Messages in this thread | | | Date | Thu, 4 Mar 2010 15:53:15 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH 4/4] cpuset,mm: use rwlock to protect task->mempolicy and mems_allowed |
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On Wed, Mar 03, 2010 at 06:52:39PM +0800, Miao Xie wrote: > if MAX_NUMNODES > BITS_PER_LONG, loading/storing task->mems_allowed or mems_allowed in > task->mempolicy are not atomic operations, and the kernel page allocator gets an empty > mems_allowed when updating task->mems_allowed or mems_allowed in task->mempolicy. So we > use a rwlock to protect them to fix this probelm.
Oh, and something else I'm also concerned about:
If MAX_NUMNODES <= BITS_PER_LONG then these locks are a noop.
> +#define read_mem_lock_irqsave(p, flags) do { (void)(flags); } while (0) > + > +#define read_mem_unlock_irqrestore(p, flags) do { (void)(flags); } while (0) > + > +/* Be used to protect task->mempolicy and mems_allowed when user reads them */
However you are appearing to use them for more than just atomically loading of the nodemasks.
> @@ -2447,11 +2503,14 @@ void cpuset_unlock(void) > int cpuset_mem_spread_node(void) > { > int node; > + unsigned long flags; > > + read_mem_lock_irqsave(current, flags); > node = next_node(current->cpuset_mem_spread_rotor, current->mems_allowed); > if (node == MAX_NUMNODES) > node = first_node(current->mems_allowed); > current->cpuset_mem_spread_rotor = node; > + read_mem_unlock_irqrestore(current, flags); > return node; > } > EXPORT_SYMBOL_GPL(cpuset_mem_spread_node);
If you are worried about doing this kind of atomic RMW on the mask, then you cannot make the lock a noop. So if you're nooping the lock in this way then you really need to cuddle it neatly around loading of the mask.
Once you do that, it would be trivial to use a seqlock.
...
> @@ -1381,8 +1434,16 @@ static struct mempolicy *get_vma_policy(struct task_struct *task, > } else if (vma->vm_policy) > pol = vma->vm_policy; > } > + if (!pol) { > + read_mem_lock_irqsave(task, irqflags); > + pol = task->mempolicy; > + mpol_get(pol); > + read_mem_unlock_irqrestore(task, irqflags); > + } > + > if (!pol) > pol = &default_policy; > + > return pol; > }
And a couple of others. It looks like you're using it here to guarantee existence of the mempolicy.... Did you mean read_mempolicy_lock? Or do you have another problem (there seems to be several cases of this).
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