linux-kernel - Re: [PATCH 13/26] ppc: Convert mmu context allocation to new IDA API

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Message-ID: <20180622145317.668cdafa@roar.ozlabs.ibm.com>
Date:   Fri, 22 Jun 2018 14:53:17 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
        Ram Pai <linuxram@...ibm.com>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 13/26] ppc: Convert mmu context allocation to new IDA
 API

On Thu, 21 Jun 2018 21:38:15 -0700
Matthew Wilcox <willy@...radead.org> wrote:

> On Fri, Jun 22, 2018 at 12:15:11PM +1000, Nicholas Piggin wrote:
> > On Thu, 21 Jun 2018 14:28:22 -0700
> > Matthew Wilcox <willy@...radead.org> wrote:  
> > >  static int alloc_context_id(int min_id, int max_id)  
> ...
> > > -	spin_lock(&mmu_context_lock);
> > > -	err = ida_get_new_above(&mmu_context_ida, min_id, &index);
> > > -	spin_unlock(&mmu_context_lock);  
> ...
> > > @@ -182,13 +148,11 @@ static void destroy_contexts(mm_context_t *ctx)
> > >  {
> > >  	int index, context_id;
> > >  
> > > -	spin_lock(&mmu_context_lock);
> > >  	for (index = 0; index < ARRAY_SIZE(ctx->extended_id); index++) {
> > >  		context_id = ctx->extended_id[index];
> > >  		if (context_id)
> > > -			ida_remove(&mmu_context_ida, context_id);
> > > +			ida_free(&mmu_context_ida, context_id);
> > >  	}
> > > -	spin_unlock(&mmu_context_lock);
> > >  }
> > >  
> > >  static void pte_frag_destroy(void *pte_frag)  
> > 
> > This hunk should be okay because the mmu_context_lock does not protect
> > the extended_id array, right Aneesh?  
> 
> That's my understanding.  The code today does this:
> 
> static inline int alloc_extended_context(struct mm_struct *mm,
>                                          unsigned long ea)
> {
>         int context_id;
> 
>         int index = ea >> MAX_EA_BITS_PER_CONTEXT;
> 
>         context_id = hash__alloc_context_id();
>         if (context_id < 0)
>                 return context_id;
> 
>         VM_WARN_ON(mm->context.extended_id[index]);
>         mm->context.extended_id[index] = context_id;
> 
> so it's not currently protected by this lock.  I suppose we are currently
> protected from destroy_contexts() being called twice simultaneously, but
> you'll notice that we don't zero the array elements in destroy_contexts(),
> so if we somehow had a code path which could call it concurrently, we'd
> be seeing warnings when the second caller tried to remove the context

Yeah that'd be an existing bug.

> IDs from the IDA.  I deduced that something else must be preventing
> this situation from occurring (like, oh i don't know, this function only
> being called on process exit, so implicitly only called once per context).

I think that's exactly right.

Thanks,
Nick