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    Subject[tip:tracing/blktrace] blktrace: fix blk_probes_ref chaos
    Commit-ID:  2311b67256cb376cdbb541c781b95397f6896911
    Gitweb: http://git.kernel.org/tip/2311b67256cb376cdbb541c781b95397f6896911
    Author: Li Zefan <lizf@cn.fujitsu.com>
    AuthorDate: Fri, 27 Mar 2009 10:20:09 +0800
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Sat, 28 Mar 2009 12:51:41 +0100

    blktrace: fix blk_probes_ref chaos

    Impact: fix mixed ioctl and ftrace-plugin blktrace use refcount bugs

    ioctl-based blktrace allocates bt and registers tracepoints when
    ioctl(BLKTRACESETUP), and do all cleanups when ioctl(BLKTRACETEARDOWN).

    while ftrace-based blktrace allocates/frees bt when:
    # echo 1/0 > /sys/block/sda/sda1/trace/enable

    and registers/unregisters tracepoints when:
    # echo blk/nop > /debugfs/tracing/current_tracer
    or
    # echo 1/0 > /debugfs/tracing/tracing_enable

    The separatation of allocation and registeration causes 2 problems:

    1. current user-space blktrace still calls ioctl(TEARDOWN) when
    ioctl(SETUP) failed:
    # echo 1 > /sys/block/sda/sda1/trace/enable
    # blktrace /dev/sda
    BLKTRACESETUP: Device or resource busy
    ^C
    and now blk_probes_ref == -1

    2. Another way to make blk_probes_ref == -1:
    # plugin sdb && mount sdb1
    # echo 1 > /sys/block/sdb/sdb1/trace/enable
    # remove sdb

    This patch does the allocation and registeration when writing
    sdaX/trace/enable.

    Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
    Cc: Jens Axboe <jens.axboe@oracle.com>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Cc: Frederic Weisbecker <fweisbec@gmail.com>
    LKML-Reference: <49CC37D9.6030201@cn.fujitsu.com>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>


    ---
    kernel/trace/blktrace.c | 15 +++++++--------
    1 files changed, 7 insertions(+), 8 deletions(-)

    diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
    index 5b28f0f..8d6bd12 100644
    --- a/kernel/trace/blktrace.c
    +++ b/kernel/trace/blktrace.c
    @@ -478,7 +478,7 @@ int do_blk_trace_setup(struct request_queue *q, char *name, dev_t dev,
    goto err;
    }

    - if (atomic_add_return(1, &blk_probes_ref) == 1)
    + if (atomic_inc_return(&blk_probes_ref) == 1)
    blk_register_tracepoints();

    return 0;
    @@ -1091,8 +1091,6 @@ static void blk_tracer_print_header(struct seq_file *m)

    static void blk_tracer_start(struct trace_array *tr)
    {
    - if (atomic_add_return(1, &blk_probes_ref) == 1)
    - blk_register_tracepoints();
    trace_flags &= ~TRACE_ITER_CONTEXT_INFO;
    }

    @@ -1107,15 +1105,10 @@ static int blk_tracer_init(struct trace_array *tr)
    static void blk_tracer_stop(struct trace_array *tr)
    {
    trace_flags |= TRACE_ITER_CONTEXT_INFO;
    - if (atomic_dec_and_test(&blk_probes_ref))
    - blk_unregister_tracepoints();
    }

    static void blk_tracer_reset(struct trace_array *tr)
    {
    - if (!atomic_read(&blk_probes_ref))
    - return;
    -
    blk_tracer_enabled = false;
    blk_tracer_stop(tr);
    }
    @@ -1254,6 +1247,9 @@ static int blk_trace_remove_queue(struct request_queue *q)
    if (bt == NULL)
    return -EINVAL;

    + if (atomic_dec_and_test(&blk_probes_ref))
    + blk_unregister_tracepoints();
    +
    kfree(bt);
    return 0;
    }
    @@ -1280,6 +1276,9 @@ static int blk_trace_setup_queue(struct request_queue *q, dev_t dev)
    return -EBUSY;
    }

    + if (atomic_inc_return(&blk_probes_ref) == 1)
    + blk_register_tracepoints();
    +
    return 0;
    }


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