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    Subject[PATCH] ide task_map_rq() preempt count imbalance
     Hello,

    2.5 kernel continued to fail on my test machine with a lot of
    scheduling while atomic messages. The offending function was
    include/linux/ide.h:task_sectors() which calls task_map_rq() followed
    by process_that_request_first(), taskfile_{input|output}_data() and
    task_unmap_rq().

    static inline void *task_map_rq(struct request *rq, unsigned long *flags)
    {
    /*
    * fs request
    */
    if (rq->cbio)
    return rq_map_buffer(rq, flags);

    /*
    * task request
    */
    return rq->buffer + blk_rq_offset(rq);
    }

    static inline void task_unmap_rq(struct request *rq, char *buffer, unsigned long *flags)
    {
    if (rq->cbio)
    rq_unmap_buffer(buffer, flags);
    }

    rq_[un]map_buffer() eventually call into k[un]map_atomic() which
    adjust preempt_count(). The problem is that rq->cbio is cleared by
    process_that_request_first() before calling task_unmap_rq(), so the
    preempt_count() isn't decremented properly.

    As it seems that rq->cbio test is to avoid the cost of calling into
    k[un]map_atomic, I just removed the optimization and it worked for me.

    I'm not familiar with ide and block layer. If I got something wrong,
    please point out.

    --
    tejun

    # This is a BitKeeper generated patch for the following project:
    # Project Name: Linux kernel tree
    # This patch format is intended for GNU patch command version 2.5 or higher.
    # This patch includes the following deltas:
    # ChangeSet 1.1412 -> 1.1413
    # include/linux/ide.h 1.72 -> 1.73
    #
    # The following is the BitKeeper ChangeSet Log
    # --------------------------------------------
    # 03/09/03 tj@atj.dyndns.org 1.1413
    # - task_[un]map_rq() preempt count imbalance fixed.
    # --------------------------------------------
    #
    diff -Nru a/include/linux/ide.h b/include/linux/ide.h
    --- a/include/linux/ide.h Wed Sep 3 14:50:20 2003
    +++ b/include/linux/ide.h Wed Sep 3 14:50:20 2003
    @@ -853,22 +853,12 @@

    static inline void *task_map_rq(struct request *rq, unsigned long *flags)
    {
    - /*
    - * fs request
    - */
    - if (rq->cbio)
    - return rq_map_buffer(rq, flags);
    -
    - /*
    - * task request
    - */
    - return rq->buffer + blk_rq_offset(rq);
    + return rq_map_buffer(rq, flags);
    }

    static inline void task_unmap_rq(struct request *rq, char *buffer, unsigned long *flags)
    {
    - if (rq->cbio)
    - rq_unmap_buffer(buffer, flags);
    + rq_unmap_buffer(buffer, flags);
    }

    #endif /* !CONFIG_IDE_TASKFILE_IO */
    -
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