Re: locking question: do_mmap(), do_munmap()

Manfred Spraul
1999-10-13T07:32:54Z

Kanoj Sarcar wrote:
> Here's a primitive patch showing the direction I am thinking of. I do not
> have any problem with a spinning lock, but I coded this against 2.2.10,
> where insert_vm_struct could go to sleep, hence I had to use sleeping
> locks to protect the vma chain.

I found a few places where I don't know how to change them.

1) arch/mips/mm/r4xx0.c:
their flush_cache_range() function internally calls find_vma().
flush_cache_range() is called by proc/mem.c, and it seems that this
function cannot get the mmap semaphore.
Currently, every caller of flush_cache_range() either owns the kernel
lock or the mmap_sem.
OTHO, this function contains a race anyway [src_vma can go away if
handle_mm_fault() sleeps, src_vma is used at the end of the function.]

2) arch/sparc/mm/fault.c:
> /* This conditional is 'interesting'. */
> if (pgd_val(*pgdp) && !(write && !(pte_val(*ptep) & _SUN4C_PAGE_WRITE))
> && (pte_val(*ptep) & _SUN4C_PAGE_VALID))
> /* Note: It is safe to not grab the MMAP semaphore here because
> * we know that update_mmu_cache() will not sleep for
> * any reason (at least not in the current implementation)
> * and therefore there is no danger of another thread getting
> * on the CPU and doing a shrink_mmap() on this vma.
> */
> sun4c_update_mmu_cache (find_vma(current->mm, address), address,
> *ptep);
> else
> do_sparc_fault(regs, text_fault, write, address);
> }
could be safe because sun4c is only UP?

3) include/ppc-asm/pgtable.h:
> extern __inline__ pte_t *find_pte(struct mm_struct *mm,unsigned long va)
> {
> pgd_t *dir;
> pmd_t *pmd;
> pte_t *pte;
>
> va &= PAGE_MASK;
>
> dir = pgd_offset( mm, va );
> if (dir)
> {
> pmd = pmd_offset(dir, va & PAGE_MASK);
> if (pmd && pmd_present(*pmd))
> {
> pte = pte_offset(pmd, va);
> if (pte && pte_present(*pte))
> {
> pte_uncache(*pte);
> flush_tlb_page(find_vma(mm,va),va);
> }
> }
> }
> return pte;
> }
Could be safe because only called for "init_mm"?

I've not yet looked at swap_out [mm/swapfile.c and
arch/m68k/atari/stram.c] and proc/array.c

--
Manfred