On 11/16/20 7:53 AM, Peter Zijlstra wrote:
On Fri, Nov 13, 2020 at 10:46:03AM -0700, Shuah Khan wrote:
+Increment interface
+-------------------
+
+Increments sequence number and returns the new value. ::
+
+ seqnum32_inc_return() --> (u32) atomic_inc_return(seqnum)
+ seqnum64_inc_return() --> (u64) atomic64_inc_return(seqnum)
Did you think about the ordering?
Looking at atomic_t.txt _inc_return() can be fully ordered without
loosing or making the intermediate state visible. This is good for
this sequence number use-case. Is there something I am overlooking?
+Fetch interface
+---------------
+
+Fetched and returns current sequence number value. ::
+
+ seqnum32_fetch() --> (u32) atomic_add_return(0, seqnum)
+ seqnum64_fetch() --> (u64) atomic64_add_return(0, seqnum)
That's horrible. Please explain how that is not broken garbage.
Per the fact that it is atomic, nothing prevents the counter being
incremented concurrently. There is no such thing as a 'current' sequence
number.
Correct. Some usages of this _fecth() in this patch series are for
printing sequence numbers in debug message and others are stats.
I will review the patches in this series and drop the ones that use
read/fetch - the reason being sequence numbers are strictly up counters
and don't need read/fetch.
thanks,
-- Shuah
