From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=202.108.3.162 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793407631; b=D9WP46jkte6pkF3mF/FZm94UbxSYbk8HJmp+ROEKZtT3BUqgP62hRt8+4kCNEcqgDuKp Wf8Jz2dgtIl1df0uUILkkvpc5Z48SmsUOjwcHwokydJzX3WX6JYdsUQVMS9d9NoG6UnY2 tfI5bsSdQky1JVbkFDz56/OSGtxu8WD/FSmAMoOizoa1yHHBjKQDZBOo8xeR5hdRFlohn 6mmpb4acKRt7X7R9zO/cwqztiUvZtewY7hbWj4NnaGcl877pAtBA8RRCqckLYSeRw8RCt Hx95nEw2x3ZjXnnQ1C3QXHZuuFpulUueZZFL50JMryaIsRV2twHCvMx1xonsru8Wqnw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793407631; h=DMARC-Filter:DKIM-Signature:X-SMAIL-HELO:Received:X-Sender:X-Auth-ID: X-SMAIL-MID:X-SMAIL-UIID:From:To:Cc:Subject:Date:Message-ID: In-Reply-To:References:MIME-Version:Content-Transfer-Encoding; bh=UCvegi+i1apDZdVVmEmHCl1vJCt7SNjmj4csEy7A/78=; b=u/GdqfW/uE7thawlynekHnBgLzqKcWxeNTCQ9tRU9SZrY/VLBtpGOnghmZyww0v/F5Ld 1fdkBieXuGpI25fvNDv++RJAqtjWYbqp+525O1kNRS52JJw42418z4WbU3H6PZz6vBZYu cyZ6gH0W5HqY5oW9noW1Qin2fAWW5Gx68up5+jOcnfIoPS4TMqki+oELuDTX1TDbKg9Wa hGx0DLTYOnHGahFIku7wEbBqIAJRnENWpsF+7VUj4r1+idy03KljzYdz8Y598vLLjfJIp pbJLsP0Y5uFWFhC4pnvsJ0A0MmpSh+nkE/Xtk1Njg95evbpV5u+bxaCTDl7mPQFjksQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=sina.com; spf=pass smtp.mailfrom=sina.com; dkim=pass header.d=sina.com header.i=@sina.com header.a=rsa-sha256 header.s=201208 header.b=hG6omZN9; arc=none smtp.remote-ip=202.108.3.162 Received: from mail3-162.sinamail.sina.com.cn (mail3-162.sinamail.sina.com.cn [202.108.3.162]) (using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BA3F3606A31A for ; Sat, 31 Oct 2026 00:47:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org BA3F3606A31A Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=sina.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=sina.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=sina.com header.i=@sina.com header.a=rsa-sha256 header.s=201208 header.b=hG6omZN9 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sina.com; s=201208; t=1793407629; bh=UCvegi+i1apDZdVVmEmHCl1vJCt7SNjmj4csEy7A/78=; h=From:Subject:Date:Message-ID; b=hG6omZN9DURI0dH/sTwnwwe+TDeUZ/ZPV1hBazZJdPMPg2RVfBNbGZLkrNyRkLSsF wl/W2/VAyTlUuJtnDgU6E9xOziVQBXjXKrxEVLUNeNRhZbb9kxgWK/k1ybk33JPZpU Pm4b/kTv7cAUi6QnqAq54aUsXwfY31d/BiP6GQQA= X-SMAIL-HELO: localhost.localdomain Received: from unknown (HELO localhost.localdomain)([114.246.236.199]) by sina.com (10.54.253.34) with ESMTP id 6AE53A860000686A; Sat, 31 Oct 2026 08:47:05 +0800 (CST) X-Sender: hdanton@sina.com X-Auth-ID: hdanton@sina.com Authentication-Results: sina.com; spf=none smtp.mailfrom=hdanton@sina.com; dkim=none header.i=none; dmarc=none action=none header.from=hdanton@sina.com X-SMAIL-MID: 8196726291870 X-SMAIL-UIID: EE7AC67ADAD14D6B94E3B3DDB2C746CE-20261031-084705-1 From: Hillf Danton To: NeilBrown Cc: "Kal Cutter Conley" , "Chuck Lever" , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sunrpc: fix lost wakeups when stopping service threads Date: Sat, 31 Oct 2026 08:46:56 +0800 Message-ID: <20261031004657.251-1-hdanton@sina.com> In-Reply-To: <179340033904.269288.4611945330659974906@noble.neil.brown.name> References: X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Sat, 31 Oct 2026 09:45:39 +1100 NeilBrown wrote: > On Sat, 31 Oct 2026, Kal Cutter Conley wrote: > > Stopping NFS can leave rpc.nfsd 0 in an uninterruptible wait with > > this stack: > > > > svc_stop_kthreads [sunrpc] > > svc_set_num_threads [sunrpc] > > nfsd_svc [nfsd] > > write_threads [nfsd] > > nfsctl_transaction_write [nfsd] > > > > SP_NEED_VICTIM and SP_VICTIM_REMAINS remain set while all remaining > > nfsd threads sleep in svc_recv() and none has RQ_VICTIM set. > > > > The stop request is published with unordered set_bit() operations. > > svc_pool_wake_idle_thread() checks the idle list and task state before > > calling wake_up_process(). If it sees an empty list or TASK_RUNNING, > > it skips the wakeup and its memory barrier. The requester can therefore > > miss a sleeping worker while that worker misses the stop request. > > > > Order work publication before both lockless checks, even when no > > wakeup occurs. Pair with set_current_state() and llist_add() in > > svc_thread_wait_for_work(), so the worker observes the pending work or > > the requester observes the idle worker and wakes it. > > > > Fixes: 60749cbe3d8a ("sunrpc: change sp_nrthreads from atomic_t to unsigned int.") > > Signed-off-by: Kal Cutter Conley > > --- > > net/sunrpc/svc.c | 9 +++++++++ > > 1 file changed, 9 insertions(+) > > > > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c > > index af8d87c1de83..f3cc63acc5e1 100644 > > --- a/net/sunrpc/svc.c > > +++ b/net/sunrpc/svc.c > > @@ -29,6 +29,8 @@ > > #include > > #include > > > > +#include > > + > > #include > > > > #include "fail.h" > > @@ -693,6 +695,13 @@ void svc_pool_wake_idle_thread(struct svc_pool *pool) > > struct svc_rqst *rqstp; > > struct llist_node *ln; > > > > + /* > > + * Order work publication before the idle-list and task-state checks, > > + * even when wake_up_process() is skipped. Pairs with the barriers in > > + * set_current_state() and llist_add() in svc_thread_wait_for_work(). > > + */ > > + smp_mb(); > > + > > I don't like this. Stand-alone barriers are, I think, harder to read > than barriers combined with operations, like smp_load_acquire() or > similar. And this one seems quite detached from anything. > > If I understand correctly, the possible race is between > svc_thread_wait_for_work which includes: > > llist_add(&rqstp->rq_idle, &pool->sp_idle_threads); > if (likely(svc_thread_should_sleep(rqstp))) > > and svc_stop_kthread which includes: > > set_bit(SP_NEED_VICTIM, &pool->sp_flags); > svc_pool_wake_idle_thread(pool); > > The first sets ->sp_idle_threads.first and then checks SP_NEED_VICTIM > and the second sets SP_NEED_VICTIM and then checks > ->sp_idle_threads.first. > > So we need barriers to ensure the values are seen is the expected order. > Have I missed anything? > > llist_add() always calls try_cmpxchg() which is fully ordered, so no > extra barriers are needed there. > > There is no barrier between the set_bit)( and the READ_ONCE() in > svc_pool_wake_idle_thread(), so you added one. I agree that is correct > and solves the problem. > But it is only this call to svc_pool_wake_idle_thread() that needs a > barrier, not all calls. So I would prefer the barrier went in > svc_stop_kthreads(). Do you have a reason for not including it there? > > We do need a full memory barrier - a "release" on the set_bit and an > "acquire" on the test of ->sp_idle_threads.first > > I would be happy with smp_load_acquire(->sp_idle_theads_first) in > svc_pool_wake_idle_thread(). I'm not sure it is needed though. > > I would like set_bit_release(SP_NEED_VICTIM) in svc_stop_kthreads() > but we don't have that. It is on the way [11]. [11] Subject: [PATCH 1/2] asm-generic/bitops: Add set_bit_release() https://lore.kernel.org/lkml/20261021095512.1345191-3-phasta@kernel.org/ > We could do > atomic_long_fetch_or_release(BIT(SP_NEED_VICTIM), (atomic_long_t*)&pool->sp_flags) > but that is ugly. > > So I think the best solution is > > set_bit(SP_NEED_VICTIM, &pool->sp_flags); > /* Order SP_NEED_VICTIM before testing sp_idle_threads, > * matching svc_thread_wait_for_work() which orders > * setting sp_idle_threads before testing the bit > */ > + smp_mb__after_atomic(); > svc_pool_wake_idle_thread(pool); > > We probably need the same in svc_wake_up() after the set_bit(). > > Does this make the barriers easier to understand? > > Thoughts? > > Thanks, > NeilBrown > > > > rcu_read_lock(); > > ln = READ_ONCE(pool->sp_idle_threads.first); > > if (ln) { > > > > base-commit: b803428098f6fcd584db9f0499217b80e465bb34 > > -- > > 2.53.0