From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail-co1nam03on0058.outbound.protection.outlook.com ([104.47.40.58]:25312 "EHLO NAM03-CO1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752767AbeFNMsg (ORCPT ); Thu, 14 Jun 2018 08:48:36 -0400 Subject: Re: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes To: Peter Zijlstra Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, Ingo Molnar , Jonathan Corbet , Gustavo Padovan , Maarten Lankhorst , Sean Paul , David Airlie , Davidlohr Bueso , "Paul E. McKenney" , Josh Triplett , Thomas Gleixner , Kate Stewart , Philippe Ombredanne , Greg Kroah-Hartman , linux-doc@vger.kernel.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org References: <20180614072922.8114-1-thellstrom@vmware.com> <20180614072922.8114-2-thellstrom@vmware.com> <20180614124129.GA12198@hirez.programming.kicks-ass.net> From: Thomas Hellstrom Message-ID: <3d73590a-13de-9164-4b32-9d7da6a1055b@vmware.com> Date: Thu, 14 Jun 2018 14:48:16 +0200 MIME-Version: 1.0 In-Reply-To: <20180614124129.GA12198@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: linux-media-owner@vger.kernel.org List-ID: Hi, Peter, On 06/14/2018 02:41 PM, Peter Zijlstra wrote: > On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote: >> +static bool __ww_mutex_wound(struct mutex *lock, >> + struct ww_acquire_ctx *ww_ctx, >> + struct ww_acquire_ctx *hold_ctx) >> +{ >> + struct task_struct *owner = __mutex_owner(lock); >> + >> + lockdep_assert_held(&lock->wait_lock); >> + >> + if (owner && hold_ctx && __ww_ctx_stamp_after(hold_ctx, ww_ctx) && >> + ww_ctx->acquired > 0) { >> + hold_ctx->wounded = 1; >> + >> + /* >> + * wake_up_process() paired with set_current_state() inserts >> + * sufficient barriers to make sure @owner either sees it's >> + * wounded or has a wakeup pending to re-read the wounded >> + * state. >> + * >> + * The value of hold_ctx->wounded in >> + * __ww_mutex_lock_check_stamp(); >> + */ >> + if (owner != current) >> + wake_up_process(owner); >> + >> + return true; >> + } >> + >> + return false; >> +} >> @@ -338,12 +377,18 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx) >> * and keep spinning, or it will acquire wait_lock, add itself >> * to waiter list and sleep. >> */ >> - smp_mb(); /* ^^^ */ >> + smp_mb(); /* See comments above and below. */ >> >> /* >> - * Check if lock is contended, if not there is nobody to wake up >> + * Check if lock is contended, if not there is nobody to wake up. >> + * We can use list_empty() unlocked here since it only compares a >> + * list_head field pointer to the address of the list head >> + * itself, similarly to how list_empty() can be considered RCU-safe. >> + * The memory barrier above pairs with the memory barrier in >> + * __ww_mutex_add_waiter and makes sure lock->ctx is visible before >> + * we check for waiters. >> */ >> - if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS))) >> + if (likely(list_empty(&lock->base.wait_list))) >> return; >> > OK, so what happens is that if we see !empty list, we take wait_lock, > if we end up in __ww_mutex_wound() we must really have !empty wait-list. > > It can however still see !owner because __mutex_unlock_slowpath() can > clear the owner field. But if owner is set, it must stay valid because > FLAG_WAITERS and we're holding wait_lock. If __ww_mutex_wound() is called from ww_mutex_set_context_fastpath() owner is the current process so we can never see !owner. However if __ww_mutex_wound() is called from __ww_mutex_add_waiter() then the above is true. > > So the wake_up_process() is in fact safe. > > Let me put that in a comment. Thanks, Thomas