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Wed, 26 Aug 2020 14:37:22 +0200 (CEST) From: Markus Armbruster To: Kevin Wolf Subject: Re: [PATCH] Simple & stupid coroutine-aware monitor_cur() References: <20200528153742.274164-1-kwolf@redhat.com> <20200528153742.274164-7-kwolf@redhat.com> <87tuxia5a9.fsf@dusky.pond.sub.org> <20200804160604.GB4860@linux.fritz.box> <87sgd15z5w.fsf@dusky.pond.sub.org> <87a6z6wqkg.fsf_-_@dusky.pond.sub.org> <87sgcyziul.fsf_-_@dusky.pond.sub.org> <20200810121914.GC14538@linux.fritz.box> Date: Wed, 26 Aug 2020 14:37:22 +0200 In-Reply-To: <20200810121914.GC14538@linux.fritz.box> (Kevin Wolf's message of "Mon, 10 Aug 2020 14:19:14 +0200") Message-ID: <87tuwp4m7x.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.3 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0.002 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=63.128.21.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/26 06:53:10 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.959, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no 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: marcandre.lureau@gmail.com, "Daniel P. =?utf-8?Q?Berrang=C3=A9?=" , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Kevin Wolf writes: > Am 07.08.2020 um 15:27 hat Markus Armbruster geschrieben: >> This is just a sketch. It's incomplete, needs comments and a real >> commit message. >> >> Support for "[PATCH v6 09/12] hmp: Add support for coroutine command >> handlers" is missing. Marked FIXME. >> >> As is, it goes on top of Kevin's series. It is meant to be squashed >> into PATCH 06, except for the FIXME, which needs to be resolved in PATCH >> 09 instead. >> >> Signed-off-by: Markus Armbruster >> --- >> monitor/monitor.c | 35 +++++++++++++++-------------------- >> 1 file changed, 15 insertions(+), 20 deletions(-) >> >> diff --git a/monitor/monitor.c b/monitor/monitor.c >> index 50fb5b20d3..8601340285 100644 >> --- a/monitor/monitor.c >> +++ b/monitor/monitor.c >> @@ -82,38 +82,34 @@ bool qmp_dispatcher_co_shutdown; >> */ >> bool qmp_dispatcher_co_busy; >> >> -/* >> - * Protects mon_list, monitor_qapi_event_state, coroutine_mon, >> - * monitor_destroyed. >> - */ >> +/* Protects mon_list, monitor_qapi_event_state, * monitor_destroyed. */ >> QemuMutex monitor_lock; >> static GHashTable *monitor_qapi_event_state; >> -static GHashTable *coroutine_mon; /* Maps Coroutine* to Monitor* */ >> >> MonitorList mon_list; >> int mon_refcount; >> static bool monitor_destroyed; >> >> +static Monitor **monitor_curp(Coroutine *co) >> +{ >> + static __thread Monitor *thread_local_mon; >> + static Monitor *qmp_dispatcher_co_mon; >> + >> + if (qemu_coroutine_self() == qmp_dispatcher_co) { >> + return &qmp_dispatcher_co_mon; >> + } >> + /* FIXME the coroutine hidden in handle_hmp_command() */ >> + return &thread_local_mon; >> +} > > Is thread_local_mon supposed to ever be set? The only callers of > monitor_set_cur() are the HMP and QMP dispatchers, which will return > something different. OOB commands are executed in @mon_iothread, outside coroutine context. qmp_dispatch() calls monitor_set_cur(), which sets thread_local_mon then. Since there is just one @mon_iothread, a @global_mon without __thread would do, but I don't see a need to exploit that here. > So should we return NULL insetad of thread_local_mon... > >> Monitor *monitor_cur(void) >> { >> - Monitor *mon; >> - >> - qemu_mutex_lock(&monitor_lock); >> - mon = g_hash_table_lookup(coroutine_mon, qemu_coroutine_self()); >> - qemu_mutex_unlock(&monitor_lock); >> - >> - return mon; >> + return *monitor_curp(qemu_coroutine_self()); >> } > > ...and return NULL here if monitor_curp() returned NULL... > >> void monitor_set_cur(Coroutine *co, Monitor *mon) >> { >> - qemu_mutex_lock(&monitor_lock); >> - if (mon) { >> - g_hash_table_replace(coroutine_mon, co, mon); >> - } else { >> - g_hash_table_remove(coroutine_mon, co); >> - } >> - qemu_mutex_unlock(&monitor_lock); >> + *monitor_curp(co) = mon; > > ...and assert(monitor_curp(co) != NULL) here? > > This approach looks workable, though the implementation of > monitor_curp() feels a bit brittle. The code is not significantly > simpler than the hash table based approach, but the assumptions it makes > are a bit more hidden. > > Saving the locks is more a theoretical improvement because all callers > are slows paths anyway. The hash table only ever has three keys: qmp_dispatcher_co, the coroutine hidden in handle_hmp_command(), and mon_iothread's leader (not in coroutine context). My version replaces the hash table by three pointer variables (two in the sketch above, because I didn't implement the third). You point out my code relies on an argument about which coroutines can execute commands. True. But I have to make that argument anyway to understand how the coroutine-enabled monitor works. On the other hand, it doesn't rely on an argument about the consistency of the hash table with the coroutines.