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Tue, 4 Aug 2020 16:06:05 +0000 (UTC) Date: Tue, 4 Aug 2020 18:06:04 +0200 From: Kevin Wolf To: Markus Armbruster Subject: Re: [PATCH v6 06/12] monitor: Make current monitor a per-coroutine property Message-ID: <20200804160604.GB4860@linux.fritz.box> References: <20200528153742.274164-1-kwolf@redhat.com> <20200528153742.274164-7-kwolf@redhat.com> <87tuxia5a9.fsf@dusky.pond.sub.org> MIME-Version: 1.0 In-Reply-To: <87tuxia5a9.fsf@dusky.pond.sub.org> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=205.139.110.61; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/08/04 01:28:16 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -40 X-Spam_score: -4.1 X-Spam_bar: ---- X-Spam_report: (-4.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1, 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_H2=-1, 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, qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Am 04.08.2020 um 15:50 hat Markus Armbruster geschrieben: > Kevin Wolf writes: > > > This way, a monitor command handler will still be able to access the > > current monitor, but when it yields, all other code code will correctly > > get NULL from monitor_cur(). > > > > Outside of coroutine context, qemu_coroutine_self() returns the leader > > coroutine of the current thread. > > Unsaid: you use it as a hash table key to map from coroutine to monitor, > and for that you need it to return a value unique to the coroutine in > coroutine context, and a value unique to the thread outside coroutine > context. Which qemu_coroutine_self() does. Correct? Correct. > The hash table works, but I hate it just as much as I hate > pthread_getspecific() / pthread_setspecific(). > > What we have here is a need for coroutine-local data. Feels like a > perfectly natural concept to me. If you have a good concept how to implement this in a generic way that doesn't impact the I/O fast path, feel free to implement it and I'll happily use it. But the hash table is simple and works for this use case, so I see little reason to invest a lot of time in something that we haven't ever had another user for. > Are we going to create another hash table whenever we need another piece > of coroutine-local data? Or shall we reuse the hash table, suitably > renamed and moved to another file? I think I would vote for separate hash tables rather than having a hash table containing a struct that mixes values from all subsystems, but this can be discussed when (if) the need arises. > Why not simply associate an opaque pointer with each coroutine? All it > takes is one more member of struct Coroutine. Whatever creates the > coroutine decides what to use it for. The monitor coroutine would use > it to point to the monitor. This doesn't work. error_report() is called from all kinds of coroutines, not just from coroutines created from the monitor, and it wants to know the current monitor. > At least, discuss the design alternatives in the commit message. *sigh* Fine. Tell me which set of alternatives to discuss. > > Signed-off-by: Kevin Wolf > > --- > > include/monitor/monitor.h | 2 +- > > monitor/hmp.c | 4 ++-- > > monitor/monitor.c | 27 +++++++++++++++++++++------ > > qapi/qmp-dispatch.c | 4 ++-- > > stubs/monitor-core.c | 2 +- > > 5 files changed, 27 insertions(+), 12 deletions(-) > > > > diff --git a/include/monitor/monitor.h b/include/monitor/monitor.h > > index 43cc746078..16072e325c 100644 > > --- a/include/monitor/monitor.h > > +++ b/include/monitor/monitor.h > > @@ -13,7 +13,7 @@ typedef struct MonitorOptions MonitorOptions; > > extern QemuOptsList qemu_mon_opts; > > > > Monitor *monitor_cur(void); > > -void monitor_set_cur(Monitor *mon); > > +void monitor_set_cur(Coroutine *co, Monitor *mon); > > bool monitor_cur_is_qmp(void); > > > > void monitor_init_globals(void); > > diff --git a/monitor/hmp.c b/monitor/hmp.c > > index 79be6f26de..3e73a4c3ce 100644 > > --- a/monitor/hmp.c > > +++ b/monitor/hmp.c > > @@ -1082,9 +1082,9 @@ void handle_hmp_command(MonitorHMP *mon, const char *cmdline) > > > > /* old_mon is non-NULL when called from qmp_human_monitor_command() */ > > old_mon = monitor_cur(); > > - monitor_set_cur(&mon->common); > > + monitor_set_cur(qemu_coroutine_self(), &mon->common); > > cmd->cmd(&mon->common, qdict); > > - monitor_set_cur(old_mon); > > + monitor_set_cur(qemu_coroutine_self(), old_mon); > > > > qobject_unref(qdict); > > } > > diff --git a/monitor/monitor.c b/monitor/monitor.c > > index 182ba136b4..35003bb486 100644 > > --- a/monitor/monitor.c > > +++ b/monitor/monitor.c > > @@ -58,24 +58,38 @@ IOThread *mon_iothread; > > /* Bottom half to dispatch the requests received from I/O thread */ > > QEMUBH *qmp_dispatcher_bh; > > > > -/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed. */ > > +/* > > + * Protects mon_list, monitor_qapi_event_state, coroutine_mon, > > + * 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 __thread Monitor *cur_monitor; > > - > > Monitor *monitor_cur(void) > > { > > - return cur_monitor; > > + Monitor *mon; > > + > > + qemu_mutex_lock(&monitor_lock); > > + mon = g_hash_table_lookup(coroutine_mon, qemu_coroutine_self()); > > + qemu_mutex_unlock(&monitor_lock); > > + > > + return mon; > > } > > > > -void monitor_set_cur(Monitor *mon) > > +void monitor_set_cur(Coroutine *co, Monitor *mon) > > { > > - cur_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); > > } > > You really need a contract now: any call to monitor_set_cur() with a > non-null @mon must be followed by a call with a null @mon. Why? g_hash_table_replace() removes the old value and replaces it with the new one. Kevin