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Fri, 30 Oct 2020 11:15:40 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 56f29214-1a0c-4b54-ae36-6013f6f44126; Fri, 30 Oct 2020 11:15:39 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 7776BB0F2; Fri, 30 Oct 2020 11:15:38 +0000 (UTC) Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1kYSNg-0006Sq-En for xen-devel@lists.xenproject.org; Fri, 30 Oct 2020 11:15:40 +0000 X-Inumbo-ID: 56f29214-1a0c-4b54-ae36-6013f6f44126 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id 56f29214-1a0c-4b54-ae36-6013f6f44126; Fri, 30 Oct 2020 11:15:39 +0000 (UTC) X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1604056538; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=fD3f5AJYmd8ppqRzHwDpJ8bn+jSd9jluVgA9JqTkT9Y=; b=BLGpfc0q1KXnjeujxDCK9YkYCyoMgTI3aDpvasmaTUWwUOjcdV3mFNz3DFw1eXf2qEgXuN pw6RcfVIpSp+zSgFqTkocnQyBvKjoO+hFBGynpC2ZTvLqbvWb+f5W1rdM9MpCW5lua+OJt QzGKdw/Et6jcWQdJE4mZYel4GE8DBhU= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 7776BB0F2; Fri, 30 Oct 2020 11:15:38 +0000 (UTC) Subject: Re: [PATCH v2 5/8] evtchn: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq() To: Julien Grall , Jan Beulich Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper , George Dunlap , Ian Jackson , Wei Liu , Stefano Stabellini , =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= References: <19babf20-3649-5c63-44a9-7edfa81835aa@suse.com> <53eb30ca-9b3f-0ef4-bc90-e1c196b716b3@suse.com> <20201022160055.nlucvj2bsxolxd5o@Air-de-Roger> <2cfcda4c-4115-e057-f401-5103f5b5b8e8@suse.com> <08108cd3-530f-3fe9-e1b2-41c7da9f98b7@xen.org> From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Message-ID: <693bb422-ed13-9327-5f22-12bd6f192916@suse.com> Date: Fri, 30 Oct 2020 12:15:37 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.12.0 MIME-Version: 1.0 In-Reply-To: <08108cd3-530f-3fe9-e1b2-41c7da9f98b7@xen.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit On 30.10.20 11:57, Julien Grall wrote: > > > On 30/10/2020 10:49, Jan Beulich wrote: >> On 30.10.2020 11:38, Julien Grall wrote: >>> On 22/10/2020 17:17, Jan Beulich wrote: >>>> On 22.10.2020 18:00, Roger Pau Monné wrote: >>>>> On Tue, Oct 20, 2020 at 04:10:09PM +0200, Jan Beulich wrote: >>>>>> --- a/xen/include/xen/event.h >>>>>> +++ b/xen/include/xen/event.h >>>>>> @@ -177,9 +177,16 @@ int evtchn_reset(struct domain *d, bool >>>>>>     * Low-level event channel port ops. >>>>>>     * >>>>>>     * All hooks have to be called with a lock held which prevents >>>>>> the channel >>>>>> - * from changing state. This may be the domain event lock, the >>>>>> per-channel >>>>>> - * lock, or in the case of sending interdomain events also the >>>>>> other side's >>>>>> - * per-channel lock. Exceptions apply in certain cases for the PV >>>>>> shim. >>>>>> + * from changing state. This may be >>>>>> + * - the domain event lock, >>>>>> + * - the per-channel lock, >>>>>> + * - in the case of sending interdomain events the other side's >>>>>> per-channel >>>>>> + *   lock, >>>>>> + * - in the case of sending non-global vIRQ-s the per-vCPU >>>>>> virq_lock (in >>>>>> + *   combination with the ordering enforced through how the vCPU's >>>>>> + *   virq_to_evtchn[] gets updated), >>>>>> + * - in the case of sending global vIRQ-s vCPU 0's virq_lock. >>>>>> + * Exceptions apply in certain cases for the PV shim. >>>>> >>>>> Having such a wide locking discipline looks dangerous to me, it's easy >>>>> to get things wrong without notice IMO. >>>> >>>> It is effectively only describing how things are (or were before >>>> XSA-343, getting restored here). >>> >>> I agree with Roger here, the new/old locking discipline is dangerous and >>> it is only a matter of time before it will bite us again. >>> >>> I think we should consider Juergen's series because the locking for the >>> event channel is easier to understand. >> >> We should, yes. The one thing I'm a little uneasy with is the >> new lock "variant" that gets introduced. Custom locking methods >> also are a common source of problems (which isn't to say I see >> any here). > > I am also unease with a new lock "variant". However, this is the best > proposal I have seen so far to unblock the issue. > > I am open to other suggestion with simple locking discipline. In theory my new lock variant could easily be replaced by a rwlock and using the try-variant for the readers only. The disadvantage of that approach would be a growth of struct evtchn. Juergen