From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 1F0F44BD375; Wed, 14 Oct 2026 13:07:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791983242; cv=none; b=KH2xpOetJH1mXTe9XeXvld+tN39dKzTaK642x5pyX3qXId0wbL25afvfsVIQ8AbyvAWyOHObicwB7T60f7uofX5qioCpo353+K6QuQft+zyvVrEZUaV4XcA6l0NrRYupWSzCaBnpachb296eNrpYlfgI1V1hPywHQ3PFROGQxcU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791983242; c=relaxed/simple; bh=tgeyzmFEqjRy/ewes1KGFcQbVOxQv8aq24/1Dxe5iq8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=B0hI2X0dVsnVI7r3g3jYgTQjTSbV99HjX/Zz23xq3LQCHX3lHr2vNFmTlpFlC7kY8IBalU1V3riJmbkp0tWxN3mNh0H7X5zqL/1tRHFuXMzCG8WRtLW2boKWwbAB+gCBTAPWovLFO1g0Q8HhgBXHfVQcv+aQVyTRsP70GxVrUU8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gJXvNhle; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="gJXvNhle" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2A5C31F000FF; Wed, 14 Oct 2026 13:07:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791983240; bh=wVcsAY5wGMlDOwmq1UZhDfW0sTXv6nSmjcnxiZJCQSs=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=gJXvNhlenmvFPChjIvsCVcupdI2lvpG74I76/6onSupzyO0JjNjLRd8nqJ5I0mzJr iqy0aBl7eU/sNq1UeTHQcq9kHlgTtxYiZmwV87gGeg9gF/6S/nInXVBK1+WAheoZrq IsAggvZ85Uz2egR5kLg2PBngfG6QygA/b7Y+0giJQQfNvt3ErLXI7YHtm4srzKTxyU 1D0sdzRuOzsf8lXOhV1QyPsJ40wzLX+by66/gY8/om4e4E12NwZy01kmORNiJoZIQd S0lEEw9xFXQeZEwJWzPJWpyO035uhqDkm6XDfP7Nsmvhg6Z2TZgUulV6X8W6/QR/yZ dT6m3qgj6lUew== Date: Wed, 14 Oct 2026 14:07:18 +0100 From: Harry Yoo To: Imre Kaloz Cc: Vlastimil Babka , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] mm/slab: sample slab_state once in kmem_cache_destroy() Message-ID: References: <20260930195110.13296-1-kaloz@kernel.org> <481e9a16-cd30-d463-14ee-b5602bcd5a21@kernel.org> 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=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Oct 13, 2026 at 06:32:26PM +0200, Imre Kaloz wrote: > Hi Harry, > > On Tue, 13 Oct 2026, Harry Yoo wrote: > > > > > > > > There is a related older window: after list_del() and before that > > > > > single read, slab_sysfs_init() could set FULL, skip this cache, and > > > > > the single read would then call both unlink and release on an > > > > > uninitialized kobject. > > > > > > > > No, with this patch slab_sysfs_init() cannot set the state to FULL > > > > because the slab_caches list and slab_state are protected by slab_mutex. > > > > > > > > > I have not hit that, and this patch does not close it. > > > > > > > > I think the patch does close two bugs. > > > > > > > > One introduced by 4ec10268ed98 (reading slab_state twice), > > > > another (reading slab_state outside slab_mutex) introduced by, hmm, > > > > not sure which commit introduced it :) should be very old. > > > > > > You're right, I got that one wrong. Sampling under slab_mutex in the > > > same section as the list_del() means slab_late_init() either sees the > > > cache on the list or we see slab_state < FULL, so the older window is > > > closed too. > > > > > > As for where the outside-mutex read came from, as far as I can tell it's > > > 0495e337b703 ("mm/slab_common: Deleting kobject in kmem_cache_destroy() > > > without holding slab_mutex/cpu_hotplug_lock"). Before that, the unlink > > > and release and their slab_state checks ran in shutdown_cache() under > > > slab_mutex. > > > > 0495e337b703 Probably yes. Not 100% sure. > > > > It's quite messy, reading slab_state has been done inside, switched > > to outside, and back to inside, and so on. > > > > But should be fine as long as we can find the first commit that > > affects -stable kernels (>= 5.10.y) > > > > > SLAB_TYPESAFE_BY_RCU caches were already released from the > > > workfn outside the mutex since 657dc2f97220. > > > > > > Want me to send a v2 with a second Fixes: for 0495e337b703, or is it > > > fine to add it when applying? > > > > Could you please verify fixing which bugs resolves the issue on sparc64 > > described in the changelog? Reading it twice, or outside slab_mutex, or > > both? > > I tested it. I built five variants of rc7 with a small test that > destroys a SLAB_TYPESAFE_BY_RCU cache while slab_late_init() runs > (same path the floppy bio slab takes), and booted each 10 times on > x86 and 8 times on sparc64. Boots that hit the WARN: > > x86 sparc64 > mainline 9/10 8/8 > one read, in kmem_cache_release() 9/10 8/8 > unlink check under slab_mutex, > release still re-reads 10/10 8/8 > one read under slab_mutex (this patch) 0/10 0/8 > one read right after mutex_unlock() 0/10 0/8 > > So it takes both. The read that hurts is the one at release time, > after rcu_barrier(); fixing only the double read keeps it, and so > does moving only the unlink check under the mutex. Reading once > right after the unlock didn't trigger it, but that is still racy, > just a much smaller window. Thanks for verifying that. > For stable, as far as I can tell the release-side read has been > outside slab_mutex since d50d82faa0c9 ("slub: fix failure when we > delete and create a slab cache") in v4.18 For SLAB_TYPESAFE_BY_RCU caches, yes. and others since 0495e337b703. I think Fixes: d50d82faa0c9 should be fine. > , so that would cover > 5.10.y. The patch doesn't apply before 6.12, so the older branches > need their own backports. Ack. Are you willing to backport it to -stable kernels as well? And could you please post v2 with updated Fixes: and the commit message ;) Thanks for fixing this! -- Cheers, Harry / Hyeonggon