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Wed, 17 Jun 2020 23:56:43 +0000 (GMT) Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 3BF28A404D; Wed, 17 Jun 2020 23:56:43 +0000 (GMT) Received: from oc2783563651 (unknown [9.145.74.214]) by d06av23.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 17 Jun 2020 23:56:43 +0000 (GMT) Date: Thu, 18 Jun 2020 01:56:41 +0200 From: Halil Pasic To: Cornelia Huck Subject: Re: [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic Message-ID: <20200618015641.1db429fc.pasic@linux.ibm.com> In-Reply-To: <20200616083333.2d4edfac.cohuck@redhat.com> References: <20200616045035.51641-1-pasic@linux.ibm.com> <20200616045035.51641-2-pasic@linux.ibm.com> <11e8278e-23cc-1e7f-4086-10ecef75b96a@de.ibm.com> <20200616083333.2d4edfac.cohuck@redhat.com> Organization: IBM X-Mailer: Claws Mail 3.11.1 (GTK+ 2.24.31; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.216, 18.0.687 definitions=2020-06-17_12:2020-06-17, 2020-06-17 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 spamscore=0 priorityscore=1501 mlxscore=0 suspectscore=0 clxscore=1015 impostorscore=0 mlxlogscore=999 lowpriorityscore=0 phishscore=0 cotscore=-2147483648 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2004280000 definitions=main-2006170173 Received-SPF: pass client-ip=148.163.158.5; envelope-from=pasic@linux.ibm.com; helo=mx0a-001b2d01.pphosted.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/17 19:56:49 X-ACL-Warn: Detected OS = Linux 3.x [generic] X-Spam_score_int: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-1, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=_AUTOLEARN 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: Thomas Huth , Matthew Rosato , "Michael S. Tsirkin" , David Hildenbrand , qemu-devel@nongnu.org, Christian Borntraeger , qemu-s390x@nongnu.org, Paolo Bonzini , Andreas Krebbel , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Tue, 16 Jun 2020 08:33:33 +0200 Cornelia Huck wrote: > > #define atomic_cmpxchg__nocheck(ptr, old, new) ({ \ > > > > typeof_strip_qual(*ptr) _old = (old); \ > > > > (void)__atomic_compare_exchange_n(ptr, &_old, new, false, \ > > > > __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST); \ > > > > _old; \ > > > > }) > > > > ind_old is copied into _old in the macro. Instead of doing the copy from the > > register the compiler reloads the value from memory. The result is that _old > > and ind_old end up having different values. _old in r1 with the bits set > > already and ind_old in r10 with the bits cleared. _old gets updated by CS > > and matches ind_old afterwards - both with the bits being 0. So the != > > compare is false and the loop is left without having set any bits. > > > > > > Paolo (to), > > I am asking myself if it would be safer to add a barrier or something like > > this in the macros in include/qemu/atomic.h. > > I'm also wondering whether this has been seen on other architectures as > well? There are also some callers in non-s390x code, and dealing with > this in common code would catch them as well. Quite a bunch of users use something like old = atomic_read(..), where atomic_read is documented as in docs/devel/atomics.rst: - ``atomic_read()`` and ``atomic_set()``; these prevent the compiler from optimizing accesses out of existence and creating unsolicited accesses, but do not otherwise impose any ordering on loads and stores: both the compiler and the processor are free to reorder them. Maybe I should have used that instead of volatile, but my problem was that I didn't fully understand what atomic_read() does, and if it does more than we need. I found the documentation just now. Another bunch uses constants as old, which is also fine. And there is a third bunch where I don't know whats up, partly because I did not dig deep enough. Regards, Halil