linux-kernel - Re: INFO: rcu detected stall in vprintk

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Message-ID: <20180402015422.GA3795@jagdpanzerIV>
Date:   Mon, 2 Apr 2018 10:54:22 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+3f28bd18291266ec826b@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        syzkaller-bugs@...glegroups.com,
        Samuel Ortiz <sameo@...ux.intel.com>,
        David Miller <davem@...emloft.net>,
        linux-wireless@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: INFO: rcu detected stall in vprintk_func

On (04/01/18 12:51), Dmitry Vyukov wrote:
[..]
> > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > Reported-by: syzbot+3f28bd18291266ec826b@...kaller.appspotmail.com
> > It will help syzbot understand when the bug is fixed. See footer for
> > details.
> > If you forward the report, please keep this part and the footer.
> >
> > llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
> > llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
> > llcp: nfc_llcp_send_ui_frame: Could not allocate PDU
> > llcp: nfc_llcp_send_ui_frame: Could not allocate PDU

Yes, this thing

	do {
		remote_miu = sock->remote_miu > LLCP_MAX_MIU ?
				local->remote_miu : sock->remote_miu;

		frag_len = min_t(size_t, remote_miu, remaining_len);

		pr_debug("Fragment %zd bytes remaining %zd",
			 frag_len, remaining_len);

		pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
					 frag_len + LLCP_HEADER_SIZE, &err);
		if (pdu == NULL) {
			pr_err("Could not allocate PDU\n");
			continue;
			^^^^^^^^
		}

is basically

	do {
		pr_err("Could not allocate PDU\n");
	} while (1)

Can cause problems sometimes. But this loop is a bit worrisome
even without the printk() call.

>From printk() side, we only can do rate limiting here. Would be
great if nfc maintainers could take a look and tweak the loop
maybe.

---

diff --git a/net/nfc/llcp_core.c b/net/nfc/llcp_core.c
index ef4026a23e80..a309a27581da 100644
--- a/net/nfc/llcp_core.c
+++ b/net/nfc/llcp_core.c
@@ -1386,7 +1386,7 @@ static void nfc_llcp_recv_agf(struct nfc_llcp_local *local, struct sk_buff *skb)
 
 		new_skb = nfc_alloc_recv_skb(pdu_len, GFP_KERNEL);
 		if (new_skb == NULL) {
-			pr_err("Could not allocate PDU\n");
+			pr_err_ratelimited("Could not allocate PDU\n");
 			return;
 		}