Messages in this thread | | | Date | Thu, 11 Aug 2022 16:43:28 +0800 | Subject | Re: [PATCH] mm/mempolicy: fix lock contention on mems_allowed | From | Abel Wu <> |
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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).
Best Regards, Abel
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