Xen devel - Dec 2010 - {PATCH] Add p2m_lock in set_shared_p2m_entry

Without the lock, set_shared_p2m_entry() will BUG when xen is compiled with
debug=y

Singed-off-by: Jui-Hao Chiang <juihaochiang <at> gmail.com>

--- a/xen/arch/x86/mm/p2m.c    Tue Dec 21 18:10:46 2010 +0000
+++ b/xen/arch/x86/mm/p2m.c    Thu Dec 23 16:40:41 2010 +0800
@@ -2665,7 +2665,9 @@ set_shared_p2m_entry(struct p2m_domain *
     set_gpfn_from_mfn(mfn_x(omfn), INVALID_M2P_ENTRY);

     P2M_DEBUG("set shared %lx %lx\n", gfn, mfn_x(mfn));
+    p2m_lock(p2m);
     rc = set_p2m_entry(p2m, gfn, mfn, 0, p2m_ram_shared);
+    p2m_unlock(p2m);
     if ( 0 == rc )
         gdprintk(XENLOG_ERR,
             "set_mmio_p2m_entry: set_p2m_entry failed! mfn=%08lx\n"


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At 00:45 +0000 on 23 Dec (1293065108), Jui-Hao Chiang
wrote:
> Without the lock, set_shared_p2m_entry() will BUG when xen is compiled with
debug=y
Thanks.  I''ll have to make a slightly more complex fix, as
set_shared_p2m_entry() can be called (via p2m_teardown() and
mem_sharing_unshare_page()) with the p2m lock already held.  :(

That''s a sign of a deeper illness which will probably lead to deadlocks
elsewhere.  I intend to give the MM locking (p2m, shadow, hap, log-dirty
and page-sharing locks) a good kicking some time soon, maybe after 4.1
is released.

Cheers,

Tim.
> Singed-off-by: Jui-Hao Chiang <juihaochiang <at>
gmail.com<http://gmail.com>>
> 
> --- a/xen/arch/x86/mm/p2m.c    Tue Dec 21 18:10:46 2010 +0000
> +++ b/xen/arch/x86/mm/p2m.c    Thu Dec 23 16:40:41 2010 +0800
> @@ -2665,7 +2665,9 @@ set_shared_p2m_entry(struct p2m_domain *
>      set_gpfn_from_mfn(mfn_x(omfn), INVALID_M2P_ENTRY);
> 
>      P2M_DEBUG("set shared %lx %lx\n", gfn, mfn_x(mfn));
> +    p2m_lock(p2m);
>      rc = set_p2m_entry(p2m, gfn, mfn, 0, p2m_ram_shared);
> +    p2m_unlock(p2m);
>      if ( 0 == rc )
>          gdprintk(XENLOG_ERR,
>              "set_mmio_p2m_entry: set_p2m_entry failed!
mfn=%08lx\n"
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> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

-- 
Tim Deegan <Tim.Deegan@citrix.com>
Principal Software Engineer, Xen Platform Team
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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Hi, Tim

Thanks.  I''ll have to make a slightly more complex fix,
as
> set_shared_p2m_entry() can be called (via p2m_teardown() and
> mem_sharing_unshare_page()) with the p2m lock already held.  :(
>
>
Actually our original problem is the mem_sharing_share_pages() doesn''t
hold
the p2m lock when calling set_shared_p2m_entry(), which causes the share
operation to die.


> That''s a sign of a deeper illness which will probably lead to
deadlocks
> elsewhere.  I intend to give the MM locking (p2m, shadow, hap, log-dirty
> and page-sharing locks) a good kicking some time soon, maybe after 4.1
> is released.
>
>
Just saw your patch
http://xenbits.xensource.com/xen-unstable.hg?rev/a8d69de8eb31.

Thanks,
Jui-Hao


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