[PATCH v2.1 15/15] xen: tmem: tmem_try_to_evict_pgp: fix a lock issue

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

From: Bob Liu <bob.liu@oracle.com>

During xen testing, below failure was triggered if dedup=0.

(XEN) Assertion '!preempt_count()' failed at preempt.c:37
(XEN) ----[ Xen-4.5-unstable  x86_64  debug=y  Not tainted ]----
(XEN) CPU:    51
(XEN) RIP:    e008:[<ffff82d08011bfef>] ASSERT_NOT_IN_ATOMIC+0x22/0x53
(XEN) RFLAGS: 0000000000010286   CONTEXT: hypervisor
(XEN) rax: ffff82d080318d20   rbx: ffff8300681ea000   rcx: 0000000000000001
(XEN) rdx: 00000033bca03300   rsi: ffff8308110da000   rdi: ffff82d080286690
(XEN) rbp: ffff83043cd0ff08   rsp: ffff83043cd0ff08   r8:  ffff8307d2beecb0
(XEN) r9:  000000000000000d   r10: 00000000deadbeef   r11: 0000000000000202
(XEN) r12: 0000000000000000   r13: 0000000000000000   r14: 0000000000000005
(XEN) r15: 0000000000000001   cr0: 0000000080050033   cr4: 00000000001526f0
(XEN) cr3: 000000005246d000   cr2: ffff880106123418
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e010   cs: e008
(XEN) Xen stack trace from rsp=ffff83043cd0ff08:
(XEN)    00007cfbc32f00c7 ffff82d0802258f0 ffff880106123418 ffffea0006156e80
(XEN)    ffff8800d0ab5368 00007faff4c83000 ffff8801bdea33e8 0000000000000002
(XEN)    0000000000000202 00000000deadbeef 0000000000000000 00000000000c3565
(XEN)    fffffffffffffff4 ffffffff810014ca ffffffff81de1000 000000000000c356
(XEN)    00000000deadbeef 0001010000000000 ffffffff810014ca 000000000000e033
(XEN)    0000000000000202 ffff8801bdea3360 000000000000e02b 000000000000beef
(XEN)    000000000000beef 000000000000beef 000000000000beef 0000000000000033
(XEN)    ffff8300681ea000 00000033bca03300 0000000000000000
(XEN) Xen call trace:
(XEN)    [<ffff82d08011bfef>] ASSERT_NOT_IN_ATOMIC+0x22/0x53
(XEN)    [<ffff82d0802258f0>] test_all_events+0x6/0x30

The root cause is there is an wronng
'write_unlock(&pcd_tree_rwlocks[firstbyte])' in function
tmem_try_to_evict_pgp().

Nobody will lock &pcd_tree_rwlocks if dedup=0, but the write_unlock() will be
executed anyway. This was introduced by a git commit
38c433d0c711406778aba1ae183a195da98656f0 ("tmem: add page deduplication with
optional compression or trailing-zero-elimination")

Signed-off-by: Bob Liu <bob.liu@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
 xen/common/tmem.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/common/tmem.c b/xen/common/tmem.c
index 66870a0..f7f828f 100644
--- a/xen/common/tmem.c
+++ b/xen/common/tmem.c
@@ -1263,7 +1263,8 @@ static bool_t tmem_try_to_evict_pgp(struct tmem_page_descriptor *pgp, bool_t *ho
             return 1;
         }
 pcd_unlock:
-        write_unlock(&pcd_tree_rwlocks[firstbyte]);
+        if ( tmem_dedup_enabled() )
+            write_unlock(&pcd_tree_rwlocks[firstbyte]);
 obj_unlock:
         spin_unlock(&obj->obj_spinlock);
     }
-- 
1.8.5.3