[Xen-changelog] [xen-unstable] Fix recursive lock p2m lock acquisition in POD code

[Xen patchbot-unstable <patchbot-unstable@xxxxxxxxxxxxxxxxxxx>]

# HG changeset patch
# User Keir Fraser <keir.fraser@xxxxxxxxxx>
# Date 1254396573 -3600
# Node ID fd3d5d66c446b3216e5e6180efd1f01db9a1b47c
# Parent  bc9f7c6ac6954d517c9988575d9ae5d5e9c8448f
Fix recursive lock p2m lock acquisition in POD code

The POD code can take the p2m lock from inside a lookup.  This causes
a crash if anyone calls gfn_to_mfn* with the p2m lock held, which is
quite a few places.  Make the POD code understand that it may be
called with the lock held, and DTRT about talking or releasing it.

Signed-off-by: Tim Deegan <Tim.Deegan@xxxxxxxxxx>
---
 xen/arch/x86/mm/p2m.c |   24 ++++++++++++++++--------
 1 files changed, 16 insertions(+), 8 deletions(-)

diff -r bc9f7c6ac695 -r fd3d5d66c446 xen/arch/x86/mm/p2m.c
--- a/xen/arch/x86/mm/p2m.c     Thu Oct 01 12:28:54 2009 +0100
+++ b/xen/arch/x86/mm/p2m.c     Thu Oct 01 12:29:33 2009 +0100
@@ -1067,21 +1067,29 @@ static int p2m_pod_check_and_populate(st
                                       l1_pgentry_t *p2m_entry, int order,
                                       p2m_query_t q)
 {
+    /* Only take the lock if we don't already have it.  Otherwise it
+     * wouldn't be safe to do p2m lookups with the p2m lock held */
+    int do_locking = !p2m_locked_by_me(d->arch.p2m);
     int r;
-    p2m_lock(d->arch.p2m);
+
+    if ( do_locking )
+        p2m_lock(d->arch.p2m);
+
     audit_p2m(d);
 
     /* Check to make sure this is still PoD */
     if ( p2m_flags_to_type(l1e_get_flags(*p2m_entry)) != 
p2m_populate_on_demand )
     {
+        if ( do_locking )
+            p2m_unlock(d->arch.p2m);
+        return 0;
+    }
+
+    r = p2m_pod_demand_populate(d, gfn, order, q);
+
+    audit_p2m(d);
+    if ( do_locking )
         p2m_unlock(d->arch.p2m);
-        return 0;
-    }
-
-    r = p2m_pod_demand_populate(d, gfn, order, q);
-
-    audit_p2m(d);
-    p2m_unlock(d->arch.p2m);
 
     return r;
 }

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