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    SubjectRe: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation
    On Sat, Apr 17, 2010 at 10:06:36PM -0500, James Bottomley wrote:
    > On Fri, 2010-04-16 at 09:45 -0700, Paul E. McKenney wrote:
    > > o mutex_lock(): Critical sections need not guarantee
    > > forward progress, as general blocking is permitted.
    > This isn't quite right. mutex critical sections must guarantee eventual
    > forward progress against the class of other potential acquirers of the
    > mutex otherwise the system will become either deadlocked or livelocked.

    If I understand you correctly, you are saying that it is OK for a given
    critical section for a given mutex to fail to make forward progress if
    nothing else happens to acquire that mutex during that time. I would
    agree, at least I would if you were to further add that the soft-lockup
    checks permit an additional 120 seconds of failure to make forward progress
    even if something -is- attempting to acquire that mutex.

    By my standards, 120 seconds is a reasonable approximation to infinity,
    hence my statement above.

    So, would you agree with the following as a more precise statement?

    o mutex_lock(): Critical sections need not guarantee
    forward progress unless some other task is waiting
    on the mutex in question, in which case critical sections
    should complete in 120 seconds.

    Thanx, Paul

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