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Re: [PATCH 6/6] mm: add preempt points into __purge_vmap_area_lazy

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On Tue, 18 Oct 2016 16:56:48 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:


> Is releasing the lock within a llist_for_each_entry_safe() actually safe? Is
> vmap_area_lock the one to protect the valist?
> 
> That is llist_for_each_entry_safe(va, n_va, valist, purg_list) does:
> 
> 	for (va = llist_entry(valist, typeof(*va), purge_list);
> 	     &va->purge_list != NULL &&
> 	     n_va = llist_entry(va->purge_list.next, typeof(*n_va),
> 				purge_list, true);
> 	     pos = n)
> 
> Thus n_va is pointing to the next element to process when we release the
> lock. Is it possible for another task to get into this same path and process
> the item that n_va is pointing to? Then when the preempted task comes back,
> grabs the vmap_area_lock, and then continues the loop with what n_va has,
> could that cause problems? That is, the next iteration after releasing the
> lock does va = n_va. What happens if n_va no longer exits?
> 
> I don't know this code that well, and perhaps vmap_area_lock is not protecting
> the list and this is all fine.
> 

Bah, nevermind. I missed the:

	valist = llist_del_all(&vmap_purge_list);

so yeah, all should be good.

Nothing to see here, move along please.

-- Steve

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[RFC] reduce latency in __purge_vmap_area_lazy Christoph Hellwig <hch@lst.de> - 2016-10-18 09:00 +0200
  [PATCH 5/6] mm: turn vmap_purge_lock into a mutex Christoph Hellwig <hch@lst.de> - 2016-10-18 09:00 +0200
  [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as potentially sleeping Christoph Hellwig <hch@lst.de> - 2016-10-18 09:00 +0200
    Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as  potentially sleeping Chris Wilson <chris@chris-wilson.co.uk> - 2016-10-18 12:40 +0200
      Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as         potentially sleeping Christoph Hellwig <hch@lst.de> - 2016-10-18 12:40 +0200
    Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as         potentially sleeping Christoph Hellwig <hch@lst.de> - 2016-10-19 16:30 +0200
      Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as  potentially sleeping Andy Lutomirski <luto@amacapital.net> - 2016-10-19 17:40 +0200
        Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as         potentially sleeping Christoph Hellwig <hch@lst.de> - 2016-10-19 18:40 +0200
          Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as  potentially sleeping Chris Wilson <chris@chris-wilson.co.uk> - 2016-10-19 21:50 +0200
          Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as  potentially sleeping Joel Fernandes <joelaf@google.com> - 2016-10-21 02:40 +0200
    Re: [PATCH 2/6] mm: mark all calls into the vmalloc subsystem as  potentially sleeping Chris Wilson <chris@chris-wilson.co.uk> - 2016-10-19 16:40 +0200
  [PATCH 1/6] mm: refactor __purge_vmap_area_lazy Christoph Hellwig <hch@lst.de> - 2016-10-18 09:00 +0200
  [PATCH 4/6] mm: remove free_unmap_vmap_area_addr Christoph Hellwig <hch@lst.de> - 2016-10-18 09:00 +0200
    Re: [PATCH 4/6] mm: remove free_unmap_vmap_area_addr Joel Fernandes <joelaf@google.com> - 2016-10-21 02:50 +0200
      Re: [PATCH 4/6] mm: remove free_unmap_vmap_area_addr Nicholas Piggin <npiggin@gmail.com> - 2016-10-21 04:00 +0200
  [PATCH 3/6] mm: remove free_unmap_vmap_area_noflush Christoph Hellwig <hch@lst.de> - 2016-10-18 09:00 +0200
  [PATCH 6/6] mm: add preempt points into __purge_vmap_area_lazy Christoph Hellwig <hch@lst.de> - 2016-10-18 09:00 +0200
    Re: [PATCH 6/6] mm: add preempt points into __purge_vmap_area_lazy Steven Rostedt <rostedt@goodmis.org> - 2016-10-18 23:00 +0200
      Re: [PATCH 6/6] mm: add preempt points into __purge_vmap_area_lazy Steven Rostedt <rostedt@goodmis.org> - 2016-10-18 23:10 +0200
  Re: [RFC] reduce latency in __purge_vmap_area_lazy Nicholas Piggin <npiggin@gmail.com> - 2016-10-18 12:50 +0200
  Re: [RFC] reduce latency in __purge_vmap_area_lazy Jisheng Zhang <jszhang@marvell.com> - 2016-10-18 13:30 +0200
  Re: [RFC] reduce latency in __purge_vmap_area_lazy Joel Fernandes <joelaf@google.com> - 2016-10-21 03:10 +0200

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