On 01/23/2014 05:07 AM, Peter Zijlstra wrote:
On Wed, Jan 22, 2014 at 04:33:55PM -0500, Waiman Long wrote:+/** + * queue_read_unlock - release read lock of a queue rwlock + * @lock : Pointer to queue rwlock structure + */ +static inline void queue_read_unlock(struct qrwlock *lock) +{ + /* + * Atomically decrement the reader count + */ + atomic_sub(_QR_BIAS,&lock->cnts.rwa); +} + +/** + * queue_write_unlock - release write lock of a queue rwlock + * @lock : Pointer to queue rwlock structure + */ +static inline void queue_write_unlock(struct qrwlock *lock) +{ + /* + * If the writer field is atomic, it can be cleared directly. + * Otherwise, an atomic subtraction will be used to clear it. + */ + if (__native_word(lock->cnts.writer)) + smp_store_release(&lock->cnts.writer, 0); + else + atomic_sub(_QW_LOCKED,&lock->cnts.rwa); +}Both these unlocks miss a barrier; atomic_sub() doesn't imply any barrier what so ever. The smp_store_release() does, but the other two are invalid release ops in generic.
I thought that all atomic RMW instructions are memory barrier. If they are not, what kind of barrier should be added?
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