100644
@@ -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;