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    SubjectRe: [PATCH v5 01/45] percpu_rwlock: Introduce the global reader-writer lock backend
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    On Tue, Jan 22, 2013 at 11:32 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
    > On Tue, 2013-01-22 at 13:03 +0530, Srivatsa S. Bhat wrote:
    >> A straight-forward (and obvious) algorithm to implement Per-CPU Reader-Writer
    >> locks can also lead to too many deadlock possibilities which can make it very
    >> hard/impossible to use. This is explained in the example below, which helps
    >> justify the need for a different algorithm to implement flexible Per-CPU
    >> Reader-Writer locks.
    >>
    >> We can use global rwlocks as shown below safely, without fear of deadlocks:
    >>
    >> Readers:
    >>
    >> CPU 0 CPU 1
    >> ------ ------
    >>
    >> 1. spin_lock(&random_lock); read_lock(&my_rwlock);
    >>
    >>
    >> 2. read_lock(&my_rwlock); spin_lock(&random_lock);
    >>
    >>
    >> Writer:
    >>
    >> CPU 2:
    >> ------
    >>
    >> write_lock(&my_rwlock);
    >>
    >
    > I thought global locks are now fair. That is, a reader will block if a
    > writer is waiting. Hence, the above should deadlock on the current
    > rwlock_t types.

    I believe you are mistaken here. struct rw_semaphore is fair (and
    blocking), but rwlock_t is unfair. The reason we can't easily make
    rwlock_t fair is because tasklist_lock currently depends on the
    rwlock_t unfairness - tasklist_lock readers typically don't disable
    local interrupts, and tasklist_lock may be acquired again from within
    an interrupt, which would deadlock if rwlock_t was fair and a writer
    was queued by the time the interrupt is processed.

    > We need to fix those locations (or better yet, remove all rwlocks ;-)

    tasklist_lock is the main remaining user. I'm not sure about removing
    rwlock_t, but I would like to at least make it fair somehow :)

    --
    Michel "Walken" Lespinasse
    A program is never fully debugged until the last user dies.


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