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Szmigiero" , Peter Xu Cc: Alex Williamson , =?utf-8?Q?C=C3=A9dric?= Le Goater , Eric Blake , Markus Armbruster , =?utf-8?Q?Daniel_P_=2E_Berrang=C3=A9?= , Avihai Horon , Joao Martins , qemu-devel@nongnu.org Subject: Re: [PATCH v2 10/17] migration/multifd: Convert multifd_send()::next_channel to atomic In-Reply-To: <76dc3ad69fa457fd1e358ad3de874474f9f64716.1724701542.git.maciej.szmigiero@oracle.com> References: <76dc3ad69fa457fd1e358ad3de874474f9f64716.1724701542.git.maciej.szmigiero@oracle.com> Date: Fri, 30 Aug 2024 15:13:36 -0300 Message-ID: <875xrhop4f.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[99.99%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MIME_TRACE(0.00)[0:+]; TO_MATCH_ENVRCPT_ALL(0.00)[]; ARC_NA(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; RCVD_TLS_ALL(0.00)[]; FROM_HAS_DN(0.00)[]; MISSING_XM_UA(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; RCPT_COUNT_SEVEN(0.00)[10]; DBL_BLOCKED_OPENRESOLVER(0.00)[oracle.com:email, suse.de:mid, imap1.dmz-prg2.suse.org:helo] Received-SPF: pass client-ip=195.135.223.130; envelope-from=farosas@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org "Maciej S. Szmigiero" writes: > From: "Maciej S. Szmigiero" > > This is necessary for multifd_send() to be able to be called > from multiple threads. > > Signed-off-by: Maciej S. Szmigiero > --- > migration/multifd.c | 24 ++++++++++++++++++------ > 1 file changed, 18 insertions(+), 6 deletions(-) > > diff --git a/migration/multifd.c b/migration/multifd.c > index d5a8e5a9c9b5..b25789dde0b3 100644 > --- a/migration/multifd.c > +++ b/migration/multifd.c > @@ -343,26 +343,38 @@ bool multifd_send(MultiFDSendData **send_data) > return false; > } > > - /* We wait here, until at least one channel is ready */ > - qemu_sem_wait(&multifd_send_state->channels_ready); > - > /* > * next_channel can remain from a previous migration that was > * using more channels, so ensure it doesn't overflow if the > * limit is lower now. > */ > - next_channel %= migrate_multifd_channels(); > - for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) { > + i = qatomic_load_acquire(&next_channel); > + if (unlikely(i >= migrate_multifd_channels())) { > + qatomic_cmpxchg(&next_channel, i, 0); > + } Do we still need this? It seems not, because the mod down below would already truncate to a value less than the number of channels. We don't need it to start at 0 always, the channels are equivalent. > + > + /* We wait here, until at least one channel is ready */ > + qemu_sem_wait(&multifd_send_state->channels_ready); > + > + while (true) { > + int i_next; > + > if (multifd_send_should_exit()) { > return false; > } > + > + i = qatomic_load_acquire(&next_channel); > + i_next = (i + 1) % migrate_multifd_channels(); > + if (qatomic_cmpxchg(&next_channel, i, i_next) != i) { > + continue; > + } Say channel 'i' is the only one that's idle. What's stopping the other thread(s) to race at this point and loop around to the same index? > + > p = &multifd_send_state->params[i]; > /* > * Lockless read to p->pending_job is safe, because only multifd > * sender thread can clear it. > */ > if (qatomic_read(&p->pending_job) == false) { With the cmpxchg your other patch adds here, then the race I mentioned above should be harmless. But we'd need to bring that code into this patch. > - next_channel = (i + 1) % migrate_multifd_channels(); > break; > } > }