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    Subject[PATCH 4.20 171/183] staging: erofs: atomic_cond_read_relaxed on ref-locked workgroup
    Date
    4.20-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Gao Xiang <gaoxiang25@huawei.com>

    commit df134b8d17b90c1e7720e318d36416b57424ff7a upstream.

    It's better to use atomic_cond_read_relaxed, which is implemented
    in hardware instructions to monitor a variable changes currently
    for ARM64, instead of open-coded busy waiting.

    Reviewed-by: Chao Yu <yuchao0@huawei.com>
    Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/staging/erofs/internal.h | 30 ++++++++++++++++++------------
    1 file changed, 18 insertions(+), 12 deletions(-)

    --- a/drivers/staging/erofs/internal.h
    +++ b/drivers/staging/erofs/internal.h
    @@ -221,23 +221,29 @@ static inline void erofs_workgroup_unfre
    preempt_enable();
    }

    +#if defined(CONFIG_SMP)
    +static inline int erofs_wait_on_workgroup_freezed(struct erofs_workgroup *grp)
    +{
    + return atomic_cond_read_relaxed(&grp->refcount,
    + VAL != EROFS_LOCKED_MAGIC);
    +}
    +#else
    +static inline int erofs_wait_on_workgroup_freezed(struct erofs_workgroup *grp)
    +{
    + int v = atomic_read(&grp->refcount);
    +
    + /* workgroup is never freezed on uniprocessor systems */
    + DBG_BUGON(v == EROFS_LOCKED_MAGIC);
    + return v;
    +}
    +#endif
    +
    static inline bool erofs_workgroup_get(struct erofs_workgroup *grp, int *ocnt)
    {
    - const int locked = (int)EROFS_LOCKED_MAGIC;
    int o;

    repeat:
    - o = atomic_read(&grp->refcount);
    -
    - /* spin if it is temporarily locked at the reclaim path */
    - if (unlikely(o == locked)) {
    -#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
    - do
    - cpu_relax();
    - while (atomic_read(&grp->refcount) == locked);
    -#endif
    - goto repeat;
    - }
    + o = erofs_wait_on_workgroup_freezed(grp);

    if (unlikely(o <= 0))
    return -1;

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