On 2022/1/20 13:08, Bob Pearson wrote: > On 1/18/22 03:02, yangx.jy@xxxxxxxxxxx wrote: >> CC: linux-rdma mail list >> >> On 2022/1/14 4:51, Bob Pearson wrote: >>> The way these queues work they can only hold 2^n - 1 entries. The reason for this is >>> to distinguish empty and full. If you let the last entry be written then producer would advance to equal consumer which is the initial empty condition. So a queue which can hold 1 entry has to have two slots (n=1) but can only hold one entry. It begins with >> Hi Bob, >> >> Thanks for your detailed example. >>> prod = cons = 0 and is empty and is*not* full > full = (cons == ((prod + 1) % 2)) = (0 == 1) = false > empty = (prod == cons) == (0 == 0) = true >>> Adding one entry gives >>> >>> slot[0] = value, prod = 1, cons = 0 and is full and not empty > full = (cons == ((prod + 1) % 2)) = (0 == 0) = true > empty = (prod == cons) == (0 == 1) = false >>> After reading this value >>> >>> slot[0] = old value, prod = cons = 1 and queue is empty again. > full = (cons == ((prod + 1) % 2)) = (1 == 0) = false > empty = (prod == cons) = (1 == 1) = true >>> Writing a new value >>> >>> slot[1] = new value, slot[0] = old value, prod = 0 and cons = 1 and queue is full again. > full = (cons == ((prod + 1) % 2)) = (1 == (0 + 1) % 2) = true > empty = (prod == cons) = (0 == 1) = false > > and x % 2^N is the same as x& (2^N - 1) or x& index_mask. Hi Bob, Agreed. either x % 2^N or x & (2^N - 1) is fine, but we use the wrong x % (2^N - 1) here. >>> The expression full = (cons == ((qp->cur_index + 1) % q->index_mask)) is correct. > Actually you are correct. If you look at queue_full() it uses > full = (cons == (prod + 1)& q->index_mask) and queue_empty() uses > empty = (prod == cons) *but* > check_qp_queue_full() uses > full = (cons == ((qp->cur_index + 1) % q->index_mask)) with % instead of&. > I was so used to looking at the first two I missed the error in the last one. > > % should be& like the others. I will update the commit message and send v2 patch. Thanks lot. Best Regards, Xiao Yang > Bob > >
