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Commit c96f49d3 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Sasha Levin
Browse files

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

[ Upstream commit f7ec1cd5cc7ef3ad964b677ba82b8b77f1c93009 ]

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@redhat.com/

	- 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.

Link: https://lkml.kernel.org/r/20240122155053.GA26214@redhat.com


Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Reported-by: default avatarDylan Hatch <dylanbhatch@google.com>
Tested-by: default avatarDylan Hatch <dylanbhatch@google.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent e24772ad

kernel/sys.c

+13 −3
Original line number Diff line number Diff line
@@ -1720,7 +1720,9 @@ 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;
@@ -1732,8 +1734,7 @@ 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:
@@ -1760,14 +1761,23 @@ 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;