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Andrzej Siewior Cc: Thomas Gleixner , linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, spasswolf@web.de Date: Tue, 17 Feb 2026 00:48:25 +0100 In-Reply-To: <20260216153745.CA3__zRc@linutronix.de> References: <20260216134333.412332-1-spasswolf@web.de> <6274de932f4a62c51b424b65fc875ef3cb5ffd60.camel@web.de> <20260216153745.CA3__zRc@linutronix.de> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.56.2-0+deb13u1 Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Provags-ID: V03:K1:45oP4sigcS+baR5+tgn8RKwYMgeXgyT4xih5Cnc9XGHmuQebVWj Xa5YJVMVARx2OETtLFox34TFqWwCV8fByzC/855sCA8RGSQ3GSevUv/CMYqhfCbXBrcU4Rk wLp0tpS2H5qNqTbGNGzx20cXHOXysdujbwPVubBcj14YyGbSJBo8I4kkCx/7Cs6+eb2WOxf AS2Tnht/wEr6JrpPOyOoA== X-Spam-Flag: NO UI-OutboundReport: notjunk:1;M01:P0:S5/+2LbUfys=;BCgc05aGT9u+GPrUfYXRy3F8TOc 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The dev_xmit_recursion*() > based counters are per-task so it should be fine. But yet the wifi > managed to repeatedly enqueue packets. This might be a real recursion, a > stack trace should tell. And then, somewhere synchronisation is missing. >=20 > > Bert Karwatzki >=20 > Sebastian The problem seems to be that different preemtible threads try to send skbs= .=20 I used this debug patch for 6.18.10: diff --git a/net/core/dev.c b/net/core/dev.c index 5b536860138d..ecfdd8e3dc99 100644 =2D-- a/net/core/dev.c +++ b/net/core/dev.c @@ -4704,6 +4704,7 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net= _device *sb_dev) =20 qdisc_pkt_len_init(skb); tcx_set_ingress(skb, false); + printk(KERN_INFO "%s 0: skb =3D %px dev =3D %s\n", __func__, skb, dev->n= ame); #ifdef CONFIG_NET_EGRESS if (static_branch_unlikely(&egress_needed_key)) { if (nf_hook_egress_active()) { @@ -4739,10 +4740,12 @@ int __dev_queue_xmit(struct sk_buff *skb, struct n= et_device *sb_dev) =20 trace_net_dev_queue(skb); if (q->enqueue) { + printk(KERN_INFO "%s 1: skb =3D %px dev =3D %s\n", __func__, skb, dev->= name); rc =3D __dev_xmit_skb(skb, q, dev, txq); goto out; } =20 + printk(KERN_INFO "%s 2: skb =3D %px dev =3D %s txq =3D %px\n", __func__,= skb, dev->name, txq); /* The device has no queue. Common case for software devices: * loopback, all the sorts of tunnels... =20 @@ -4761,15 +4764,20 @@ int __dev_queue_xmit(struct sk_buff *skb, struct n= et_device *sb_dev) /* Other cpus might concurrently change txq->xmit_lock_owner * to -1 or to their cpu id, but not to our id. */ + printk(KERN_INFO "%s: cpu =3D %d xmit_lock_owner =3D %d\n", __func__, c= pu, READ_ONCE(txq->xmit_lock_owner)); if (READ_ONCE(txq->xmit_lock_owner) !=3D cpu) { - if (dev_xmit_recursion()) + printk(KERN_INFO "%s 3: skb =3D %px dev =3D %s txq =3D %px\n", __func_= _, skb, dev->name, txq); + if (dev_xmit_recursion()) { + printk(KERN_INFO "%s: recursion alert for device %s!\n", __func__, de= v->name); goto recursion_alert; + } =20 skb =3D validate_xmit_skb(skb, dev, &again); if (!skb) goto out; =20 HARD_TX_LOCK(dev, txq, cpu); + printk(KERN_INFO "%s 4: skb =3D %px dev =3D %s txq =3D %px\n", __func_= _, skb, dev->name, txq); =20 if (!netif_xmit_stopped(txq)) { dev_xmit_recursion_inc(); @@ -4777,6 +4785,7 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net= _device *sb_dev) dev_xmit_recursion_dec(); if (dev_xmit_complete(rc)) { HARD_TX_UNLOCK(dev, txq); + printk(KERN_INFO "%s 5: skb =3D %px dev =3D %s txq =3D %px\n", __fun= c__, skb, dev->name, txq); goto out; } } The normal path of an skb is this: 2026-02-17T00:29:24.124757+01:00 [ T1522] __dev_queue_xmit 0: skb =3D ffff= 8c11ed06bd00 dev =3D wlp4s0 2026-02-17T00:29:24.124845+01:00 [ T1522] __dev_queue_xmit 2: skb =3D ffff= 8c11ed06bd00 dev =3D wlp4s0 txq =3D ffff8c1145320200 2026-02-17T00:29:24.124851+01:00 [ T1522] __dev_queue_xmit: cpu =3D 7 xmit= _lock_owner =3D -1 2026-02-17T00:29:24.124853+01:00 [ T1522] __dev_queue_xmit 3: skb =3D ffff= 8c11ed06bd00 dev =3D wlp4s0 txq =3D ffff8c1145320200 2026-02-17T00:29:24.124855+01:00 [ T1522] __dev_queue_xmit 4: skb =3D ffff= 8c11ed06bd00 dev =3D wlp4s0 txq =3D ffff8c1145320200 2026-02-17T00:29:24.124857+01:00 [ T1522] __dev_queue_xmit 5: skb =3D 0000= 000000000000 dev =3D wlp4s0 txq =3D ffff8c1145320200 This is the situation which produces the error messages: T1522 tries to send an skb on CPU 7: 2026-02-17T00:29:24.212215+01:00 [ T1522] __dev_queue_xmit 0: skb =3D ffff= 8c11ed06b100 dev =3D wlp4s0 2026-02-17T00:29:24.212217+01:00 [ T1522] __dev_queue_xmit 2: skb =3D ffff= 8c11ed06b100 dev =3D wlp4s0 txq =3D ffff8c1145320200 2026-02-17T00:29:24.212219+01:00 [ T1522] __dev_queue_xmit: cpu =3D 7 xmit= _lock_owner =3D -1 2026-02-17T00:29:24.212221+01:00 [ T1522] __dev_queue_xmit 3: skb =3D ffff= 8c11ed06b100 dev =3D wlp4s0 txq =3D ffff8c1145320200 2026-02-17T00:29:24.212223+01:00 [ T1522] __dev_queue_xmit 4: skb =3D ffff= 8c11ed06b100 dev =3D wlp4s0 txq =3D ffff8c1145320200=20 Here T1522 gets preempted and T1513 is executed on CPU 7 and also tries to= send an skb: 2026-02-17T00:29:24.212225+01:00 [ T1513] __dev_queue_xmit 0: skb =3D ffff= 8c11ed06a300 dev =3D wlp4s0 2026-02-17T00:29:24.212228+01:00 [ T1513] __dev_queue_xmit 2: skb =3D ffff= 8c11ed06a300 dev =3D wlp4s0 txq =3D ffff8c1145320200 2026-02-17T00:29:24.212230+01:00 [ T1513] __dev_queue_xmit: cpu =3D 7 xmit= _lock_owner =3D 7 2026-02-17T00:29:24.212231+01:00 [ T1513] Dead loop on virtual device wlp4= s0, fix it urgently! 2026-02-17T00:29:24.212234+01:00 [ T1513] __dev_queue_xmit 0: skb =3D ffff= 8c11ed06a300 dev =3D wlp4s0 2026-02-17T00:29:24.212236+01:00 [ T1513] __dev_queue_xmit 2: skb =3D ffff= 8c11ed06a300 dev =3D wlp4s0 txq =3D ffff8c1145320200 2026-02-17T00:29:24.212238+01:00 [ T1513] __dev_queue_xmit: cpu =3D 7 xmit= _lock_owner =3D 7 2026-02-17T00:29:24.212240+01:00 [ T1513] Dead loop on virtual device wlp4= s0, fix it urgently! 2026-02-17T00:29:24.212242+01:00 [ T1513] __dev_queue_xmit 0: skb =3D ffff= 8c11ed06a300 dev =3D wlp4s0 2026-02-17T00:29:24.212244+01:00 [ T1513] __dev_queue_xmit 2: skb =3D ffff= 8c11ed06a300 dev =3D wlp4s0 txq =3D ffff8c1145320200 2026-02-17T00:29:24.212246+01:00 [ T1513] __dev_queue_xmit: cpu =3D 7 xmit= _lock_owner =3D 7 2026-02-17T00:29:24.212247+01:00 [ T1513] Dead loop on virtual device wlp4= s0, fix it urgently! T1513 gets preempted and T1522 finishes processing the skb from above: 2026-02-17T00:29:24.212249+01:00 [ T1522] __dev_queue_xmit 5: skb =3D 0000= 000000000000 dev =3D wlp4s0 txq =3D ffff8c1145320200 Bert Karwatzki