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Thu, 26 Mar 2020 22:05:10 +0000 (GMT) Message-ID: Subject: Re: [RFC PATCH 1/1] ppc/smp: Replace unnecessary 'while' by 'if' From: Leonardo Bras To: Paul Mackerras Date: Thu, 26 Mar 2020 19:05:03 -0300 In-Reply-To: <20200326214005.GB9894@blackberry> References: <20200326203752.497029-1-leonardo@linux.ibm.com> <20200326214005.GB9894@blackberry> Content-Type: multipart/signed; micalg="pgp-sha256"; protocol="application/pgp-signature"; boundary="=-GTdlyedGwk3mGYQKKQZt" User-Agent: Evolution 3.34.4 (3.34.4-1.fc31) MIME-Version: 1.0 X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.138, 18.0.645 definitions=2020-03-26_13:2020-03-26, 2020-03-26 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 malwarescore=0 adultscore=0 phishscore=0 suspectscore=0 mlxlogscore=838 bulkscore=0 lowpriorityscore=0 priorityscore=1501 impostorscore=0 spamscore=0 clxscore=1011 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2003020000 definitions=main-2003260160 X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-kernel@vger.kernel.org, Greg Kroah-Hartman , Thomas Gleixner , linuxppc-dev@lists.ozlabs.org, Allison Randal Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" --=-GTdlyedGwk3mGYQKKQZt Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2020-03-27 at 08:40 +1100, Paul Mackerras wrote: > On Thu, Mar 26, 2020 at 05:37:52PM -0300, Leonardo Bras wrote: > > spin_until_cond() will wait until nmi_ipi_busy =3D=3D false, and > > nmi_ipi_lock_start() does not seem to change nmi_ipi_busy, so there is > > no way this while will ever repeat. > >=20 > > Replace this 'while' by an 'if', so it does not look like it can repeat= . >=20 > Nack, it can repeat. The scenario is that cpu A is in this code, > inside spin_until_cond(); cpu B has previously set nmi_ipi_busy, and > cpu C is also waiting for nmi_ipi_busy to be cleared, like cpu A. > When cpu B clears nmi_ipi_busy, both cpu A and cpu C will see that and > will race inside nmi_ipi_lock_start(). One of them, say cpu C, will > take the lock and proceed to set nmi_ipi_busy and then call > nmi_ipi_unlock(). Then the other cpu (cpu A) will then take the lock > and return from nmi_ipi_lock_start() and find nmi_ipi_busy =3D=3D true. > At that point it needs to go through the while loop body once more. >=20 > Paul. Ok, got it. Thanks for explaining Paul! --=-GTdlyedGwk3mGYQKKQZt Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- iQIzBAABCAAdFiEEMdeUgIzgjf6YmUyOlQYWtz9SttQFAl59JxAACgkQlQYWtz9S ttTSdRAAttveHAUp1FuyI8cjRY3YA64LGrCYQ/GJZfAZljGAMyg78+BO1orF0p7b n+wYBDvQzqmLMgL5fUxeHG+CQzPX7unA/9n+rn7y4j+BVZbupjLHOnPXswoDda3Y Ycs5eFcEJ2d5Fs0psXJs/aOVtF6cd8uKNzdpk0tVGM3VMkoB509DtJqdkjY96n1g Oi7+eDKF+yUNIxmOjEeTaJwR9fSrzLRJhdnLLpOmR0/42QvOcGeh8q3p2qtCyfgo H6qEXVCq5E1nYLGuvJskD46vaRRXGZ29myCPzFDTUpaP5JMH36sUw4Ei6UjNgz8x DhXNqoqSKQ4/mP91fMXgfyBGER2l4f4dTn1fmkXRC6IAt49XIFzRTVHf7GxS3i2z jHQQHrkWmCnFeRjzbGZDZUeNU/XmNMXKponwH7Q2KvPmtiaG9YDsPnB5cKGfFyKN uOGH0fjq1yHDq3O4epERX+o+2qTxKcEUQOakkQ4chpJ6eC/EgM2KPEwadZdZ1BAf DQM4nCzsPQP+zptJO+LjJqDjZ1JXBwoeJYfPJBfB8pAw8VgvIXcnXVfHwoSH7WGf 1JRmT583BLGzc3cqD89kyzZ2NbL+XEGNTWcEAEHzg6Nf+2t4AQQY3T7U4Cluj2Fl cXrBonqlCQPiR7ShKkwkYgQ7ZuYeuV1JdKsDjVXy85WBnxcAugI= =iE5j -----END PGP SIGNATURE----- --=-GTdlyedGwk3mGYQKKQZt--