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    SubjectRe: changing mm->mmap_sem (was: Re: system call for process information?)

    On Fri, Mar 16, 2001 at 08:50:25AM -0300, Rik van Riel wrote:
    > On Fri, 16 Mar 2001, Stephen C. Tweedie wrote:
    > > > Write locks would be used in the code where we actually want
    > > > to change the VMA list and page faults would use an extra lock
    > > > to protect against each other (possibly a per-pagetable lock
    > >
    > > Why do we need another lock? The critical section where we do the
    > > final update on the pte _already_ takes the page table spinlock to
    > > avoid races against the swapper.
    > The problem is that mmap_sem seems to be protecting the list
    > of VMAs, so taking _only_ the page_table_lock could let a VMA
    > change under us while a page fault is underway ...

    Right, I'm not suggesting removing that: making the mmap_sem
    read/write is fine, but yes, we still need that semaphore. But as for
    the "page faults would use an extra lock to protect against each
    other" bit --- we already have another lock, the page table lock,
    which can be used in this way, so ANOTHER lock should be unnecessary.

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