From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from shelob.surriel.com (shelob.surriel.com [96.67.55.147]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EA0341802A5 for ; Wed, 17 Jul 2024 13:48:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=96.67.55.147 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721224106; cv=none; b=NWEQtgdTSfc29KFgsiUsCMXQjz54eU2z18HTYkrRDtDStyBYFBnf+pxtnGcSgnNPsxw/LVqF75fFvxC3XMtc42jmLQFbjwdoJIjHh1czVTaSb6IZ0TpJCr+CcTA+98hqqsd7nBQ1NgUWeDVLj2csYKAZqYCIs+ymd7XU0/Ya0fk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1721224106; c=relaxed/simple; bh=WNXsZ/OA49D2wMn7rHbOLQ0vu8l6tOxdGmDc0yYy8RM=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=gROtuJ1A/iseH4R/Z4wZKsLV/kSaP9cZ09ZX1owviBs2Gqfvoh+IXzjbZjhPOjAB32E79M47atplZkUd7b1fDhFzKMoR383QjuimamnK9NxvUWE7FYuf/1/AkJvuc7gdhv9cq8WYBpKQz4zMzGcdDneZ8XSBh3YLCQdWwPPaeSI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=surriel.com; spf=pass smtp.mailfrom=shelob.surriel.com; arc=none smtp.client-ip=96.67.55.147 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=surriel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=shelob.surriel.com Received: from [2601:18c:9101:a8b6:6e0b:84ff:fee2:98bb] (helo=imladris.surriel.com) by shelob.surriel.com with esmtpsa (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.97.1) (envelope-from ) id 1sU50W-000000008VQ-1VFX; Wed, 17 Jul 2024 09:47:48 -0400 Message-ID: <93155b2ccafa43ed4845ae450451c6b8e27509cc.camel@surriel.com> Subject: Re: [RFC PATCH] nmi,printk: fix ABBA deadlock between nmi_backtrace and dump_stack_lvl From: Rik van Riel To: John Ogness , Andrew Morton Cc: Omar Sandoval , linux-kernel@vger.kernel.org, Petr Mladek , Steven Rostedt , Sergey Senozhatsky , kernel-team Date: Wed, 17 Jul 2024 09:47:48 -0400 In-Reply-To: <87plrcqyii.fsf@jogness.linutronix.de> References: <20240715232052.73eb7fb1@imladris.surriel.com> <87plrcqyii.fsf@jogness.linutronix.de> Autocrypt: addr=riel@surriel.com; prefer-encrypt=mutual; keydata=mQENBFIt3aUBCADCK0LicyCYyMa0E1lodCDUBf6G+6C5UXKG1jEYwQu49cc/gUBTTk33Aeo2hjn4JinVaPF3zfZprnKMEGGv4dHvEOCPWiNhlz5RtqH3SKJllq2dpeMS9RqbMvDA36rlJIIo47Z/nl6IA8MDhSqyqdnTY8z7LnQHqq16jAqwo7Ll9qALXz4yG1ZdSCmo80VPetBZZPw7WMjo+1hByv/lvdFnLfiQ52tayuuC1r9x2qZ/SYWd2M4p/f5CLmvG9UcnkbYFsKWz8bwOBWKg1PQcaYHLx06sHGdYdIDaeVvkIfMFwAprSo5EFU+aes2VB2ZjugOTbkkW2aPSWTRsBhPHhV6dABEBAAG0HlJpayB2YW4gUmllbCA8cmllbEByZWRoYXQuY29tPokBHwQwAQIACQUCW5LcVgIdIAAKCRDOed6ShMTeg05SB/986ogEgdq4byrtaBQKFg5LWfd8e+h+QzLOg/T8mSS3dJzFXe5JBOfvYg7Bj47xXi9I5sM+I9Lu9+1XVb/r2rGJrU1DwA09TnmyFtK76bgMF0sBEh1ECILYNQTEIemzNFwOWLZZlEhZFRJsZyX+mtEp/WQIygHVWjwuP69VJw+fPQvLOGn4j8W9QXuvhha7u1QJ7mYx4dLGHrZlHdwDsqpvWsW+3rsIqs1BBe5/Itz9o6y9gLNtQzwmSDioV8KhF85VmYInslhv5tUtMEppfdTLyX4SUKh8ftNIVmH9mXyRCZclSoa6IMd635Jq1Pj2/Lp64tOzSvN5Y9zaiCc5FucXtB9SaWsgdmFuIFJpZWwgPHJpZWxAc3VycmllbC5jb20+iQE+BBMBAgAoBQJSLd2lAhsjBQkSzAMABgsJCAcDAgYVCAIJCgsEFgIDAQIeAQIXgAAKCRDOed6ShMTeg4PpB/0ZivKYFt0LaB22ssWUrBoeNWCP1NY/lkq2QbPhR3agLB7ZXI97PF2z/5QD9Fuy/FD/j 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.4 (3.50.4-1.fc39) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Sender: riel@surriel.com On Wed, 2024-07-17 at 09:22 +0206, John Ogness wrote: >=20 > The purpose of printk_cpu_sync_get_irqsave() is to avoid having the > different backtraces being interleaved in the _ringbuffer_. It really > isn't necessary that they are printed in that context. And indeed, > there > is no guarantee that they will be printed in that context anyway. >=20 > Perhaps a simple solution would be for printk_cpu_sync_get_irqsave() > to > call printk_deferred_enter/_exit. Something like the below patch. >=20 I think that would do the trick. The nmi_backtrace() printk is already deferred, because of the check for in_nmi() in vprintk(), and this change would put all the other users of printk_cpu_sync_get_irqsave() on the exact same footing as nmi_backtrace(). Combing through the code a little, it looks like that would remove the potential for this deadlock to happen again. >=20 >=20 > diff --git a/include/linux/printk.h b/include/linux/printk.h > index 65c5184470f1..1a6f5aac28bf 100644 > --- a/include/linux/printk.h > +++ b/include/linux/printk.h > @@ -315,8 +315,10 @@ extern void __printk_cpu_sync_put(void); > =C2=A0#define printk_cpu_sync_get_irqsave(flags) \ > =C2=A0 for (;;) { \ > =C2=A0 local_irq_save(flags); \ > + printk_deferred_enter(); \ > =C2=A0 if (__printk_cpu_sync_try_get()) \ > =C2=A0 break; \ > + printk_deferred_exit(); \ > =C2=A0 local_irq_restore(flags); \ > =C2=A0 __printk_cpu_sync_wait(); \ > =C2=A0 } > @@ -329,6 +331,7 @@ extern void __printk_cpu_sync_put(void); > =C2=A0#define printk_cpu_sync_put_irqrestore(flags) \ > =C2=A0 do { \ > =C2=A0 __printk_cpu_sync_put(); \ > + printk_deferred_exit(); \ > =C2=A0 local_irq_restore(flags); \ > =C2=A0 } while (0) > =C2=A0 >=20 --=20 All Rights Reversed.