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 7937837E5EF; Tue, 21 Jul 2026 10:46:43 +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=1784630820; cv=none; b=Up8XWvSsfgkkW0qyaI6SnXsoNy2pvF4xzyLqYpchvfHlJABvRRyyBbGYdTJb0bJXeOrk8Z966BjwZvg6lDgc1BasnGSqVuNkjFxRSA+L48jA06n4CzraUgIrX2X+bsdzPk2vi683IBh7/MVwkDGCD0hbsY5771im7I9YadquH1c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784630820; c=relaxed/simple; bh=1vm5b1f4nHNmH0W7NEQir6F8Wl4czzLDJkUbQm1YMUo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gj4j8E3pBYdbCaTnjdE6WzYZvTS4blUSzTx1jLRNs/P8OT76S/eN/ppHSqYVi9oMMYFOBZhXzMlKbhiL0ZeAs800AkYmK2XHQJzfYnZ1uNUMXQdYuKvhbQXceiMytiSyc1+7ZiofgtG8OVQhBD5VjO5hUj17S5j5jytEpAPiASc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LpSb/m/n; 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="LpSb/m/n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8B50A1F00A3F; Tue, 21 Jul 2026 10:46:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784630803; bh=dO+PVrm5HMtA1iQW+bv64CElxpl8z6fE8uq5ZpX/a00=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LpSb/m/nFwv6Fxf5E4wwMzA7a+zOz1RBkSBx/X2VMxQsjr3Jh5IKmhspJL2hbmV7V /o2P27drWcfPuhXrKLBUxWzh7xlRF/f+KelYKfm1dhIsE4iZzJS9mR4Jsif7unblPh XyXCRDg8gkRqH/5VgHF31/P3ht3P9mDAaZJolDrrk+J09luZY75llAHOiBc3u95Fkt s1NRRgaO1p+mVXyxHrRrkvd/sVLJLVghF6Vce1FP+c02FIOQ/dt4gh4lLaj+s0nv0N YrVAtJf5Yjlmnu8I1sxzExiRnxIVsfghFGLy6EL3LrC82sPMhJhLsAqM80fja5mayC rxhrYrO/l9lgQ== Message-ID: <12c63386-f96e-421e-b34c-00548d016f69@kernel.org> Date: Tue, 21 Jul 2026 12:46:35 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH slab/for-next v4 5/8] mm/slab: allow kfree_rcu_sheaf() on PREEMPT_RT Content-Language: en-US To: "Harry Yoo (Oracle)" , Andrew Morton , Hao Li , Christoph Lameter , David Rientjes , Roman Gushchin , Alexei Starovoitov , Andrii Nakryiko , Puranjay Mohan , Amery Hung , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Boqun Feng , Uladzislau Rezki , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Pedro Falcato , Suren Baghdasaryan , Shengming Hu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, rcu@vger.kernel.org, bpf@vger.kernel.org References: <20260720-kfree_rcu_nolock-v4-0-964e03c41a4e@kernel.org> <20260720-kfree_rcu_nolock-v4-5-964e03c41a4e@kernel.org> From: "Vlastimil Babka (SUSE)" Autocrypt: addr=vbabka@kernel.org; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSNWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBrZXJuZWwub3JnPsLBsAQTAQoAWhYhBKlA1DSZLC6OmRA9UCJPp+fM gqZkBQJqFFy6GxSAAAAAAAQADm1hbnUyLDIuNSsxLjEyLDIsMgIbAwUJGtCBUAULCQgHAwUV CgkICwUWAgMBAAIeBQIXgAAKCRAiT6fnzIKmZJIUEADFx/tREzUImHrEwVHeSvDFmA7tJysI UVrlvrM09E7GIuzphzv7jYmo8n3ANpCczLEVr4G0syYQdTigaZgv3+FQDIIzhKih1IHhu1Ei XHlywNWKnQxxQEUNi5Mwx43wQz5XVw9F1A7gtKBKNtfogO511hAbrzagrYajyQacEJ/+sfhZ 9Da8ltHIXD8pcYaHUfQgEusCgmEd9+KrUwrTbckFKmYq5chuE6yJ4J0EmWknL096jIE6CnzF FRslQ3B1UKDjxVsm1ZHfir5NeWszLkTvGFsddFaWTgh8UycESG6VQzKXjjewXu2pG7YQYRpj QKm1W5X2TkwWkXRBZTmfmbhxIUMh3+zf5wQ463rSmDN/8v81tdqBtAW6rH/kzg1GvkaTHXn0 507yEHFzBksk2viAuIxxr7km8+/KARYLIdGtx30EG8cKzAUZOK6WqxtNCsXUJNrVE8CWrCaD icoNu7Fs1c5hmPHdSTnU48ce67449DdnO4neLSNhRiGlMHJgfJUmgrxu/hcYeOZ3haWmEQ2w uW1Mh01OHi8QZHCEyAbABrPs9GUgccc/4eYXX9hIgxfSkYzn8f+8NuIFPWl/0uTvjgqU29FQ SbzOLxHq9439Ox40G5mS5eZXRGxITYR+6TXvRGI6P/264jvflnr/pDGUttaikU+0W+1uxgKH cmYbEc7ATQRbGTU1AQgAn0H6UrFiWcovkh6EXVcl+SeqyO6JHOPm+e9Wu0Vw+VIUvXZVUVVQ La1PQDUi6j00ChlcR66g9/V0sPIcSutacPKfdKYOBvzd4rlhL8rfrdEsQw5ApZxrA8kYZVMh FmBRKAa6wos25moTlMKpCWzTH84+WO5+ziCTsTUZASAToz3RdunTD+vQcHj0GqNTPAHK63sf bAB2I0BslZkXkY1RLb/YhuA6E7JyEd2pilZOrIuBGl/5q2qSakgnAVFWFBR/DO27JuAksYnq +aH8vI0xGvwn75KqSk4UzAkDzWSmO4ZHuahKtQgZNsMYV+PGayRBX9b9zbldzopoLBdqHc4n jQARAQABwsF8BBgBCgAmAhsMFiEEqUDUNJksLo6ZED1QIk+n58yCpmQFAmfIHFQFCRYU6J8A CgkQIk+n58yCpmS2PA//bqN1LfcotmArgElsa+0EGZSQlYgK48pm8WAeTXTngudP9IJ4SuKY HR5RNjHcBeqN+Me0zxRqYzRb8nGanHEkDyf4Im8DQM8d6vbyU+FcPmG4skud4kgS1zMHnlVd SXfSIwKC/hKgdHG8aBV7545Lz9X6Iohea+94wneD0aw/hqF+QWewGZhWJriWAZtvEkzNjQOi 4U9F/trLten/x7bpphDSnDMKJtITbtzATT1Dq7o7VpIUK1nCTQALMuMjKCdi8OdU/+V+R3O4 0PXWvX8qrvqYapVbZ+9KqT74FsuB0Ya9uXwgBF2Q6cRuETZk5vqaqKxzqoQZCO8AOz/58j6O 2RHNy/mZEN+7tJ5Tsq42zVJ4jxsT8b9YplavCMsnBgDeRWhcbYhCyttoL7nYISyWg4kQYZ/P wIV3OuNv2f8iKYsxNsRuClOAF82+gvqOy1/1pprFjy8uo2pkoOrb63aOP3vO5VHnRKgra6dq NcaZ+c6J4H+nEJGi2SkHAUJz5oBzuThvPudLvPA/SK8sKoM01IRxSihev/S/5WLazXB1PGem OCbvzC1IjWJJraxiDJ5IygokapUa2RP7+WBR22skQ3SSl6G107QgWKSyTOGWEaRmV53vxQLV jXuCmzSSasTL60zq5yGrT4/DYQVSNEUiUbG4pYekxJujNeEDkUlky0Y= In-Reply-To: <20260720-kfree_rcu_nolock-v4-5-964e03c41a4e@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 7/20/26 14:44, Harry Yoo (Oracle) wrote: > As suggested by Vlastimil Babka [1], kfree_rcu_sheaf() can be used > on PREEMPT_RT if we always assume spinning is not allowed on PREEMPT_RT. > This is because local_trylock and spinlock_t are safe to use with > trylock and unlock as long as the kernel does not spin and the context > is not NMI and not hardirq. > > Now that __kfree_rcu_sheaf() knows how to handle SLAB_FREE_NOLOCK, > relax the limitation and try the sheaves path on PREEMPT_RT as well. > > Keep the lockdep map on non RT kernels. However, do not use the lockdep > map on PREEMPT_RT to avoid suppressing valid lockdep warnings. > > As pointed by Vlastimil Babka [2], on PREEMPT_RT it is unnecessary to > defer call_rcu() under IRQ-disabled section or raw spinlock. However, > let us avoid adding more complexity as the scenario is not supposed > to be common on PREEMPT_RT, with a hope that call_rcu_nolock() will be > soon supported in RCU. > > Link: https://lore.kernel.org/linux-mm/6811cc17-8ee4-48c8-8cbf-6bf4d9f98162@kernel.org [1] > Link: https://lore.kernel.org/linux-mm/40591888-3a87-433e-b3d2-cda1cab543be@kernel.org [2] > Suggested-by: Vlastimil Babka (SUSE) > Signed-off-by: Harry Yoo (Oracle) Reviewed-by: Vlastimil Babka (SUSE) Nit: > --- > mm/slab_common.c | 12 ++++++++++-- > mm/slub.c | 17 ++++++++++------- > 2 files changed, 20 insertions(+), 9 deletions(-) > > diff --git a/mm/slab_common.c b/mm/slab_common.c > index 9c9ae1384f47..8c631bf97cd5 100644 > --- a/mm/slab_common.c > +++ b/mm/slab_common.c > @@ -1595,6 +1595,14 @@ static bool kfree_rcu_sheaf(void *obj) > { > struct kmem_cache *s; > struct slab *slab; > + unsigned int free_flags = SLAB_FREE_DEFAULT; > + > + /* > + * It is not safe to spin on PREEMPT_RT because the kernel might be > + * holding a raw spinlock and slab acquires sleeping locks. > + */ > + if (IS_ENABLED(CONFIG_PREEMPT_RT)) > + free_flags = SLAB_FREE_NOLOCK; > > if (is_vmalloc_addr(obj)) > return false; > @@ -1605,7 +1613,7 @@ static bool kfree_rcu_sheaf(void *obj) > > s = slab->slab_cache; > if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id())) > - return __kfree_rcu_sheaf(s, obj, SLAB_FREE_DEFAULT); > + return __kfree_rcu_sheaf(s, obj, free_flags); > > return false; > } > @@ -1954,7 +1962,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr) > if (!head) > might_sleep(); > > - if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr)) > + if (kfree_rcu_sheaf(ptr)) > return; > > // Queue the object but don't yet schedule the batch. > diff --git a/mm/slub.c b/mm/slub.c > index 8afa6b47b1f2..deac315d0f23 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -6065,12 +6065,13 @@ static void rcu_free_sheaf(struct rcu_head *head) > * kvfree_call_rcu() can be called while holding a raw_spinlock_t. Since > * __kfree_rcu_sheaf() may acquire a spinlock_t (sleeping lock on PREEMPT_RT), > * this would violate lock nesting rules. Therefore, kvfree_call_rcu() avoids > - * this problem by bypassing the sheaves layer entirely on PREEMPT_RT. > + * this problem by passing allow_spin = false on PREEMPT_RT. by passing SLAB_FREE_NOLOCK ? > * > * However, lockdep still complains that it is invalid to acquire spinlock_t > * while holding raw_spinlock_t, even on !PREEMPT_RT where spinlock_t is a > * spinning lock. Tell lockdep that acquiring spinlock_t is valid here > - * by temporarily raising the wait-type to LD_WAIT_CONFIG. > + * by temporarily raising the wait-type to LD_WAIT_CONFIG. Skip the lockdep map > + * on PREEMPT_RT to avoid suppressing valid lockdep warnings. > */ > static DEFINE_WAIT_OVERRIDE_MAP(kfree_rcu_sheaf_map, LD_WAIT_CONFIG); > > @@ -6080,10 +6081,10 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, unsigned int free_flags) > struct slab_sheaf *rcu_sheaf; > bool allow_spin = free_flags_allow_spinning(free_flags); > > - if (WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT))) > - return false; > + VM_WARN_ON_ONCE(IS_ENABLED(CONFIG_PREEMPT_RT) && allow_spin); > > - lock_map_acquire_try(&kfree_rcu_sheaf_map); > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > + lock_map_acquire_try(&kfree_rcu_sheaf_map); > > if (!local_trylock(&s->cpu_sheaves->lock)) > goto fail; > @@ -6181,12 +6182,14 @@ bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj, unsigned int free_flags) > local_unlock(&s->cpu_sheaves->lock); > > stat(s, FREE_RCU_SHEAF); > - lock_map_release(&kfree_rcu_sheaf_map); > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > + lock_map_release(&kfree_rcu_sheaf_map); > return true; > > fail: > stat(s, FREE_RCU_SHEAF_FAIL); > - lock_map_release(&kfree_rcu_sheaf_map); > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) > + lock_map_release(&kfree_rcu_sheaf_map); > return false; > } > >