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received-spf: None (protection.outlook.com: microsoft.com does not designate permitted sender hosts) authentication-results: spf=none (sender IP is ) smtp.mailfrom=decui@microsoft.com; x-ms-exchange-senderadcheck: 1 x-microsoft-antispam-message-info: fLTc86fQgN1tc7HsQlUrN94fUCZYeLKCCjkNyKoNELf1+T7U5sy2fwFsm6EPEsCj+nWy2p+rFgipQ5cdrmLTrGgHE6C8lCsP56c4LloPwhg/I879q1VIC6k22Zr/PqbvRLW1j4AiLdr6JiyKX/28dhptNK2s6u/q/363+UNn7MGnuEtQA6aEsYPa11obA7R6Ohj+g4mPb4yUutv0G3UoYE79WDs8zlmxqzbu1o+LY98SLuBQW+MP3MhjibBtgh78bnOL8PKashkvJ4Gay7dEaRmEpk2oN0acgH0x0DFXd4hinhqTVtaveNw40Yzw688t spamdiagnosticoutput: 1:99 spamdiagnosticmetadata: NSPM Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: microsoft.com X-MS-Exchange-CrossTenant-Network-Message-Id: 54c9b8f8-411c-49ed-f97b-08d676afe49a X-MS-Exchange-CrossTenant-originalarrivaltime: 10 Jan 2019 03:58:34.2497 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 72f988bf-86f1-41af-91ab-2d7cd011db47 X-MS-Exchange-Transport-CrossTenantHeadersStamped: PU1P153MB0139 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > From: Michael Kelley > Sent: Saturday, January 5, 2019 1:01 PM > > From: Kimberly Brown Sent: Friday, January 4, > > 2019 8:35 PM > > > > static inline void set_channel_pending_send_size(struct vmbus_channel = *c, > > u32 size) > > { > > + if (size) { > > + ++c->out_full_total; > > + > > + if (!c->out_full_flag) { > > + ++c->out_full_first; > > + c->out_full_flag =3D true; > > + } > > + } else { > > + c->out_full_flag =3D false; > > + } > > + > > c->outbound.ring_buffer->pending_send_sz =3D size; > > } > > >=20 > I think there may be an atomicity problem with the above code. I looked > in the hv_sock code, and didn't see any locks being held when > set_channel_pending_send_size() is called. The original code doesn't ne= ed > a lock because it is just storing a single value into pending_send_sz. > > In the similar code in hv_ringbuffer_write(), the ring buffer spin lock i= s held > while the counts are incremented and the out_full_flag is maintained, so = all is > good there. But some locking may be needed here. Dexuan knows the > hv_sock > code best and can comment on whether there is any higher level > synchronization > that prevents multiple threads from running the above code on the same > channel. > Even if there is such higher level synchronization, this code probably sh= ouldn't > depend on it for correctness. >=20 > Michael Yes, there is indeed a higher level per-"sock" lock, e.g. in the code path vsock_stream_sendmsg() -> vsock_stream_has_space() ->=20 transport->stream_has_space() -> hvs_stream_has_space() ->=20 hvs_set_channel_pending_send_size(), we acquire the lock by lock_sock(sk) at the beginning of vsock_stream_sendmsg(), and call release_sock(sk) at the end of the function. So we don't have an atomicity issue here for hv_sock, which is the only use= r of set_channel_pending_send_size(), so far. Thanks, -- Dexuan