forked from Minki/linux
eb3b7b848f
Like the common queued rwlock code the s390 implementation uses the queued spinlock code on a spinlock_t embedded in the rwlock_t to achieve the queueing. The encoding of the rwlock_t differs though, the counter field in the rwlock_t is split into two parts. The upper two bytes hold the write bit and the write wait counter, the lower two bytes hold the read counter. The arch_read_lock operation works exactly like the common qrwlock but the enqueue operation for a writer follows a diffent logic. After the failed inline try to get the rwlock in write, the writer first increases the write wait counter, acquires the wait spin_lock for the queueing, and then loops until there are no readers and the write bit is zero. Without the write wait counter a CPU that just released the rwlock could immediately reacquire the lock in the inline code, bypassing all outstanding read and write waiters. For s390 this would cause massive imbalances in favour of writers in case of a contended rwlock. Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com> |
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