[lkml]   [2010]   [Aug]   [27]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] mm: fix hang on anon_vma->root->lock
On Fri, Aug 27, 2010 at 10:13 AM, Christoph Lameter <> wrote:

> The basic problem with SLAB_DESTROY_BY_RCU is that you get a reference to
> an object that is guaranteed only to have the same type (the instance may
> fluctuate and be replaced from under you unless other measures are taken).

(I wouldn't describe that as a "problem with SLAB_DESTROY_BY_RCU":
it's precisely the nature of SLAB_DESTROY_BY_RCU, what makes it useful
in solving backward-locking problems elsewhere.)

> Typically one must take a lock within the memory structure to pin down
> the object (or take a refcount). Only then can you follow pointers and
> such. It is only possible to verify that the right object has been
> reached *after* locking. Following a pointer without having determined
> that we hit the right object should not occur.
> A solution here would be to take the anon_vma->lock (prevents the object
> switching under us) and then verify that the mapping is the one we are
> looking for and that the pointer points to the right root. Then take the
> root lock.
> Hughs solution takes a global spinlock which will limit scalability.

Eh? My solution was a second page_mapped(page) test i.e. testing an atomic.


 \ /
  Last update: 2010-08-27 19:57    [W:0.058 / U:6.088 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site