linux-kernel - Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp

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Date:   Tue, 4 Apr 2023 18:00:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yair Podemsky <ypodemsk@...hat.com>
Cc:     linux@...linux.org.uk, mpe@...erman.id.au, npiggin@...il.com,
        christophe.leroy@...roup.eu, hca@...ux.ibm.com, gor@...ux.ibm.com,
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        Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode

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.