Messages in this thread | | | Date | Thu, 21 Jun 2018 21:38:15 -0700 | From | Matthew Wilcox <> | Subject | Re: [PATCH 13/26] ppc: Convert mmu context allocation to new IDA API |
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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@infradead.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 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).
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Thanks.
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