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dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:48198 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1kNgVM-000628-8u for qemu-devel@archiver.kernel.org; Wed, 30 Sep 2020 14:07:04 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:58788) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1kNgSH-0003SX-4s for qemu-devel@nongnu.org; Wed, 30 Sep 2020 14:03:53 -0400 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:34070) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1kNgSD-00062i-JF for qemu-devel@nongnu.org; Wed, 30 Sep 2020 14:03:52 -0400 Dkim-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1601489023; 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; 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auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mlevitsk@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=mlevitsk@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/30 00:31:59 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -25 X-Spam_score: -2.6 X-Spam_bar: -- X-Spam_report: (-2.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.469, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: stefanha@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, 2020-09-30 at 19:44 +0200, Paolo Bonzini wrote: > On 30/09/20 16:31, Maxim Levitsky wrote: > > > + > > > + qatomic_set(&dev->realized, value); > > > + /* > > > + * Ensure that concurrent users see this update prior to > > > + * any other changes done by unrealize. > > > + */ > > > + smp_wmb(); > > > > I''l probably never fully understand where to use read/write/full barrier. > > If I understand corrctly, read barrier prevents reads done by this thread to be reordered, > > by the CPU and write barrier prevents writes done by this CPU to be re-ordered. > > I must say that the above is not really satisfactory. The right thing > to do would be to say which changes are done by unrealize; then you > should make sure that *after* reading something that unrealize could > undo you check if dev->realized is still true. I didn't fully understand this to be honest. I just wanted to explain what I know and what I don't know about hardware barriers. I know that read barriers should be paired with write barriers, like if one CPU has a write barrier, which ensures the orders of writes to two memory locations, the other CPU can then use a read barrier to ensure that it sees those writes in this order. I thus think that reads of dev->realized should be paired with read barrier, but here a full memory barrier isn't really needed. Best regards, Maxim Levitsky > > scsi_device_find is one such case, but I'm not convinced it is enough. > > Paolo > > > Both (depending on the macro) usually imply compiler barrier (to avoid compilier re-ordering > > stuff...)