From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bali.collaboradmins.com (bali.collaboradmins.com [148.251.105.195]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 02769426EB1; Wed, 6 May 2026 14:34:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=148.251.105.195 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778078046; cv=none; b=LHjsw4Flj3RejmabW3wCroQQyHa1f/z2/lnvPdo9mbjDmcBXK7z8gDFXI16OErpw09UP9SMG90DqT/t/jC+e5/qki9aKXi7d1SyCgMkFCPZ19MpdqMrgNHg8U6R4BsNgN8oaaUObKYuE0X3r7aurqgL3MhwNeyt8xuihcXDrQEc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778078046; c=relaxed/simple; bh=hgvN1jV6xsRIZYoJS/nwWzGkDdd5X1ml/V1oqoZcQcs=; h=Date:From:To:Cc:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=ORbEx67c7olsFHN7SHCAaVKqnhuPIoBNPPiUHs8FhNIBTJHqvSNqigiaIo8rcyhf5G1s5qVy6bOV6lZtuqsqK6azb5gNHS5/M/+4XJXaD+lkbKR+7nO1aszM/m/oyvJS/EBC+U6wXo4tZIgxwmzZHjuB+XFYxdl1l/zbghWU3II= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=collabora.com; spf=pass smtp.mailfrom=collabora.com; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b=IUsp6/vZ; arc=none smtp.client-ip=148.251.105.195 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=collabora.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=collabora.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=collabora.com header.i=@collabora.com header.b="IUsp6/vZ" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=collabora.com; s=mail; t=1778078043; bh=hgvN1jV6xsRIZYoJS/nwWzGkDdd5X1ml/V1oqoZcQcs=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=IUsp6/vZTnL428w38dQ33J4n60kD/ExAAsffIhADT5i8y64f55SvhQtXraAGvm21A ntW/h7Sd5brHtRWAR5EC/b6lIsBiQPrLv0AN82sr0IeoW3KoPBrpiScSzopTUchpIN +q2j7tZr8KShGYv6Lii9IUW0Uue/VAA+noI7oZklTezHeG6s2/+BLOowpKDQZoyRtZ HIkTbEDGszR9FA8hzXAoX4KidtHXHCmxhzly1PMLwTW7CoaJmLh0BHcY5x36uUhcwS eYtTqP+tZ5ZRtZG2FAHSU7P+PvjXDDpOkosvB3SoTcEUdB1zjArla4/xa+rNloDyjt TaWtC581dNciQ== Received: from fedora (unknown [100.64.0.11]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange ECDHE (prime256v1) server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) (Authenticated sender: bbrezillon) by bali.collaboradmins.com (Postfix) with ESMTPSA id 4697917E0610; Wed, 6 May 2026 16:34:02 +0200 (CEST) Date: Wed, 6 May 2026 16:33:56 +0200 From: Boris Brezillon To: Rob Clark Cc: Steven Price , Liviu Dudau , Dmitry Osipenko , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Akash Goel , Chia-I Wu , Dmitry Baryshkov , Abhinav Kumar , Jessica Zhang , Sean Paul , Marijn Suijten , linux-arm-msm@vger.kernel.org, freedreno@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] drm/gem: Fix a race between drm_gem_lru_scan() and drm_gem_object_release() Message-ID: <20260506163356.3287ce42@fedora> In-Reply-To: References: <20260506-panthor-shrinker-fixes-v1-0-e7721526de96@collabora.com> <20260506-panthor-shrinker-fixes-v1-2-e7721526de96@collabora.com> Organization: Collabora X-Mailer: Claws Mail 4.4.0 (GTK 3.24.52; x86_64-redhat-linux-gnu) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wed, 6 May 2026 06:21:42 -0700 Rob Clark wrote: > On Wed, May 6, 2026 at 5:16=E2=80=AFAM Boris Brezillon > wrote: > > > > The following race can currently happen: > > > > | Thread 0 in `drm_gem_lru_scan` | Thread 1 in `drm_gem_o= bject_release` | > > | - | - = | > > | move obj1 with refcount=3D=3D0 to `still_in_lru` | = | > > | move obj2 with refcount!=3D0 to `still_in_lru` | = | > > | mutex_unlock | = | > > | shrink obj2 | = | > > | | lru =3D obj1->lru; // = `still_in_lru` | > > | mutex_lock | = | > > | move obj1 back to the original lru | = | > > | mutex_unlock | = | > > | return | = | > > | | dereference `still_in_= lru` | > > > > Move the drm_gem_lru_move_tail_locked() after the > > kref_get_unless_zero() check so that we don't end up with a > > vanishing LRU when we hit drm_gem_object_release(). We also need to > > remove the skipped object from its LRU, otherwise we'll keep hitting > > it on subsequent loop iterations until it's actually removed from the > > list in the drm_gem_release(). > > > > Fixes: e7c2af13f811 ("drm/gem: Add LRU/shrinker helper") > > Reported-by: Chia-I Wu > > Closes: https://gitlab.freedesktop.org/panfrost/linux/-/work_items/86 > > Signed-off-by: Boris Brezillon > > Reviewed-by: Chia-I Wu > > --- > > drivers/gpu/drm/drm_gem.c | 14 +++++++++----- > > 1 file changed, 9 insertions(+), 5 deletions(-) > > > > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c > > index fca42949eb2b..97cf63de0112 100644 > > --- a/drivers/gpu/drm/drm_gem.c > > +++ b/drivers/gpu/drm/drm_gem.c > > @@ -1660,15 +1660,19 @@ drm_gem_lru_scan(struct drm_gem_lru *lru, > > if (!obj) > > break; > > > > - drm_gem_lru_move_tail_locked(&still_in_lru, obj); > > - > > /* > > * If it's in the process of being freed, gem_object->f= ree() > > - * may be blocked on lock waiting to remove it. So just > > - * skip it. > > + * may be blocked on lock waiting to remove it. So jus= t remove > > + * it from its current LRU and skip it. > > */ > > - if (!kref_get_unless_zero(&obj->refcount)) > > + if (!kref_get_unless_zero(&obj->refcount)) { > > + if (obj->lru) > > + drm_gem_lru_remove_locked(obj); =20 >=20 > if we are iterating a LRU.. and lru->lock is held, shouldn't obj->lru > always be non-null? Right, the !=3D NULL check is not needed here.