From patchwork Tue Mar 9 03:06:03 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Zheng Yejian X-Patchwork-Id: 397458 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9526FC433E9 for ; Tue, 9 Mar 2021 03:00:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 41527652A8 for ; Tue, 9 Mar 2021 03:00:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230053AbhCIC71 (ORCPT ); Mon, 8 Mar 2021 21:59:27 -0500 Received: from szxga07-in.huawei.com ([45.249.212.35]:13874 "EHLO szxga07-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229589AbhCIC7R (ORCPT ); Mon, 8 Mar 2021 21:59:17 -0500 Received: from DGGEMS410-HUB.china.huawei.com (unknown [172.30.72.59]) by szxga07-in.huawei.com (SkyGuard) with ESMTP id 4DvfyJ61ZNz8vQl; Tue, 9 Mar 2021 10:57:28 +0800 (CST) Received: from code-website.localdomain (10.175.127.227) by DGGEMS410-HUB.china.huawei.com (10.3.19.210) with Microsoft SMTP Server id 14.3.498.0; Tue, 9 Mar 2021 10:59:05 +0800 From: Zheng Yejian To: , , , CC: , , , , Subject: [PATCH 4.4 1/3] futex: Change locking rules Date: Tue, 9 Mar 2021 11:06:03 +0800 Message-ID: <20210309030605.3295183-2-zhengyejian1@huawei.com> X-Mailer: git-send-email 2.25.4 In-Reply-To: <20210309030605.3295183-1-zhengyejian1@huawei.com> References: <20210309030605.3295183-1-zhengyejian1@huawei.com> MIME-Version: 1.0 X-Originating-IP: [10.175.127.227] X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Peter Zijlstra Currently futex-pi relies on hb->lock to serialize everything. But hb->lock creates another set of problems, especially priority inversions on RT where hb->lock becomes a rt_mutex itself. The rt_mutex::wait_lock is the most obvious protection for keeping the futex user space value and the kernel internal pi_state in sync. Rework and document the locking so rt_mutex::wait_lock is held accross all operations which modify the user space value and the pi state. This allows to invoke rt_mutex_unlock() (including deboost) without holding hb->lock as a next step. Nothing yet relies on the new locking rules. Signed-off-by: Peter Zijlstra (Intel) Cc: juri.lelli@arm.com Cc: bigeasy@linutronix.de Cc: xlpang@redhat.com Cc: rostedt@goodmis.org Cc: mathieu.desnoyers@efficios.com Cc: jdesfossez@efficios.com Cc: dvhart@infradead.org Cc: bristot@redhat.com Link: http://lkml.kernel.org/r/20170322104151.751993333@infradead.org Signed-off-by: Thomas Gleixner [Lee: Back-ported in support of a previous futex back-port attempt] Signed-off-by: Lee Jones Signed-off-by: Greg Kroah-Hartman Signed-off-by: Zheng Yejian --- kernel/futex.c | 138 +++++++++++++++++++++++++++++++++++++++---------- 1 file changed, 112 insertions(+), 26 deletions(-) diff --git a/kernel/futex.c b/kernel/futex.c index a14b7ef90e5c..b410752f5ad1 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1014,6 +1014,39 @@ static void exit_pi_state_list(struct task_struct *curr) * [10] There is no transient state which leaves owner and user space * TID out of sync. Except one error case where the kernel is denied * write access to the user address, see fixup_pi_state_owner(). + * + * + * Serialization and lifetime rules: + * + * hb->lock: + * + * hb -> futex_q, relation + * futex_q -> pi_state, relation + * + * (cannot be raw because hb can contain arbitrary amount + * of futex_q's) + * + * pi_mutex->wait_lock: + * + * {uval, pi_state} + * + * (and pi_mutex 'obviously') + * + * p->pi_lock: + * + * p->pi_state_list -> pi_state->list, relation + * + * pi_state->refcount: + * + * pi_state lifetime + * + * + * Lock order: + * + * hb->lock + * pi_mutex->wait_lock + * p->pi_lock + * */ /* @@ -1021,10 +1054,12 @@ static void exit_pi_state_list(struct task_struct *curr) * the pi_state against the user space value. If correct, attach to * it. */ -static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, +static int attach_to_pi_state(u32 __user *uaddr, u32 uval, + struct futex_pi_state *pi_state, struct futex_pi_state **ps) { pid_t pid = uval & FUTEX_TID_MASK; + int ret, uval2; /* * Userspace might have messed up non-PI and PI futexes [3] @@ -1032,8 +1067,33 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, if (unlikely(!pi_state)) return -EINVAL; + /* + * We get here with hb->lock held, and having found a + * futex_top_waiter(). This means that futex_lock_pi() of said futex_q + * has dropped the hb->lock in between queue_me() and unqueue_me_pi(), + * which in turn means that futex_lock_pi() still has a reference on + * our pi_state. + */ WARN_ON(!atomic_read(&pi_state->refcount)); + /* + * Now that we have a pi_state, we can acquire wait_lock + * and do the state validation. + */ + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); + + /* + * Since {uval, pi_state} is serialized by wait_lock, and our current + * uval was read without holding it, it can have changed. Verify it + * still is what we expect it to be, otherwise retry the entire + * operation. + */ + if (get_futex_value_locked(&uval2, uaddr)) + goto out_efault; + + if (uval != uval2) + goto out_eagain; + /* * Handle the owner died case: */ @@ -1049,11 +1109,11 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, * is not 0. Inconsistent state. [5] */ if (pid) - return -EINVAL; + goto out_einval; /* * Take a ref on the state and return success. [4] */ - goto out_state; + goto out_attach; } /* @@ -1065,14 +1125,14 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, * Take a ref on the state and return success. [6] */ if (!pid) - goto out_state; + goto out_attach; } else { /* * If the owner died bit is not set, then the pi_state * must have an owner. [7] */ if (!pi_state->owner) - return -EINVAL; + goto out_einval; } /* @@ -1081,11 +1141,29 @@ static int attach_to_pi_state(u32 uval, struct futex_pi_state *pi_state, * user space TID. [9/10] */ if (pid != task_pid_vnr(pi_state->owner)) - return -EINVAL; -out_state: + goto out_einval; + +out_attach: atomic_inc(&pi_state->refcount); + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); *ps = pi_state; return 0; + +out_einval: + ret = -EINVAL; + goto out_error; + +out_eagain: + ret = -EAGAIN; + goto out_error; + +out_efault: + ret = -EFAULT; + goto out_error; + +out_error: + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); + return ret; } /** @@ -1178,6 +1256,9 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key, /* * No existing pi state. First waiter. [2] + * + * This creates pi_state, we have hb->lock held, this means nothing can + * observe this state, wait_lock is irrelevant. */ pi_state = alloc_pi_state(); @@ -1202,7 +1283,8 @@ static int attach_to_pi_owner(u32 uval, union futex_key *key, return 0; } -static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, +static int lookup_pi_state(u32 __user *uaddr, u32 uval, + struct futex_hash_bucket *hb, union futex_key *key, struct futex_pi_state **ps, struct task_struct **exiting) { @@ -1213,7 +1295,7 @@ static int lookup_pi_state(u32 uval, struct futex_hash_bucket *hb, * attach to the pi_state when the validation succeeds. */ if (match) - return attach_to_pi_state(uval, match->pi_state, ps); + return attach_to_pi_state(uaddr, uval, match->pi_state, ps); /* * We are the first waiter - try to look up the owner based on @@ -1232,7 +1314,7 @@ static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval) if (unlikely(cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))) return -EFAULT; - /*If user space value changed, let the caller retry */ + /* If user space value changed, let the caller retry */ return curval != uval ? -EAGAIN : 0; } @@ -1296,7 +1378,7 @@ static int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb, */ match = futex_top_waiter(hb, key); if (match) - return attach_to_pi_state(uval, match->pi_state, ps); + return attach_to_pi_state(uaddr, uval, match->pi_state, ps); /* * No waiter and user TID is 0. We are here because the @@ -1436,6 +1518,7 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this, if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) { ret = -EFAULT; + } else if (curval != uval) { /* * If a unconditional UNLOCK_PI operation (user space did not @@ -1969,7 +2052,7 @@ retry_private: * rereading and handing potential crap to * lookup_pi_state. */ - ret = lookup_pi_state(ret, hb2, &key2, + ret = lookup_pi_state(uaddr2, ret, hb2, &key2, &pi_state, &exiting); } @@ -2247,7 +2330,6 @@ static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q, int err = 0; oldowner = pi_state->owner; - /* * We are here because either: * @@ -2266,11 +2348,10 @@ static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q, * because we can fault here. Imagine swapped out pages or a fork * that marked all the anonymous memory readonly for cow. * - * Modifying pi_state _before_ the user space value would - * leave the pi_state in an inconsistent state when we fault - * here, because we need to drop the hash bucket lock to - * handle the fault. This might be observed in the PID check - * in lookup_pi_state. + * Modifying pi_state _before_ the user space value would leave the + * pi_state in an inconsistent state when we fault here, because we + * need to drop the locks to handle the fault. This might be observed + * in the PID check in lookup_pi_state. */ retry: if (!argowner) { @@ -2331,21 +2412,26 @@ retry: return argowner == current; /* - * To handle the page fault we need to drop the hash bucket - * lock here. That gives the other task (either the highest priority - * waiter itself or the task which stole the rtmutex) the - * chance to try the fixup of the pi_state. So once we are - * back from handling the fault we need to check the pi_state - * after reacquiring the hash bucket lock and before trying to - * do another fixup. When the fixup has been done already we - * simply return. + * To handle the page fault we need to drop the locks here. That gives + * the other task (either the highest priority waiter itself or the + * task which stole the rtmutex) the chance to try the fixup of the + * pi_state. So once we are back from handling the fault we need to + * check the pi_state after reacquiring the locks and before trying to + * do another fixup. When the fixup has been done already we simply + * return. + * + * Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely + * drop hb->lock since the caller owns the hb -> futex_q relation. + * Dropping the pi_mutex->wait_lock requires the state revalidate. */ handle_fault: + raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock); spin_unlock(q->lock_ptr); err = fault_in_user_writeable(uaddr); spin_lock(q->lock_ptr); + raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock); /* * Check if someone else fixed it for us: