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[23.128.96.18]) by mx.google.com with ESMTP id g15si29937103edy.571.2021.07.22.12.04.37; Thu, 22 Jul 2021 12:05:02 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@ffwll.ch header.s=google header.b=GlYfJXLD; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229710AbhGVSWM (ORCPT + 99 others); Thu, 22 Jul 2021 14:22:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:53282 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229510AbhGVSWL (ORCPT ); Thu, 22 Jul 2021 14:22:11 -0400 Received: from mail-wr1-x42a.google.com (mail-wr1-x42a.google.com [IPv6:2a00:1450:4864:20::42a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3B674C061575 for ; Thu, 22 Jul 2021 12:02:45 -0700 (PDT) Received: by mail-wr1-x42a.google.com with SMTP id l7so7012532wrv.7 for ; Thu, 22 Jul 2021 12:02:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ffwll.ch; s=google; h=date:from:to:cc:subject:message-id:mail-followup-to:references :mime-version:content-disposition:in-reply-to; bh=NxAWTtoyjsf5ed4GT6Mn+COejMtj0Ij6Ef+0zC6Fp0I=; b=GlYfJXLDmHKYXvI8nkXkzG+gacX7+T76k4N1wPXrW28aQH7doyu/scQErhKLXXeT5j rjL1Zy95ogbryL7A6ufHOhbOYRJpkseGhqFKNJOtnO4d2rxbs/aeimJHRJADUojQ7bYT yW1mQ5ZWHeoMzezARa4AkB8kHrYLFWHY/XjdI= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id :mail-followup-to:references:mime-version:content-disposition :in-reply-to; bh=NxAWTtoyjsf5ed4GT6Mn+COejMtj0Ij6Ef+0zC6Fp0I=; b=B6JidbPO8X369ckY2dzW4nBzg6suVlvj4bvCi/yMXRsrbyjj/bw7w3LqIanz0GEhN2 EnkMobeyByaT/ogZkkJjUzxbOBlby0Ti27VL0dtIb8gj6w+kxDkj4HmUqUZbtg0HAU2U 9qqDGC8qd8j5hNb+6JrwGhFmpHye2a0rQV5DgLAlWEAe4BHcHem1gE/sOPYd+Z3HWcVr abVyE9HUVBN30Pf20gDpRnCyJdwdp+3p3//BXmvalbL5kFxYu4q+8eOBObhgwcYJL4IN HLRD2X4IU3doqVI5mWVtLrbJigjgenvHf086/4+JKaJWHMizG419MVkYcVHNHU6MG5wQ bKAg== X-Gm-Message-State: AOAM533Sx5j1oeTDYDbsZWHUeM+4hastHNiR22wNJYU+2IiWafBKJpFa DBHzeWIX8jrb2T/wLTQecNWSuA== X-Received: by 2002:adf:eb43:: with SMTP id u3mr1433153wrn.83.1626980563826; Thu, 22 Jul 2021 12:02:43 -0700 (PDT) Received: from phenom.ffwll.local ([2a02:168:57f4:0:efd0:b9e5:5ae6:c2fa]) by smtp.gmail.com with ESMTPSA id c10sm25624941wmb.40.2021.07.22.12.02.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Jul 2021 12:02:43 -0700 (PDT) Date: Thu, 22 Jul 2021 21:02:41 +0200 From: Daniel Vetter To: Boqun Feng Cc: Desmond Cheong Zhi Xi , LKML , Peter Zijlstra , VMware Graphics , Zack Rusin , Dave Airlie , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , dri-devel , intel-gfx , Shuah Khan , Greg KH , linux-kernel-mentees@lists.linuxfoundation.org Subject: Re: [PATCH 1/3] drm: use the lookup lock in drm_is_current_master Message-ID: Mail-Followup-To: Boqun Feng , Desmond Cheong Zhi Xi , LKML , Peter Zijlstra , VMware Graphics , Zack Rusin , Dave Airlie , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , dri-devel , intel-gfx , Shuah Khan , Greg KH , linux-kernel-mentees@lists.linuxfoundation.org References: <20210722092929.244629-1-desmondcheongzx@gmail.com> <20210722092929.244629-2-desmondcheongzx@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 22, 2021 at 6:00 PM Boqun Feng wrote: > > On Thu, Jul 22, 2021 at 12:38:10PM +0200, Daniel Vetter wrote: > > On Thu, Jul 22, 2021 at 05:29:27PM +0800, Desmond Cheong Zhi Xi wrote: > > > Inside drm_is_current_master, using the outer drm_device.master_mutex > > > to protect reads of drm_file.master makes the function prone to creating > > > lock hierarchy inversions. Instead, we can use the > > > drm_file.master_lookup_lock that sits at the bottom of the lock > > > hierarchy. > > > > > > Reported-by: Daniel Vetter > > > Signed-off-by: Desmond Cheong Zhi Xi > > > --- > > > drivers/gpu/drm/drm_auth.c | 9 +++++---- > > > 1 file changed, 5 insertions(+), 4 deletions(-) > > > > > > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c > > > index f00354bec3fb..9c24b8cc8e36 100644 > > > --- a/drivers/gpu/drm/drm_auth.c > > > +++ b/drivers/gpu/drm/drm_auth.c > > > @@ -63,8 +63,9 @@ > > > > > > static bool drm_is_current_master_locked(struct drm_file *fpriv) > > > { > > > - lockdep_assert_held_once(&fpriv->minor->dev->master_mutex); > > > - > > > + /* Either drm_device.master_mutex or drm_file.master_lookup_lock > > > + * should be held here. > > > + */ > > > > Disappointing that lockdep can't check or conditions for us, a > > lockdep_assert_held_either would be really neat in some cases. > > > > The implementation is not hard but I don't understand the usage, for > example, if we have a global variable x, and two locks L1 and L2, and > the function > > void do_something_to_x(void) > { > lockdep_assert_held_either(L1, L2); > x++; > } > > and two call sites: > > void f(void) > { > lock(L1); > do_something_to_x(); > unlock(L1); > } > > void g(void) > { > lock(L2); > do_something_to_x(); > unlock(L2); > } > > , wouldn't it be racy if f() and g() called by two threads at the same > time? Usually I would expect there exists a third synchronazition > mechanism (say M), which synchronizes the calls to f() and g(), and we > put M in the lockdep_assert_held() check inside do_something_to_x() > like: > > void do_something_to_x(void) > { > lockdep_assert_held_once(M); > x++; > } > > But of course, M may not be a lock, so we cannot put the assert there. > > My cscope failed to find ->master_lookup_lock in -rc2 and seems it's not > introduced in the patchset either, could you point me the branch this > patchset is based on, so that I could understand this better, and maybe > come up with a solution? Thanks ;-) The use case is essentially 2 nesting locks, and only the innermost is used to update a field. So when you only read this field, it's safe if either of these two locks are held. Essentially this is a read/write lock type of thing, except for various reasons the two locks might not be of the same type (like here where the write lock is a mutex, but the read lock is a spinlock). It's a bit like the rcu_derefence macro where it's ok to either be in a rcu_read_lock() section, or holding the relevant lock that's used to update the value. We do _not_ have two different locks that allow writing to the same X. Does that make it clearer what's the use-case here? In an example: void * interesting_pointer. do_update_interesting_pointer() { mutex_lock(A); /* do more stuff to prepare things */ spin_lock(B); interesting_pointer = new_value; spin_unlock(B); mutex_unlock(A); } read_interesting_thing_locked() { lockdep_assert_held_either(A, B); return interesting_pointer->thing; } read_interesting_thing() { int thing; spin_lock(B); thing = interesting_pointer->thing; spin_unlock(B); return B; } spinlock might also be irqsafe here if this can be called from irq context. Cheers, Daniel > Regards, > Boqun > > > Adding lockdep folks, maybe they have ideas. > > > > On the patch: > > > > Reviewed-by: Daniel Vetter > > > > > return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master; > > > } > > > > > > @@ -82,9 +83,9 @@ bool drm_is_current_master(struct drm_file *fpriv) > > > { > > > bool ret; > > > > > > - mutex_lock(&fpriv->minor->dev->master_mutex); > > > + spin_lock(&fpriv->master_lookup_lock); > > > ret = drm_is_current_master_locked(fpriv); > > > - mutex_unlock(&fpriv->minor->dev->master_mutex); > > > + spin_unlock(&fpriv->master_lookup_lock); > > > > > > return ret; > > > } > > > -- > > > 2.25.1 > > > > > > > -- > > Daniel Vetter > > Software Engineer, Intel Corporation > > http://blog.ffwll.ch -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch