From mboxrd@z Thu Jan 1 00:00:00 1970 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=-12.7 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 0FE2DC48BE8 for ; Wed, 16 Jun 2021 04:40:40 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DC68D61375 for ; Wed, 16 Jun 2021 04:40:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229642AbhFPEit (ORCPT ); Wed, 16 Jun 2021 00:38:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60350 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229476AbhFPEis (ORCPT ); Wed, 16 Jun 2021 00:38:48 -0400 Received: from mail-pj1-x1029.google.com (mail-pj1-x1029.google.com [IPv6:2607:f8b0:4864:20::1029]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 76B33C061574 for ; Tue, 15 Jun 2021 21:36:43 -0700 (PDT) Received: by mail-pj1-x1029.google.com with SMTP id s17-20020a17090a8811b029016e89654f93so3205142pjn.1 for ; Tue, 15 Jun 2021 21:36:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:subject:to:cc:references:in-reply-to:mime-version :message-id:content-transfer-encoding; bh=4m2dd+0KAlT5LpJaa+DedRGeManljjf9yOejn/TMdrY=; b=fcUjWSTdc1OF7h+LKvI+SW6XJPSEsYQUhjK6rqUeBjvHrKv9YNmGsUopO5TZH80snW jFtbGvGb2ErVoYoRfoG2Dxd7lHMyIkge9uLP8l2Hx97lhT1AJRgy3KowUeCk2dytmrZx yP9m/eV6hIOs/GKDtgEjah421cGpPhRdA2pq8dyjuLT3Eb3Og49eMMbje2aiW1Ow+oo9 WYTQfrsrzqyOzARIPJESXlPm/CdVLXWFenjA6pQg5AwJoCzxmH6hiBIf15FW+roxx75c Avb6eJMVU1gKPZewBK3TP3ZMTuL6s+Kp/G7VHb6mLuQavmRcVPKkdftjr92xnbCuSxOe tHfw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:subject:to:cc:references:in-reply-to :mime-version:message-id:content-transfer-encoding; bh=4m2dd+0KAlT5LpJaa+DedRGeManljjf9yOejn/TMdrY=; b=gIW/2zF0Hry3syLfm7x8nkmoP7f5tnsDy3PIX5Kxc0E4ZVbeIK5qaBT7J8WuvMZ8xF icwHjgtJLZYXFN+/PJSyGx8Hg0BEkf0BRpC4AFQ+5VSBOgTTGPLTAQtbmtwUEcFBTFHX 8H0PJOiyuR0RR6n2zNwggJ4tM/w0BC4xzMiWFTlFnSCW8/O0tTWNa/PkY/GnhXJqldLt CVuA33CdH4vBSPsSNR9XJTfukewZ6Vdj0enEjGvYrNbjUxbyaHVCVgwbGly2Vd1hpl7h 1LrsCslELET2VQw0uYX6C5u8bF4PzC7tydSxmX47A4ZWoI15n4AS5J+YCIpAkJMBvoqX uUeQ== X-Gm-Message-State: AOAM532BOe+UoOOkxmFqqOCQ+VJQl0jzGgTUg2G/mSmKUk2i1bTWnebT uF1ElP8nT3Fh/kS+JF2//Lc= X-Google-Smtp-Source: ABdhPJzqLenEbcG/d7dt/tltfloS9RqI+xRkYpJkS9mZPlBqv4i9E5MBf7wHTBBXcmjQjNLhVJztXA== X-Received: by 2002:a17:902:6b09:b029:112:1e7a:a227 with SMTP id o9-20020a1709026b09b02901121e7aa227mr7516516plk.31.1623818203022; Tue, 15 Jun 2021 21:36:43 -0700 (PDT) Received: from localhost (60-242-147-73.tpgi.com.au. [60.242.147.73]) by smtp.gmail.com with ESMTPSA id i2sm3908261pjj.25.2021.06.15.21.36.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 15 Jun 2021 21:36:42 -0700 (PDT) Date: Wed, 16 Jun 2021 14:36:37 +1000 From: Nicholas Piggin Subject: Re: [PATCH 6/8] powerpc/membarrier: Remove special barrier on mm switch To: Andy Lutomirski , x86@kernel.org Cc: Andrew Morton , Benjamin Herrenschmidt , Dave Hansen , LKML , linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org, Mathieu Desnoyers , Michael Ellerman , Paul Mackerras , Peter Zijlstra References: In-Reply-To: MIME-Version: 1.0 Message-Id: <1623817894.lsnlb539ub.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Excerpts from Andy Lutomirski's message of June 16, 2021 1:21 pm: > powerpc did the following on some, but not all, paths through > switch_mm_irqs_off(): >=20 > /* > * Only need the full barrier when switching between processes. > * Barrier when switching from kernel to userspace is not > * required here, given that it is implied by mmdrop(). Barrier > * when switching from userspace to kernel is not needed after > * store to rq->curr. > */ > if (likely(!(atomic_read(&next->membarrier_state) & > (MEMBARRIER_STATE_PRIVATE_EXPEDITED | > MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev)) > return; >=20 > This is puzzling: if !prev, then one might expect that we are switching > from kernel to user, not user to kernel, which is inconsistent with the > comment. But this is all nonsense, because the one and only caller would > never have prev =3D=3D NULL and would, in fact, OOPS if prev =3D=3D NULL. Yeah that's strange, code definitely doesn't match comment. Good catch. >=20 > In any event, this code is unnecessary, since the new generic > membarrier_finish_switch_mm() provides the same barrier without arch help= . If that's merged then I think this could be too. I'll do a bit more=20 digging into this too. Thanks, Nick >=20 > Cc: Michael Ellerman > Cc: Benjamin Herrenschmidt > Cc: Paul Mackerras > Cc: linuxppc-dev@lists.ozlabs.org > Cc: Nicholas Piggin > Cc: Mathieu Desnoyers > Cc: Peter Zijlstra > Signed-off-by: Andy Lutomirski > --- > arch/powerpc/include/asm/membarrier.h | 27 --------------------------- > arch/powerpc/mm/mmu_context.c | 2 -- > 2 files changed, 29 deletions(-) > delete mode 100644 arch/powerpc/include/asm/membarrier.h >=20 > diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include= /asm/membarrier.h > deleted file mode 100644 > index 6e20bb5c74ea..000000000000 > --- a/arch/powerpc/include/asm/membarrier.h > +++ /dev/null > @@ -1,27 +0,0 @@ > -#ifndef _ASM_POWERPC_MEMBARRIER_H > -#define _ASM_POWERPC_MEMBARRIER_H > - > -static inline void membarrier_arch_switch_mm(struct mm_struct *prev, > - struct mm_struct *next, > - struct task_struct *tsk) > -{ > - /* > - * Only need the full barrier when switching between processes. > - * Barrier when switching from kernel to userspace is not > - * required here, given that it is implied by mmdrop(). Barrier > - * when switching from userspace to kernel is not needed after > - * store to rq->curr. > - */ > - if (likely(!(atomic_read(&next->membarrier_state) & > - (MEMBARRIER_STATE_PRIVATE_EXPEDITED | > - MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev)) > - return; > - > - /* > - * The membarrier system call requires a full memory barrier > - * after storing to rq->curr, before going back to user-space. > - */ > - smp_mb(); > -} > - > -#endif /* _ASM_POWERPC_MEMBARRIER_H */ > diff --git a/arch/powerpc/mm/mmu_context.c b/arch/powerpc/mm/mmu_context.= c > index a857af401738..8daa95b3162b 100644 > --- a/arch/powerpc/mm/mmu_context.c > +++ b/arch/powerpc/mm/mmu_context.c > @@ -85,8 +85,6 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct = mm_struct *next, > =20 > if (new_on_cpu) > radix_kvm_prefetch_workaround(next); > - else > - membarrier_arch_switch_mm(prev, next, tsk); > =20 > /* > * The actual HW switching method differs between the various > --=20 > 2.31.1 >=20 >=20