From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order Date: Tue, 6 Dec 2016 17:55:44 +0100 Message-ID: <20161206165544.GX3045@worktop.programming.kicks-ass.net> References: <1480601214-26583-1-git-send-email-nhaehnle@gmail.com> <1480601214-26583-6-git-send-email-nhaehnle@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Return-path: Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2001:1868:205::9]) by gabe.freedesktop.org (Postfix) with ESMTPS id 102F06E3D1 for ; Tue, 6 Dec 2016 16:55:50 +0000 (UTC) Content-Disposition: inline In-Reply-To: <1480601214-26583-6-git-send-email-nhaehnle@gmail.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" To: Nicolai =?iso-8859-1?Q?H=E4hnle?= Cc: Maarten Lankhorst , Nicolai =?iso-8859-1?Q?H=E4hnle?= , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, Ingo Molnar List-Id: dri-devel@lists.freedesktop.org T24gVGh1LCBEZWMgMDEsIDIwMTYgYXQgMDM6MDY6NDhQTSArMDEwMCwgTmljb2xhaSBIw6Robmxl IHdyb3RlOgo+IEBAIC02OTMsOCArNzQ4LDEyIEBAIF9fbXV0ZXhfbG9ja19jb21tb24oc3RydWN0 IG11dGV4ICpsb2NrLCBsb25nIHN0YXRlLCB1bnNpZ25lZCBpbnQgc3ViY2xhc3MsCj4gIAkJICog bXV0ZXhfdW5sb2NrKCkgaGFuZGluZyB0aGUgbG9jayBvZmYgdG8gdXMsIGRvIGEgdHJ5bG9jawo+ ICAJCSAqIGJlZm9yZSB0ZXN0aW5nIHRoZSBlcnJvciBjb25kaXRpb25zIHRvIG1ha2Ugc3VyZSB3 ZSBwaWNrIHVwCj4gIAkJICogdGhlIGhhbmRvZmYuCj4gKwkJICoKPiArCQkgKiBGb3Igdy93IGxv Y2tzLCB3ZSBhbHdheXMgbmVlZCB0byBkbyB0aGlzIGV2ZW4gaWYgd2UncmUgbm90Cj4gKwkJICog Y3VycmVudGx5IHRoZSBmaXJzdCB3YWl0ZXIsIGJlY2F1c2Ugd2UgbWF5IGhhdmUgYmVlbiB0aGUK PiArCQkgKiBmaXJzdCB3YWl0ZXIgZHVyaW5nIHRoZSB1bmxvY2suCj4gIAkJICovCj4gLQkJaWYg KF9fbXV0ZXhfdHJ5bG9jayhsb2NrLCBmaXJzdCkpCj4gKwkJaWYgKF9fbXV0ZXhfdHJ5bG9jayhs b2NrLCB1c2Vfd3dfY3R4IHx8IGZpcnN0KSkKPiAgCQkJZ290byBhY3F1aXJlZDsKClNvIEknbSBz b21ld2hhdCB1bmNvbWZvcnRhYmxlIHdpdGggdGhpcy4gVGhlIHBvaW50IGlzIHRoYXQgd2l0aCB0 aGUKLmhhbmRvZmYgbG9naWMgaXQgaXMgdmVyeSBlYXN5IHRvIGFjY2lkZW50YWxseSBhbGxvdzoK CgltdXRleF9sb2NrKCZhKTsKCW11dGV4X2xvY2soJmEpOwoKQW5kIEknbSBub3Qgc3VyZSB0aGlz IGRvZXNuJ3QgbWFrZSB0aGF0IGhhcHBlbiBmb3Igd3dfbXV0ZXhlcy4gV2UgZ2V0IHRvCnRoaXMg X19tdXRleF90cnlsb2NrKCkgd2l0aG91dCBmaXJzdCBoYXZpbmcgYmxvY2tlZC4KCgo+ICAJCS8q Cj4gQEAgLTcxNiw3ICs3NzUsMjAgQEAgX19tdXRleF9sb2NrX2NvbW1vbihzdHJ1Y3QgbXV0ZXgg KmxvY2ssIGxvbmcgc3RhdGUsIHVuc2lnbmVkIGludCBzdWJjbGFzcywKPiAgCQlzcGluX3VubG9j a19tdXRleCgmbG9jay0+d2FpdF9sb2NrLCBmbGFncyk7Cj4gIAkJc2NoZWR1bGVfcHJlZW1wdF9k aXNhYmxlZCgpOwo+ICAKPiAtCQlpZiAoIWZpcnN0ICYmIF9fbXV0ZXhfd2FpdGVyX2lzX2ZpcnN0 KGxvY2ssICZ3YWl0ZXIpKSB7Cj4gKwkJaWYgKHVzZV93d19jdHggJiYgd3dfY3R4KSB7Cj4gKwkJ CS8qCj4gKwkJCSAqIEFsd2F5cyByZS1jaGVjayB3aGV0aGVyIHdlJ3JlIGluIGZpcnN0IHBvc2l0 aW9uLiBXZQo+ICsJCQkgKiBkb24ndCB3YW50IHRvIHNwaW4gaWYgYW5vdGhlciB0YXNrIHdpdGgg YSBsb3dlcgo+ICsJCQkgKiBzdGFtcCBoYXMgdGFrZW4gb3VyIHBvc2l0aW9uLgo+ICsJCQkgKgo+ ICsJCQkgKiBXZSBhbHNvIG1heSBoYXZlIHRvIHNldCB0aGUgaGFuZG9mZiBmbGFnIGFnYWluLCBp Zgo+ICsJCQkgKiBvdXIgcG9zaXRpb24gYXQgdGhlIGhlYWQgd2FzIHRlbXBvcmFyaWx5IHRha2Vu IGF3YXkuCj4gKwkJCSAqLwo+ICsJCQlmaXJzdCA9IF9fbXV0ZXhfd2FpdGVyX2lzX2ZpcnN0KGxv Y2ssICZ3YWl0ZXIpOwo+ICsKPiArCQkJaWYgKGZpcnN0KQo+ICsJCQkJX19tdXRleF9zZXRfZmxh Zyhsb2NrLCBNVVRFWF9GTEFHX0hBTkRPRkYpOwo+ICsJCX0gZWxzZSBpZiAoIWZpcnN0ICYmIF9f bXV0ZXhfd2FpdGVyX2lzX2ZpcnN0KGxvY2ssICZ3YWl0ZXIpKSB7Cj4gIAkJCWZpcnN0ID0gdHJ1 ZTsKPiAgCQkJX19tdXRleF9zZXRfZmxhZyhsb2NrLCBNVVRFWF9GTEFHX0hBTkRPRkYpOwo+ICAJ CX0KClNvIHRoZSBwb2ludCBpcyB0aGF0ICF3d19jdHggZW50cmllcyBhcmUgJ3NraXBwZWQnIGR1 cmluZyB0aGUgaW5zZXJ0aW9uCmFuZCB0aGVyZWZvcmUsIGlmIG9uZSBiZWNvbWVzIGZpcnN0LCBp dCBtdXN0IHN0YXkgZmlyc3Q/Cgo+IEBAIC03MjgsNyArODAwLDcgQEAgX19tdXRleF9sb2NrX2Nv bW1vbihzdHJ1Y3QgbXV0ZXggKmxvY2ssIGxvbmcgc3RhdGUsIHVuc2lnbmVkIGludCBzdWJjbGFz cywKPiAgCQkgKiBvciB3ZSBtdXN0IHNlZSBpdHMgdW5sb2NrIGFuZCBhY3F1aXJlLgo+ICAJCSAq Lwo+ICAJCWlmICgoZmlyc3QgJiYgbXV0ZXhfb3B0aW1pc3RpY19zcGluKGxvY2ssIHd3X2N0eCwg dXNlX3d3X2N0eCwgdHJ1ZSkpIHx8Cj4gLQkJICAgICBfX211dGV4X3RyeWxvY2sobG9jaywgZmly c3QpKQo+ICsJCSAgICAgX19tdXRleF90cnlsb2NrKGxvY2ssIHVzZV93d19jdHggfHwgZmlyc3Qp KQo+ICAJCQlicmVhazsKPiAgCj4gIAkJc3Bpbl9sb2NrX211dGV4KCZsb2NrLT53YWl0X2xvY2ss IGZsYWdzKTsKCgpfX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19f XwpkcmktZGV2ZWwgbWFpbGluZyBsaXN0CmRyaS1kZXZlbEBsaXN0cy5mcmVlZGVza3RvcC5vcmcK aHR0cHM6Ly9saXN0cy5mcmVlZGVza3RvcC5vcmcvbWFpbG1hbi9saXN0aW5mby9kcmktZGV2ZWwK From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753998AbcLFQ4A (ORCPT ); Tue, 6 Dec 2016 11:56:00 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:32956 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753904AbcLFQz4 (ORCPT ); Tue, 6 Dec 2016 11:55:56 -0500 Date: Tue, 6 Dec 2016 17:55:44 +0100 From: Peter Zijlstra To: Nicolai =?iso-8859-1?Q?H=E4hnle?= Cc: linux-kernel@vger.kernel.org, Nicolai =?iso-8859-1?Q?H=E4hnle?= , Ingo Molnar , Maarten Lankhorst , Daniel Vetter , Chris Wilson , dri-devel@lists.freedesktop.org Subject: Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order Message-ID: <20161206165544.GX3045@worktop.programming.kicks-ass.net> References: <1480601214-26583-1-git-send-email-nhaehnle@gmail.com> <1480601214-26583-6-git-send-email-nhaehnle@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1480601214-26583-6-git-send-email-nhaehnle@gmail.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 01, 2016 at 03:06:48PM +0100, Nicolai Hähnle wrote: > @@ -693,8 +748,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > * mutex_unlock() handing the lock off to us, do a trylock > * before testing the error conditions to make sure we pick up > * the handoff. > + * > + * For w/w locks, we always need to do this even if we're not > + * currently the first waiter, because we may have been the > + * first waiter during the unlock. > */ > - if (__mutex_trylock(lock, first)) > + if (__mutex_trylock(lock, use_ww_ctx || first)) > goto acquired; So I'm somewhat uncomfortable with this. The point is that with the .handoff logic it is very easy to accidentally allow: mutex_lock(&a); mutex_lock(&a); And I'm not sure this doesn't make that happen for ww_mutexes. We get to this __mutex_trylock() without first having blocked. > /* > @@ -716,7 +775,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > spin_unlock_mutex(&lock->wait_lock, flags); > schedule_preempt_disabled(); > > - if (!first && __mutex_waiter_is_first(lock, &waiter)) { > + if (use_ww_ctx && ww_ctx) { > + /* > + * Always re-check whether we're in first position. We > + * don't want to spin if another task with a lower > + * stamp has taken our position. > + * > + * We also may have to set the handoff flag again, if > + * our position at the head was temporarily taken away. > + */ > + first = __mutex_waiter_is_first(lock, &waiter); > + > + if (first) > + __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); > + } else if (!first && __mutex_waiter_is_first(lock, &waiter)) { > first = true; > __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF); > } So the point is that !ww_ctx entries are 'skipped' during the insertion and therefore, if one becomes first, it must stay first? > @@ -728,7 +800,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, > * or we must see its unlock and acquire. > */ > if ((first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)) || > - __mutex_trylock(lock, first)) > + __mutex_trylock(lock, use_ww_ctx || first)) > break; > > spin_lock_mutex(&lock->wait_lock, flags);