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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier
    On Thu, Jan 07, 2010 at 01:59:42PM -0500, Steven Rostedt wrote:
    > On Thu, 2010-01-07 at 10:39 -0800, Paul E. McKenney wrote:
    >
    > > > sys_membarrier() should "insert" mb() on behalf of B "instead"
    > > > of barrier(), right? But, if we send IPI, B enters kernel mode
    > > > and returns to user-mode. Should this imply mb() in any case?
    > >
    > > Hello, Oleg,
    > >
    > > The issue is with some suggested optimizations that would avoid sending
    > > the IPI to CPUs that are not running threads in the same process as the
    > > thread executing the sys_membarrier(). Some forms of these optimizations
    > > sample ->mm without locking, and the question is whether this is safe.
    >
    > Note, we are not suggesting optimizations. It has nothing to do with
    > performance of the syscall. We just can't allow one process to be DoSing
    > another process on another cpu by it sending out millions of IPIs.
    > Mathieu already showed that you could cause a 2x slowdown to the
    > unrelated tasks.

    I would have said that we are trying to optimize our way out of a DoS
    situation, but point taken. Whatever we choose to call it, the discussion
    is on the suggested modifications, not strictly on the original patch. ;-)

    Thanx, Paul


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