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    Subject[tip:perfcounters/core] perf_counter: fix the output lock
    Commit-ID:  22c1558e51c210787c6cf75d8905246fc91ec030
    Gitweb: http://git.kernel.org/tip/22c1558e51c210787c6cf75d8905246fc91ec030
    Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
    AuthorDate: Tue, 5 May 2009 17:50:25 +0200
    Committer: Ingo Molnar <mingo@elte.hu>
    CommitDate: Tue, 5 May 2009 20:18:32 +0200

    perf_counter: fix the output lock

    Use -1 instead of 0 as unlocked, since 0 is a valid cpu number.

    ( This is not an issue right now but will be once we allow multiple
    counters to output to the same mmap area. )

    [ Impact: prepare code for multi-counter profile output ]

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Paul Mackerras <paulus@samba.org>
    Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
    LKML-Reference: <20090505155437.232686598@chello.nl>
    Signed-off-by: Ingo Molnar <mingo@elte.hu>


    ---
    kernel/perf_counter.c | 7 ++++---
    1 files changed, 4 insertions(+), 3 deletions(-)

    diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c
    index 2d13427..c881afe 100644
    --- a/kernel/perf_counter.c
    +++ b/kernel/perf_counter.c
    @@ -1409,6 +1409,7 @@ static int perf_mmap_data_alloc(struct perf_counter *counter, int nr_pages)
    }

    data->nr_pages = nr_pages;
    + atomic_set(&data->lock, -1);

    rcu_assign_pointer(counter->data, data);

    @@ -1755,7 +1756,7 @@ static void perf_output_lock(struct perf_output_handle *handle)
    if (in_nmi() && atomic_read(&data->lock) == cpu)
    return;

    - while (atomic_cmpxchg(&data->lock, 0, cpu) != 0)
    + while (atomic_cmpxchg(&data->lock, -1, cpu) != -1)
    cpu_relax();

    handle->locked = 1;
    @@ -1784,7 +1785,7 @@ again:
    * NMI can happen here, which means we can miss a done_head update.
    */

    - cpu = atomic_xchg(&data->lock, 0);
    + cpu = atomic_xchg(&data->lock, -1);
    WARN_ON_ONCE(cpu != smp_processor_id());

    /*
    @@ -1794,7 +1795,7 @@ again:
    /*
    * Since we had it locked, we can lock it again.
    */
    - while (atomic_cmpxchg(&data->lock, 0, cpu) != 0)
    + while (atomic_cmpxchg(&data->lock, -1, cpu) != -1)
    cpu_relax();

    goto again;

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