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Sat, 29 Jun 2019 08:42:24 -0700 (PDT) Subject: Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg To: Jens Axboe , osandov@fb.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <722e1d561f0a49dc464d3a2b1be4c077f7502726.1558625912.git.asml.silence@gmail.com> From: Pavel Begunkov Openpgp: preference=signencrypt Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Message-ID: <0979d23f-bc31-b653-51d0-867dd52bf7ee@gmail.com> Date: Sat, 29 Jun 2019 08:42:23 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 MIME-Version: 1.0 In-Reply-To: <722e1d561f0a49dc464d3a2b1be4c077f7502726.1558625912.git.asml.silence@gmail.com> Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="uzz2TY5uZnI06xCKH3n3Lp4BlDS8ROpf4" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --uzz2TY5uZnI06xCKH3n3Lp4BlDS8ROpf4 Content-Type: multipart/mixed; boundary="FuhgTVb8yfbQS7dziWodTAhy0by58eNYo"; protected-headers="v1" From: Pavel Begunkov To: Jens Axboe , osandov@fb.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org Message-ID: <0979d23f-bc31-b653-51d0-867dd52bf7ee@gmail.com> Subject: Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg References: <722e1d561f0a49dc464d3a2b1be4c077f7502726.1558625912.git.asml.silence@gmail.com> In-Reply-To: <722e1d561f0a49dc464d3a2b1be4c077f7502726.1558625912.git.asml.silence@gmail.com> --FuhgTVb8yfbQS7dziWodTAhy0by58eNYo Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: quoted-printable Ping? On 23/05/2019 08:39, Pavel Begunkov (Silence) wrote: > From: Pavel Begunkov >=20 > cmpxchg() with an immediate value could be replaced with less expensive= > xchg(). The same true if new value don't _depend_ on the old one. >=20 > In the second block, atomic_cmpxchg() return value isn't checked, so > after atomic_cmpxchg() -> atomic_xchg() conversion it could be replace= d > with atomic_set(). Comparison with atomic_read() in the second chunk wa= s > left as an optimisation (if that was the initial intention). >=20 > Signed-off-by: Pavel Begunkov > --- > lib/sbitmap.c | 10 +++------- > 1 file changed, 3 insertions(+), 7 deletions(-) >=20 > diff --git a/lib/sbitmap.c b/lib/sbitmap.c > index 155fe38756ec..7d7e0e278523 100644 > --- a/lib/sbitmap.c > +++ b/lib/sbitmap.c > @@ -37,9 +37,7 @@ static inline bool sbitmap_deferred_clear(struct sbit= map *sb, int index) > /* > * First get a stable cleared mask, setting the old mask to 0. > */ > - do { > - mask =3D sb->map[index].cleared; > - } while (cmpxchg(&sb->map[index].cleared, mask, 0) !=3D mask); > + mask =3D xchg(&sb->map[index].cleared, 0); > =20 > /* > * Now clear the masked bits in our free word > @@ -527,10 +525,8 @@ static struct sbq_wait_state *sbq_wake_ptr(struct = sbitmap_queue *sbq) > struct sbq_wait_state *ws =3D &sbq->ws[wake_index]; > =20 > if (waitqueue_active(&ws->wait)) { > - int o =3D atomic_read(&sbq->wake_index); > - > - if (wake_index !=3D o) > - atomic_cmpxchg(&sbq->wake_index, o, wake_index); > + if (wake_index !=3D atomic_read(&sbq->wake_index)) > + atomic_set(&sbq->wake_index, wake_index); > return ws; > } > =20 >=20 --=20 Yours sincerely, Pavel Begunkov --FuhgTVb8yfbQS7dziWodTAhy0by58eNYo-- --uzz2TY5uZnI06xCKH3n3Lp4BlDS8ROpf4 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEE+6JuPTjTbx479o3OWt5b1Glr+6UFAl0Xht8ACgkQWt5b1Glr +6UtHw/+NA/Cl2kUbo540HN/b2x/QpS31C5Jq/4YyHKBPMidCw83E8RS+OB2/Sql 59OOugg8Bvw1ZAym2gM7/Ab6XmZpeWwg+kRSlRzKL9GPjJRg+U+kpz9oDF5WO4w8 qAI8gk+v74rjH8PVuNuwDQPnFiuyVWJA9QmbOf6uTZpuojAXVPWXjqTT2w3bCh1j XkOFR5LqdXgQEToCF4YJdpgXV4M3Peotk0lr2nA0ojf9NywB9kh/zJTnk1qPq8RY fSuX4bJwa45y1aduX+AI0jL0l140VUqFx8jcCqyma7nq2eOfTFstkx/4rm6b3Zbg 54OIisWekQy2hiUKDRic/GbnbToDuaFnzebyHacUKX1i/LWDxFtbt2SGF4S4g0kg Upq0VExbzi9Hfng7FF4NgzbVHkQGOVvZlBtDiW00k3PRzgwq8fPcPywluh1daOle qyEzKJlGIngBZARMfI9oFL7zpQ4FC+EnHh/m+CQtYzZqW6pcBH8Dyk+myYvxkFpE CNY1Fjk112Lq5lIcE4ZqNnGDxCsrFq3Bx4Ntq7R72+BjGchaM6yUVpfdQ37YK9m9 +g7u6zKZGN7/vWAaEtcFaSQEcIHPMAUzve5xONylGAJ03zcRq4skYEd7u5rkuWSd pJUMvgkJUWIoEUZ8HoMhBNmnwyFywcmOlU59F+hMa+AaPOMBNXc= =UP90 -----END PGP SIGNATURE----- --uzz2TY5uZnI06xCKH3n3Lp4BlDS8ROpf4--