From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 1043A1849 for ; Mon, 16 Sep 2024 00:24:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726446291; cv=none; b=o2g+o9AUHbqPG7FSMuhFrZqx9/9OU23iP6GDspQswHeort9JEygTaJmbwo1H4ZYAlljNoHDnQjzjy4QFNf/P5iheuRsqCHMAMKsVb1bO+yjOkEONq7lLuG/YxhsuG1CvhpDfQl3C8NMX86QN71JPYRnU3cHKc/ypaWTYaVAVPo0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726446291; c=relaxed/simple; bh=c4drn7cnGXSnPICCASiqet/+DEdzauX/Zgd/4HmdryI=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=j36AlE7AWlNtQSmewlNEkKrzeUkbyzDtb3X2H7qQhIrTPOyMeVuFFd5/jZBbAr+0VY1if+De4v+oayxoM6HmI9rT8WjpgL8k4MxZLK5yRkbuy8eLBV0vLV8SgSG6U6D6N0J4l2eGsQBaexfTWlSTqrPhFIQL2HleY0O3Sl1g/lE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Hz7XaeIY; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Hz7XaeIY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F3612C4CEC3; Mon, 16 Sep 2024 00:24:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726446290; bh=c4drn7cnGXSnPICCASiqet/+DEdzauX/Zgd/4HmdryI=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=Hz7XaeIYE3zg00gQPrQgJQlWignyty5Vl7dXFL2UUgcc+jOkIyCkU7Ix7DD3JPFVC 8GCeCHOH5DzGsYRku3LOX9HAeowFUWFSxi1z72n+bI9A72fVTBVPQlJ2feTdVJT0vf us5z3rTK2pAF2Xq7MOZK6BzbSIHaBUSfp0v61+R0G7TXL9Nl5vgm3YNYz2SUoP0mQU 26mjfm1FGZ+UOt7NZxz1jdQBHMfaSpFDdxHwn/liNziEl4B1ctfHsOr0Tef3AuKlci GbTg1g+mKs6qKXnhl0zh6A6/K54YZv8eOHceSOrvbmL6DwVDvA0S5aLwBCyt4jyOZj FUVPYRHyHVVaw== Message-ID: <1ada154324d020a86aff7a7f4d155c13227abc7d.camel@kernel.org> Subject: Re: [PATCH nfsd-next] SQUASH sunrpc: allow svc threads to fail initialisation cleanly From: Jeff Layton To: NeilBrown , Chuck Lever III Cc: Linux NFS Mailing List , "Olga Kornievskaia Olga Kornievskaia" , Dai Ngo , Tom Talpey Date: Sun, 15 Sep 2024 20:24:48 -0400 In-Reply-To: <172644394073.17050.16376953609629336068@noble.neil.brown.name> References: <172644394073.17050.16376953609629336068@noble.neil.brown.name> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Mon, 2024-09-16 at 09:45 +1000, NeilBrown wrote: > The memory barriers in this patch were all wrong. > smp_store_release / smp_load_acquire ensures that all changes written > before the store_release are equally visible after the acquire. > These are not needed here as the *only* value that > svc_thread_init_status() or its caller sets that is of any interest to > svc_start_kthread() is ->rq_err. The barrier wold effect writes before > ->rq_err is written and reads after ->rq_err is read. >=20 > However we DO need a full memory barrier between setting ->rq_err and > before the the waitqueue_active() read in wake_up_var(). This is a > barrier between a write and a read, hence a full barrier: smb_mb(). >=20 > This addresses a race if wait_var_event() adds itself to the wait queue > and tests ->rq_err such that the read in waitqueue_active() happens > earlier and doesn't see that the task has been added, and the ->rq_err > write is delayed so that the waiting task doesn't see it. In this case > the wake_up never happens. >=20 > Signed-off-by: NeilBrown > --- > include/linux/sunrpc/svc.h | 7 +++++-- > net/sunrpc/svc.c | 3 +-- > 2 files changed, 6 insertions(+), 4 deletions(-) >=20 > diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h > index 437672bcaa22..558e5ae03103 100644 > --- a/include/linux/sunrpc/svc.h > +++ b/include/linux/sunrpc/svc.h > @@ -326,8 +326,11 @@ static inline bool svc_thread_should_stop(struct svc= _rqst *rqstp) > */ > static inline void svc_thread_init_status(struct svc_rqst *rqstp, int er= r) > { > - /* store_release ensures svc_start_kthreads() sees the error */ > - smp_store_release(&rqstp->rq_err, err); > + rqstp->rq_err =3D err; > + /* memory barrier ensures assignment to error above is visible before > + * waitqueue_active() test below completes. > + */ > + smb_mb(); > wake_up_var(&rqstp->rq_err); > if (err) > kthread_exit(1); > diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c > index ff6f3e35b36d..9aff845196ce 100644 > --- a/net/sunrpc/svc.c > +++ b/net/sunrpc/svc.c > @@ -818,8 +818,7 @@ svc_start_kthreads(struct svc_serv *serv, struct svc_= pool *pool, int nrservs) > svc_sock_update_bufs(serv); > wake_up_process(task); > =20 > - /* load_acquire ensures we get value stored in svc_thread_init_status(= ) */ > - wait_var_event(&rqstp->rq_err, smp_load_acquire(&rqstp->rq_err) !=3D -= EAGAIN); > + wait_var_event(&rqstp->rq_err, rqstp->rq_err !=3D -EAGAIN); > err =3D rqstp->rq_err; > if (err) { > svc_exit_thread(rqstp); Makes sense. Reviewed-by: Jeff Layton