On Thu 11-08-22 16:43:28, Abel Wu wrote:
> On 8/9/22 8:11 PM, Michal Hocko Wrote:
> > On Tue 09-08-22 18:49:27, Abel Wu wrote:
> > > The mems_allowed field can be modified by other tasks, so it
> > > isn't safe to access it with alloc_lock unlocked even in the
> > > current process context.
> >
> > It would be useful to describe the racing scenario and the effect it
> > would have. 78b132e9bae9 hasn't really explained thinking behind and why
> > it was considered safe to drop the lock. I assume it was based on the
> > fact that the operation happens on the current task but this is hard to
> > tell.
> >
>
> Sorry for my poor description. Say there are two tasks: A from cpusetA
> is performing set_mempolicy(2), and B is changing cpusetA's cpuset.mems.
>
> A (set_mempolicy) B (echo xx > cpuset.mems)
>
> pol = mpol_new();
> update_tasks_nodemask(cpusetA) {
> foreach t in cpusetA {
> cpuset_change_task_nodemask(t) {
> task_lock(t); // t could be A
> mpol_set_nodemask(pol) {
> new = f(A->mems_allowed);
> update t->mems_allowed;
> pol.create(pol, new);
> }
> task_unlock(t);
> task_lock(A);
> A->mempolicy = pol;
> task_unlock(A);
> }
> }
> }
>
> In this case A's pol->nodes is computed by old mems_allowed, and could
> be inconsistent with A's new mems_allowed.
>
> While it is different when replacing vmas' policy: the pol->nodes is
> gone wild only when current_cpuset_is_being_rebound():
>
> A (mbind) B (echo xx > cpuset.mems)
>
> cpuset_being_rebound = cpusetA;
> update_tasks_nodemask(cpusetA) {
> foreach t in cpusetA {
> cpuset_change_task_nodemask(t) {
> task_lock(t); // t could be A
> pol = mpol_new();
> mmap_write_lock(A->mm);
> mpol_set_nodemask(pol) {
> mask = f(A->mems_allowed);
> update t->mems_allowed;
> pol.create(pol, mask);
> }
> task_unlock(t);
> }
> foreach v in A->mm {
> if (current_cpuset_is_being_rebound())
> pol.rebind(pol, cpuset.mems);
> v->vma_policy = pol;
> }
> mmap_write_unlock(A->mm);
> mmap_write_lock(t->mm);
> mpol_rebind_mm(t->mm);
> mmap_write_unlock(t->mm);
> }
> }
> cpuset_being_rebound = NULL;
>
> In this case, the cpuset.mems, which has already done updating, is
> finally used for calculating pol->nodes, rather than A->mems_allowed.
> So it is OK to call mpol_set_nodemask() with alloc_lock unlocked when
> doing mbind(2).
Please add this to the patch changelog.
Thanks!
--
Michal Hocko
SUSE Labs
