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[80.187.96.150]) by smtp.gmail.com with ESMTPSA id p21sm1153030wmc.11.2021.10.26.10.26.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 26 Oct 2021 10:26:17 -0700 (PDT) Subject: Re: [RFC 0/2] tls: add macros for coroutine-safe TLS variables To: Richard Henderson , Stefan Hajnoczi References: <20211025140716.166971-1-stefanha@redhat.com> <2ca9c094-61e6-54b8-89a8-6dad22514c96@linaro.org> <85df7de0-cebb-efe9-d26b-f459a37a6621@linaro.org> <4b8a5b71-0ec2-7c2c-6929-de303ec29a53@linaro.org> From: Thomas Huth Message-ID: Date: Tue, 26 Oct 2021 19:26:16 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.14.0 MIME-Version: 1.0 In-Reply-To: <4b8a5b71-0ec2-7c2c-6929-de303ec29a53@linaro.org> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=thuth@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=thuth@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.215, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-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: fweimer@redhat.com, Kevin Wolf , "open list:Block layer core" , QEMU Developers , Fam Zheng , Warner Losh , sguelton@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 26/10/2021 19.10, Richard Henderson wrote: > On 10/26/21 9:34 AM, Stefan Hajnoczi wrote: >> On Tue, Oct 26, 2021 at 08:10:16AM -0700, Richard Henderson wrote: >>> On 10/26/21 6:22 AM, Stefan Hajnoczi wrote: >>>> If "safe" TLS variables are opt-in then we'll likely have obscure bugs >>>> when code changes to access a TLS variable that was previously never >>>> accessed from a coroutine. There is no compiler error and no way to >>>> detect this. When it happens debugging it is painful. >>> >>> Co-routines are never used in user-only builds. >> >> If developers have the choice of using __thread then bugs can slip >> through. > > Huh?  How.  No, really. > >> Are you concerned about performance, the awkwardness of calling >> getters/setters, or something else for qemu-user? > > Awkwardness first, performance second. > > I'll also note that coroutines never run on vcpu threads, only io threads. > So I'll resist any use of these interfaces in TCG as well. Would it maybe make sense to tweak check_patch.pl to forbid __thread in certain folders only, e.g. block/ and util/ (i.e. where we know that there might be code that the iothreads are using)? Thomas