From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 2AFEEC678D4 for ; Thu, 2 Mar 2023 15:03:02 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pXkSA-0001BL-Kz; Thu, 02 Mar 2023 10:02:44 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pXkS1-000153-TM for qemu-devel@nongnu.org; Thu, 02 Mar 2023 10:02:34 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1pXkRz-0004uY-NQ for qemu-devel@nongnu.org; Thu, 02 Mar 2023 10:02:33 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1677769348; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=8wUlAmbkeiive3tr45UwwXyfR/QPhGo1RznOWahl5n0=; b=VmPakh9DyjqOhx3ONpAHrZnLlAmV6d6Szl3pfwTxyvdbICmpB5iyPBsQykKpiWSYXPfyF9 ovFTjN7tZ3+pqokCsCv0WdymbQ8f66qjbkT0Rwq/XyYi/DDUdJIRgN+eiVjcY5BGSc6qF4 +tGP80pIxM/NsgccLuEwGg9Nfx8N+4w= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-217-iboxR91LMMi_xDQHR0G5Sg-1; Thu, 02 Mar 2023 10:02:25 -0500 X-MC-Unique: iboxR91LMMi_xDQHR0G5Sg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id CD50E18E0A6A; Thu, 2 Mar 2023 15:02:23 +0000 (UTC) Received: from blackfin.pond.sub.org (unknown [10.39.193.92]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8CC3040C83B6; Thu, 2 Mar 2023 15:02:23 +0000 (UTC) Received: by blackfin.pond.sub.org (Postfix, from userid 1000) id 830CD21E6A1F; Thu, 2 Mar 2023 16:02:22 +0100 (CET) From: Markus Armbruster To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Fam Zheng , qemu-block@nongnu.org, Emanuele Giuseppe Esposito , Kevin Wolf , "Dr. David Alan Gilbert" , Hanna Reitz Subject: Re: [PATCH 5/6] hmp: convert handle_hmp_command() to AIO_WAIT_WHILE_UNLOCKED() References: <20230301205801.2453491-1-stefanha@redhat.com> <20230301205801.2453491-6-stefanha@redhat.com> <87ttz3sju0.fsf@pond.sub.org> <20230302132205.GB2485531@fedora> Date: Thu, 02 Mar 2023 16:02:22 +0100 In-Reply-To: <20230302132205.GB2485531@fedora> (Stefan Hajnoczi's message of "Thu, 2 Mar 2023 08:22:05 -0500") Message-ID: <87v8jjjiwx.fsf@pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-Scanned-By: MIMEDefang 3.1 on 10.11.54.1 Received-SPF: pass client-ip=170.10.129.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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=-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: 2.1.29 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 Stefan Hajnoczi writes: > On Thu, Mar 02, 2023 at 08:17:43AM +0100, Markus Armbruster wrote: >> Stefan Hajnoczi writes: >> >> > The HMP monitor runs in the main loop thread. Calling >> >> Correct. >> >> > AIO_WAIT_WHILE(qemu_get_aio_context(), ...) from the main loop thread is >> > equivalent to AIO_WAIT_WHILE_UNLOCKED(NULL, ...) because neither unlocks >> > the AioContext and the latter's assertion that we're in the main loop >> > succeeds. >> > >> > Signed-off-by: Stefan Hajnoczi >> > --- >> > monitor/hmp.c | 2 +- >> > 1 file changed, 1 insertion(+), 1 deletion(-) >> > >> > diff --git a/monitor/hmp.c b/monitor/hmp.c >> > index 2aa85d3982..5ecbdac802 100644 >> > --- a/monitor/hmp.c >> > +++ b/monitor/hmp.c >> > @@ -1167,7 +1167,7 @@ void handle_hmp_command(MonitorHMP *mon, const char *cmdline) >> > Coroutine *co = qemu_coroutine_create(handle_hmp_command_co, &data); >> > monitor_set_cur(co, &mon->common); >> > aio_co_enter(qemu_get_aio_context(), co); >> > - AIO_WAIT_WHILE(qemu_get_aio_context(), !data.done); >> > + AIO_WAIT_WHILE_UNLOCKED(NULL, !data.done); >> > } >> > >> > qobject_unref(qdict); >> >> Acked-by: Markus Armbruster >> >> For an R-by, I need to understand this in more detail. I'm not familiar >> with the innards of AIO_WAIT_WHILE() & friends, so I need to go real >> slow. >> >> We change >> >> ctx from qemu_get_aio_context() to NULL >> unlock from true to false >> >> in >> >> bool waited_ = false; \ >> AioWait *wait_ = &global_aio_wait; \ >> AioContext *ctx_ = (ctx); \ >> /* Increment wait_->num_waiters before evaluating cond. */ \ >> qatomic_inc(&wait_->num_waiters); \ >> /* Paired with smp_mb in aio_wait_kick(). */ \ >> smp_mb(); \ >> if (ctx_ && in_aio_context_home_thread(ctx_)) { \ >> while ((cond)) { \ >> aio_poll(ctx_, true); \ >> waited_ = true; \ >> } \ >> } else { \ >> assert(qemu_get_current_aio_context() == \ >> qemu_get_aio_context()); \ >> while ((cond)) { \ >> if (unlock && ctx_) { \ >> aio_context_release(ctx_); \ >> } \ >> aio_poll(qemu_get_aio_context(), true); \ >> if (unlock && ctx_) { \ >> aio_context_acquire(ctx_); \ >> } \ >> waited_ = true; \ >> } \ >> } \ >> qatomic_dec(&wait_->num_waiters); \ >> waited_; }) >> >> qemu_get_aio_context() is non-null here, correct? > > qemu_get_aio_context() always returns the main loop thread's AioContext. So it's non-null. > qemu_get_current_aio_context() returns the AioContext that was most > recently set in the my_aiocontext thread-local variable for IOThreads, > the main loop's AioContext for BQL threads, or NULL for threads > that don't use AioContext at all. > >> What's the value of in_aio_context_home_thread(qemu_get_aio_context())? > > This function checks whether the given AioContext is associated with > this thread. In a BQL thread it returns true if the context is the main > loop's AioContext. In an IOThread it returns true if the context is the > IOThread's AioContext. Otherwise it returns false. I guess that means in_aio_context_home_thread(qemu_get_aio_context()) is true in the main thread. Before the patch, the if's condition is true, and we execute while ((cond)) { \ aio_poll(ctx_, true); \ waited_ = true; \ } \ Afterwards, it's false, and we execute >> } \ >> qatomic_dec(&wait_->num_waiters); \ >> waited_; }) >> >> qemu_get_aio_context() is non-null here, correct? > > qemu_get_aio_context() always returns the main loop thread's AioContext. So it's non-null. > qemu_get_current_aio_context() returns the AioContext that was most > recently set in the my_aiocontext thread-local variable for IOThreads, > the main loop's AioContext for BQL threads, or NULL for threads > that don't use AioContext at all. > >> What's the value of in_aio_context_home_thread(qemu_get_aio_context())? > > This function checks whether the given AioContext is associated with > this thread. In a BQL thread it returns true if the context is the main > loop's AioContext. In an IOThread it returns true if the context is the > IOThread's AioContext. Otherwise it returns false. I guess that means in_aio_context_home_thread(qemu_get_aio_context()) is true in the main thread. Before the patch, the if's condition is true, and we execute while ((cond)) { \ aio_poll(ctx_, true); \ waited_ = true; \ } \ Afterwards, it's false, and we instead execute assert(qemu_get_current_aio_context() == \ qemu_get_aio_context()); \ while ((cond)) { \ if (unlock && ctx_) { \ aio_context_release(ctx_); \ } \ aio_poll(qemu_get_aio_context(), true); \ if (unlock && ctx_) { \ aio_context_acquire(ctx_); \ } \ waited_ = true; \ } \ The assertion is true: both operands of == are the main loop's AioContext. The if conditions are false, because unlock is. Therefore, we execute the exact same code. All correct?