On 9 February 2016 at 18:24, Jan Kara <jack@xxxxxxx> wrote: > Hello, > > I was thinking about current issues with DAX fault locking [1] (data > corruption due to racing faults allocating blocks) and also races which > currently don't allow us to clear dirty tags in the radix tree due to races > between faults and cache flushing [2]. Both of these exist because we don't > have an equivalent of page lock available for DAX. While we have a > reasonable solution available for problem [1], so far I'm not aware of a > decent solution for [2]. After briefly discussing the issue with Mel he had > a bright idea that we could used hashed locks to deal with [2] (and I think > we can solve [1] with them as well). So my proposal looks as follows: > > DAX will have an array of mutexes One folly here: Arrays of mutexes NEVER work unless you manage to align them to occupy one complete L2/L3 cache line each. Otherwise the CPUS will fight over cache lines each time they touch (read or write) a mutex, and it then becomes a O^n-like scalability problem if multiple mutexes occupy one cache line. It becomes WORSE as more mutexes fit into a single cache line and even more worse with the number of CPUS accessing such contested lines. Ced -- Cedric Blancher <cedric.blancher@xxxxxxxxx> Institute Pasteur -- 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>