On Wed, Jul 26, 2017 at 10:02 AM, Christopher Lameter <cl@xxxxxxxxx> wrote:
On Wed, 26 Jul 2017, Dima Zavin wrote:
> The fix is to cache the value that's returned by cpusets_enabled() at the
> top of the loop, and only operate on the seqlock (both begin and retry) if
> it was true.
I think the proper fix would be to ensure that the calls to
read_mems_allowed_{begin,retry} cannot cause the deadlock. Otherwise you
have to fix this in multiple places.
Maybe read_mems_allowed_* can do some form of synchronization or *_retry
can implictly rely on the results of cpusets_enabled() by *_begin?
Thanks for the quick reply!
I can turn the cookie into a uint64, put the sequence into the low order 32 bits and put the enabled state into bit 33 (or 63 :) ). Then retry will not query cpusets_enabled() and will just look at the enabled bit. This means that *_retry will always have a conditional jump (i.e. lose the whole static_branch optimization) but maybe that's ok since that's pretty rare and the *_begin() will still benefit from it?
--Dima
