Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Tue, 4 Apr 2023 18:00:38 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Yair Podemsky <ypodemsk@redhat.com>
Cc:     linux@armlinux.org.uk, mpe@ellerman.id.au, npiggin@gmail.com,
        christophe.leroy@csgroup.eu, hca@linux.ibm.com, gor@linux.ibm.com,
        agordeev@linux.ibm.com, borntraeger@linux.ibm.com,
        svens@linux.ibm.com, davem@davemloft.net, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org, hpa@zytor.com, will@kernel.org,
        aneesh.kumar@linux.ibm.com, akpm@linux-foundation.org,
        arnd@arndb.de, keescook@chromium.org, paulmck@kernel.org,
        jpoimboe@kernel.org, samitolvanen@google.com, frederic@kernel.org,
        ardb@kernel.org, juerg.haefliger@canonical.com,
        rmk+kernel@armlinux.org.uk, geert+renesas@glider.be,
        tony@atomide.com, linus.walleij@linaro.org,
        sebastian.reichel@collabora.com, nick.hawkins@hpe.com,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        sparclinux@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-mm@kvack.org, mtosatti@redhat.com, vschneid@redhat.com,
        dhildenb@redhat.com, alougovs@redhat.com,
        Frederic Weisbecker <fweisbec@gmail.com>
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode
Message-ID: <20230404160038.GB38236@hirez.programming.kicks-ass.net>
References: <20230404134224.137038-1-ypodemsk@redhat.com>
 <20230404134224.137038-4-ypodemsk@redhat.com>
 <20230404151217.GB297936@hirez.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230404151217.GB297936@hirez.programming.kicks-ass.net>
Precedence: bulk

On Tue, Apr 04, 2023 at 05:12:17PM +0200, Peter Zijlstra wrote:
> > case 2:
> > CPU-A                                             CPU-B
> > 
> > modify pagetables
> > tlb_flush (memory barrier)
> >                                                   state == CONTEXT_USER
> > int state = atomic_read(&ct->state);
> >                                                   Kernel-enter:
> >                                                   state == CONTEXT_KERNEL
> >                                                   READ(pagetable values)
> > if (state & CT_STATE_MASK == CONTEXT_USER)
> > 


Hmm, hold up; what about memory ordering, we need a store-load ordering
between the page-table write and the context trackng load, and a
store-load order on the context tracking update and software page-table
walker loads.

Now, iirc page-table modification is done under pte_lock (or
page_table_lock) and that only provides a RELEASE barrier on this end,
which is insufficient to order against a later load.

Is there anything else?

On the state tracking side, we have ct_state_inc() which is
atomic_add_return() which should provide full barrier and is sufficient.