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Tue, 26 Nov 2019 05:32:34 GMT Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by userp3020.oracle.com with ESMTP id 2wgvfhujmq-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 26 Nov 2019 05:32:33 +0000 Received: from abhmp0004.oracle.com (abhmp0004.oracle.com [141.146.116.10]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id xAQ5WVqE027572; Tue, 26 Nov 2019 05:32:31 GMT Received: from ca-dmjordan1.us.oracle.com (/10.211.9.48) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Mon, 25 Nov 2019 21:32:30 -0800 Date: Tue, 26 Nov 2019 00:32:38 -0500 From: Daniel Jordan To: Herbert Xu Cc: Eric Biggers , Linux Crypto Mailing List , Steffen Klassert , Daniel Jordan Subject: Re: [v2 PATCH] crypto: pcrypt - Avoid deadlock by using per-instance padata queues Message-ID: <20191126053238.yxhtfbt5okcjycuy@ca-dmjordan1.us.oracle.com> References: <20191119130556.dso2ni6qlks3lr23@gondor.apana.org.au> <20191119173732.GB819@sol.localdomain> <20191119185827.nerskpvddkcsih25@gondor.apana.org.au> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191119185827.nerskpvddkcsih25@gondor.apana.org.au> User-Agent: NeoMutt/20180716 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9452 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=2 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=860 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1911140001 definitions=main-1911260046 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9452 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=2 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=926 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1911140001 definitions=main-1911260046 Sender: linux-crypto-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-crypto@vger.kernel.org On Wed, Nov 20, 2019 at 02:58:27AM +0800, Herbert Xu wrote: > /* Replace the internal control structure with a new one. */ > -static void padata_replace(struct padata_instance *pinst, > - struct parallel_data *pd_new) > +static int padata_replace_one(struct padata_shell *ps) > { > - struct parallel_data *pd_old = pinst->pd; > + struct parallel_data *pd_old = rcu_dereference_protected(ps->pd, 1); > + struct parallel_data *pd_new; > int notification_mask = 0; > > - pinst->flags |= PADATA_RESET; > - > - rcu_assign_pointer(pinst->pd, pd_new); > + pd_new = padata_alloc_pd(ps); > + if (!pd_new) > + return -ENOMEM; > > - synchronize_rcu(); > + rcu_assign_pointer(ps->pd, pd_new); > > if (!cpumask_equal(pd_old->cpumask.pcpu, pd_new->cpumask.pcpu)) > notification_mask |= PADATA_CPU_PARALLEL; > @@ -510,10 +515,25 @@ static void padata_replace(struct padata_instance *pinst, > if (atomic_dec_and_test(&pd_old->refcnt)) > padata_free_pd(pd_old); > > + return notification_mask; > +} > + > +static void padata_replace(struct padata_instance *pinst) > +{ > + int notification_mask = 0; > + struct padata_shell *ps; > + > + pinst->flags |= PADATA_RESET; > + > + list_for_each_entry(ps, &pinst->pslist, list) > + notification_mask |= padata_replace_one(ps); > + > + synchronize_rcu(); > + > if (notification_mask) > blocking_notifier_call_chain(&pinst->cpumask_change_notifier, > notification_mask, > - &pd_new->cpumask); > + &pinst->cpumask); > > pinst->flags &= ~PADATA_RESET; > } I think it's possible for a task in padata_do_parallel() racing with another in padata_replace() to use a pd after free. The synchronize_rcu() comes after the pd_old->refcnt's are dec'd. padata_do_parallel() rcu_dereference_bh(ps->pd) // doesn't see PADATA_RESET set padata_replace() pinst->flags |= PADATA_RESET padata_replace_one() rcu_assign_pointer(ps->pd, pd_new) atomic_dec_and_test(&pd_old->refcnt) padata_free_pd() atomic_inc(&pd->refcnt) // too late If I'm not missing something, one way out is adding a list_head to parallel_data for remembering the old pd's on a local list in padata_replace() so that this function can loop over it and drop the refs after synchronize_rcu(). A padata_do_parallel() call will have then had a chance to take a ref on the pd it's using. And, not this patch, but with the removal of flushing it seems there's no need for PADATA_RESET, so it and its EBUSY error can go away.