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Message-ID: <facf3c932c05cfc0c8287465a94f7919bbf6ad58.camel@gmx.de>
Date: Wed, 09 Dec 2020 11:25:34 +0100
From: Mike Galbraith <efault@....de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
tglx@...utronix.de, linux-rt-users@...r.kernel.org,
lkml <linux-kernel@...r.kernel.org>,
Boqun Feng <boqun.feng@...il.com>,
Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>
Subject: Re: [RT] 5.9-rt14 softirq_ctrl.lock vs listening_hash[i].lock
lockdep splat
On Wed, 2020-12-09 at 11:05 +0100, Peter Zijlstra wrote:
>
> > [ 47.844585] Possible unsafe locking scenario:
> >
> > [ 47.844586] CPU0 CPU1
> > [ 47.844586] ---- ----
> > [ 47.844587] lock(&h->listening_hash[i].lock);
> > [ 47.844588] lock((softirq_ctrl.lock).lock);
> > [ 47.844588] lock(&h->listening_hash[i].lock);
> > [ 47.844589] lock((softirq_ctrl.lock).lock);
> > [ 47.844590]
> > *** DEADLOCK ***
> >
> >
> So I've been looking at these local_lock vs lockdep splats for a bit,
> and unlike the IRQ inversions as reported here:
>
> https://lore.kernel.org/linux-usb/20201029174348.omqiwjqy64tebg5z@linutronix.de/
>
> I think the above is an actual real problem (for RT).
>
> AFAICT the above translates to:
>
> inet_listen()
> lock_sock()
> spin_lock_bh(&sk->sk_lock.slock);
> acquire(softirq_ctrl);
> acquire(&sk->sk_lock.slock);
>
> inet_csk_listen_start()
> sk->sk_prot->hash() := inet_hash()
> local_bh_disable()
> __inet_hash()
> spin_lock(&ilb->lock);
> acquire(&ilb->lock);
>
> ----
>
> tcp4_seq_next()
> listening_get_next()
> spin_lock(&ilb->lock);
> acquire(&ilb->lock);
>
> tcp4_seq_show()
> get_tcp4_sock()
> sock_i_ino()
> read_lock_bh(&sk->sk_callback_lock);
> acquire(softirq_ctrl) // <---- whoops
> acquire(&sk->sk_callback_lock)
>
>
> Which you can run in two tasks on the same CPU (and thus get the same
> softirq_ctrl local_lock), and deadlock.
>
> By holding softirq_ctrl we serialize against softirq-context
> (in-softirq) but that isn't relevant here, since neither context is
> that.
>
> On !RT there isn't a problem because softirq_ctrl isn't an actual lock,
> but the moment that turns into a real lock (like on RT) you're up a
> creek.
>
> In general we have the rule that as long as a lock is only ever used
> from task context (like the above ilb->lock, afaict) then it doesn't
> matter if you also take it with (soft)irqs disabled or not. But this
> softirq scheme breaks that. If you ever take a lock with BH disabled,
> you must now always take it with BH disabled, otherwise you risk
> deadlocks against the softirq_ctrl lock.
>
> Or am I missing something obvious (again) ?
Sebastian fixed this via...
From 0fe43be6c32e05d0dd692069d41a40c5453a2195 Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Date: Mon, 12 Oct 2020 17:33:54 +0200
Subject: tcp: Remove superfluous BH-disable around listening_hash
Commit
9652dc2eb9e40 ("tcp: relax listening_hash operations")
removed the need to disable bottom half while acquiring
listening_hash.lock. There are still two callers left which disable
bottom half before the lock is acquired.
Drop local_bh_disable() around __inet_hash() which acquires
listening_hash->lock, invoke inet_ehash_nolisten() with disabled BH.
inet_unhash() conditionally acquires listening_hash->lock.
Reported-by: Mike Galbraith <efault@....de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Link: https://lore.kernel.org/linux-rt-users/12d6f9879a97cd56c09fb53dee343cbb14f7f1f7.camel@gmx.de/
Signed-off-by: Mike Galbraith <efault@....de>
---
net/ipv4/inet_hashtables.c | 19 ++++++++++++-------
net/ipv6/inet6_hashtables.c | 5 +----
2 files changed, 13 insertions(+), 11 deletions(-)
diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 45fb450b4522..5fb95030e7c0 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -635,7 +635,9 @@ int __inet_hash(struct sock *sk, struct sock *osk)
int err = 0;
if (sk->sk_state != TCP_LISTEN) {
+ local_bh_disable();
inet_ehash_nolisten(sk, osk, NULL);
+ local_bh_enable();
return 0;
}
WARN_ON(!sk_unhashed(sk));
@@ -667,11 +669,8 @@ int inet_hash(struct sock *sk)
{
int err = 0;
- if (sk->sk_state != TCP_CLOSE) {
- local_bh_disable();
+ if (sk->sk_state != TCP_CLOSE)
err = __inet_hash(sk, NULL);
- local_bh_enable();
- }
return err;
}
@@ -682,17 +681,20 @@ void inet_unhash(struct sock *sk)
struct inet_hashinfo *hashinfo = sk->sk_prot->h.hashinfo;
struct inet_listen_hashbucket *ilb = NULL;
spinlock_t *lock;
+ bool state_listen;
if (sk_unhashed(sk))
return;
if (sk->sk_state == TCP_LISTEN) {
+ state_listen = true;
ilb = &hashinfo->listening_hash[inet_sk_listen_hashfn(sk)];
- lock = &ilb->lock;
+ spin_lock(&ilb->lock);
} else {
+ state_listen = false;
lock = inet_ehash_lockp(hashinfo, sk->sk_hash);
+ spin_lock_bh(lock);
}
- spin_lock_bh(lock);
if (sk_unhashed(sk))
goto unlock;
@@ -705,7 +707,10 @@ void inet_unhash(struct sock *sk)
__sk_nulls_del_node_init_rcu(sk);
sock_prot_inuse_add(sock_net(sk), sk->sk_prot, -1);
unlock:
- spin_unlock_bh(lock);
+ if (state_listen)
+ spin_unlock(&ilb->lock);
+ else
+ spin_unlock_bh(lock);
}
EXPORT_SYMBOL_GPL(inet_unhash);
diff --git a/net/ipv6/inet6_hashtables.c b/net/ipv6/inet6_hashtables.c
index 55c290d55605..9bad345cba9a 100644
--- a/net/ipv6/inet6_hashtables.c
+++ b/net/ipv6/inet6_hashtables.c
@@ -333,11 +333,8 @@ int inet6_hash(struct sock *sk)
{
int err = 0;
- if (sk->sk_state != TCP_CLOSE) {
- local_bh_disable();
+ if (sk->sk_state != TCP_CLOSE)
err = __inet_hash(sk, NULL);
- local_bh_enable();
- }
return err;
}
--
2.29.2
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