From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail-dm3nam03on0060.outbound.protection.outlook.com ([104.47.41.60]:52125 "EHLO NAM03-DM3-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1755010AbeFNNTS (ORCPT ); Thu, 14 Jun 2018 09:19:18 -0400 Subject: Re: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes From: Thomas Hellstrom 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> <3d73590a-13de-9164-4b32-9d7da6a1055b@vmware.com> Message-ID: Date: Thu, 14 Jun 2018 15:18:56 +0200 MIME-Version: 1.0 In-Reply-To: <3d73590a-13de-9164-4b32-9d7da6a1055b@vmware.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-media-owner@vger.kernel.org List-ID: On 06/14/2018 02:48 PM, Thomas Hellstrom wrote: > 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. Or actually it was intended to be true, but FLAG_WAITERS is set too late. It needs to be moved to just after we actually add the waiter to the list. Then the hunk that replaces a FLAG_WAITERS read with a lockless list_empty() can also be ditched. /Thomas > >> >> So the wake_up_process() is in fact safe. >> >> Let me put that in a comment. > > > Thanks, > > Thomas > >