On 18.1.2016 8:39, Sergey Senozhatsky wrote: > On (01/18/16 16:11), Minchan Kim wrote: > [..] >>> so, even if clear_bit_unlock/test_and_set_bit_lock do smp_mb or >>> barrier(), there is no corresponding barrier from record_obj()->WRITE_ONCE(). >>> so I don't think WRITE_ONCE() will help the compiler, or am I missing >>> something? >> >> We need two things >> 2. memory barrier. >> >> As compiler barrier, WRITE_ONCE works to prevent store tearing here >> by compiler. >> However, if we omit unpin_tag here, we lose memory barrier(e,g, smp_mb) >> so another CPU could see stale data caused CPU memory reordering. > > oh... good find! lost release semantic of unpin_tag()... Ah, release semantic, good point indeed. OK then we need the v2 approach again, with WRITE_ONCE() in record_obj(). Or some kind of record_obj_release() with release semantic, which would be a bit more effective, but I guess migration is not that critical path to be worth introducing it. Thanks, Vlastimil > > -ss > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>