Re: [PATCH RESEND 2/4] cpuset: simplify cpuset_node_allowed API

Vladimir Davydov
2014-10-27T15:37:08Z

Hi Tejun,

On Mon, Oct 27, 2014 at 11:18:06AM -0400, Tejun Heo wrote:
> On Mon, Oct 20, 2014 at 03:50:30PM +0400, Vladimir Davydov wrote:
> > Current cpuset API for checking if a zone/node is allowed to allocate
> > from looks rather awkward. We have hardwall and softwall versions of
> > cpuset_node_allowed with the softwall version doing literally the same
> > as the hardwall version if __GFP_HARDWALL is passed to it in gfp flags.
> > If it isn't, the softwall version may check the given node against the
> > enclosing hardwall cpuset, which it needs to take the callback lock to
> > do.
> >
> > Such a distinction was introduced by commit 02a0e53d8227 ("cpuset:
> > rework cpuset_zone_allowed api"). Before, we had the only version with
> > the __GFP_HARDWALL flag determining its behavior. The purpose of the
> > commit was to avoid sleep-in-atomic bugs when someone would mistakenly
> > call the function without the __GFP_HARDWALL flag for an atomic
> > allocation. The suffixes introduced were intended to make the callers
> > think before using the function.
> >
> > However, since the callback lock was converted from mutex to spinlock by
> > the previous patch, the softwall check function cannot sleep, and these
> > precautions are no longer necessary.
> >
> > So let's simplify the API back to the single check.
> >
> > Suggested-by: David Rientjes <rientjes@google.com>
> > Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
> > Acked-by: Christoph Lameter <cl@linux.com>
> > Acked-by: Zefan Li <lizefan@huawei.com>
>
> Applied 1-2 to cgroup/for-3.19-cpuset-api-simplification which
> contains only these two patches on top of v3.18-rc2 and will stay
> stable. sl[au]b trees can pull it in or I can take the other two
> patches too. Please let me know how the other two should be routed.

JFYI, Andrew merged all four patches in his mmotm tree.

FWIW, there's a typo in this patch recently found and fixed by Dan
Carpenter. The fix is below.

Thanks,
Vladimir

---
From: Dan Carpenter <dan.carpenter@oracle.com>

This will deadlock instead of unlocking.

Fixes: f73eae8d8384 ('cpuset: simplify cpuset_node_allowed API')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Vladimir Davydov <vdavydov@parallels.com>

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 38f7433..4eaa203 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1992,7 +1992,7 @@ static int cpuset_css_online(struct cgroup_subsys_state *css)
spin_lock_irq(&callback_lock);
cs->mems_allowed = parent->mems_allowed;
cpumask_copy(cs->cpus_allowed, parent->cpus_allowed);
- spin_lock_irq(&callback_lock);
+ spin_unlock_irq(&callback_lock);
out_unlock:
mutex_unlock(&cpuset_mutex);
return 0;