On Thu, Apr 26, 2012 at 08:43:06PM -0400, Steven Rostedt wrote: > On Thu, Apr 26, 2012 at 07:51:05PM -0300, Glauber Costa wrote: > > In mem cgroup, we need to guarantee that two concurrent updates > > of the jump_label interface wait for each other. IOW, we can't have > > other updates returning while the first one is still patching the > > kernel around, otherwise we'll race. > > But it shouldn't. The code as is should prevent that. > > > > > I believe this is something that can fit well in the static branch > > API, without noticeable disadvantages: > > > > * in the common case, it will be a quite simple lock/unlock operation > > * Every context that calls static_branch_slow* already expects to be > > in sleeping context because it will mutex_lock the unlikely case. > > * static_key_slow_inc is not expected to be called in any fast path, > > otherwise it would be expected to have quite a different name. Therefore > > the mutex + atomic combination instead of just an atomic should not kill > > us. > > > > Signed-off-by: Glauber Costa <[email protected]> > > CC: Tejun Heo <[email protected]> > > CC: Li Zefan <[email protected]> > > CC: Kamezawa Hiroyuki <[email protected]> > > CC: Johannes Weiner <[email protected]> > > CC: Michal Hocko <[email protected]> > > CC: Ingo Molnar <[email protected]> > > CC: Jason Baron <[email protected]> > > --- > > kernel/jump_label.c | 21 +++++++++++---------- > > 1 files changed, 11 insertions(+), 10 deletions(-) > > > > diff --git a/kernel/jump_label.c b/kernel/jump_label.c > > index 4304919..5d09cb4 100644 > > --- a/kernel/jump_label.c > > +++ b/kernel/jump_label.c > > @@ -57,17 +57,16 @@ static void jump_label_update(struct static_key *key, > > int enable); > > > > void static_key_slow_inc(struct static_key *key) > > { > > + jump_label_lock(); > > if (atomic_inc_not_zero(&key->enabled)) > > - return; > > If key->enabled is not zero, there's nothing to be done. As the jump > label has already been enabled. Note, the key->enabled doesn't get set > until after the jump label is updated. Thus, if two tasks were to come > in, they both would be locked on the jump_label_lock(). >
Right, for x86 which uses stop_machine currently, we guarantee that all cpus are going to see the updated code, before the inc of key->enabled. However, other arches (sparc, mips, powerpc, for example), seem to be using much lighter weight updates, which I hope are ok :) Thanks, -Jason _______________________________________________ Devel mailing list [email protected] https://openvz.org/mailman/listinfo/devel
