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    SubjectRe: [RFC][PATCH RT] rwsem_rt: Another (more sane) approach to mulit reader rt locks
    On Thu, May 17, 2012 at 06:17:47PM +0200, Peter Zijlstra wrote:
    > On Thu, 2012-05-17 at 08:47 -0700, Paul E. McKenney wrote:
    > > On Thu, May 17, 2012 at 05:32:59PM +0200, Peter Zijlstra wrote:
    > > > On Thu, 2012-05-17 at 08:18 -0700, Paul E. McKenney wrote:
    > > > > Some researchers at MIT RCU-ified this lock:
    > > > >
    > > > > http://people.csail.mit.edu/nickolai/papers/clements-bonsai.pdf
    > > >
    > > > Ah, as have I [1].. and they seem to have gotten about as far as I have,
    > > > which means almost there but not quite [2] :-)
    > >
    > > I had forgotten about that -- that was the first call for call_srcu(),
    > > if I remember correctly.
    > >
    > > > The most interesting case is file maps and they simply ignored those.
    > > > While I appreciate that from an academic pov, -- they can still write a
    > > > paper on the other interesting bits -- I don't really like it from a
    > > > practical point.
    > > >
    > > > [1] https://lkml.org/lkml/2010/1/4/257
    > > > [2] https://lkml.org/lkml/2010/1/4/532
    > >
    > > Hmmm... Do the recent dcache changes cover some of the things that
    > > Linus called out? Probably not, but some at least.
    >
    > No, and the points viro made:
    >
    > https://lkml.org/lkml/2010/1/5/194
    >
    > are still very much an issue, you really don't want to do fput() from an
    > asynchronous context. Which means you have to do synchronize_rcu() or
    > similar from munmap() which will be rather unpopular :/

    I don't claim to understand all of the code, but I am also unafraid to
    ask stupid questions. ;-)

    So, is it possible to do something like the following?

    1. Schedule a workqueue from an RCU callback, and to have that
    workqueue do the fput.

    2. Make things like unmount() do rcu_barrier() followed by
    flush_workqueue(), or probably multiple flush_workqueue()s.

    3. If someone concurrently does munmap() and a write to the
    to-be-unmapped region, then the write can legally happen.

    4. Acquire mmap_sem in the fault path, but only if the fault
    requires blocking, and recheck the situation under
    mmap_sem -- the hope being to prevent long-lived page
    faults from messing things up.

    Fire away! ;-)

    > Since we should not use per-cpu data for either files or processes
    > (there are simply too many of those around) the alternative is
    > horrendously hideous things like:
    >
    > https://lkml.org/lkml/2010/1/6/136
    >
    > which one cannot get away with either.
    >
    > The whole thing is very vexing indeed since all of this is only needed
    > for ill-behaved applications since a well-constructed application will
    > never fault in a range it is concurrently unmapping.
    >
    > Most annoying.

    No argument there!!!

    Thanx, Paul



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