>> +#define buflock_write(bw, buf, size, item) \
>> + do { \
>> + bw.next_head = (bw.head + 1) & ((size) - 1); \
>> + smp_wmb(); \
>
> Why do we need the write barrier here?
I believe my first answer to this question was foggy indeed, so allow me to go
again, with a time line:
Scenario 1, correct write order:
writer store_next_head store_buf store_head
reader load_head load_buf load_next_head
Result: head != next_head, incoherent read detected
Scenario 2, incorrect write order:
writer store_buf store_next_head store_head
reader load_head load_buf load_next_head
Result: head == next_head, incoherent read not detected
Based on the assumption that scenario 2 could happen if the smp_wmb() is not
present, the barrier is needed.
Henrik
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