[xen staging] xen/spinlock: move debug helpers inside the locked regions

[patchbot@xxxxxxx]

commit 98bed5de1de3352c63cfe29a00f17e8d9ce72689
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Wed Jul 29 13:13:30 2020 +0200
Commit:     Julien Grall <jgrall@xxxxxxxxxx>
CommitDate: Thu Jul 30 19:27:08 2020 +0100

    xen/spinlock: move debug helpers inside the locked regions
    
    Debug helpers such as lock profiling or the invariant pCPU assertions
    must strictly be performed inside the exclusive locked region, or else
    races might happen.
    
    Note the issue was not strictly introduced by the pointed commit in
    the Fixes tag, since lock stats where already incremented before the
    barrier, but that commit made it more apparent as manipulating the cpu
    field could happen outside of the locked regions and thus trigger the
    BUG_ON on rel_lock(). This is only enabled on debug builds, and thus
    releases are not affected.
    
    Fixes: 80cba391a35 ('spinlocks: in debug builds store cpu holding the lock')
    Reported-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 xen/common/spinlock.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
index 17f4519fc7..ce3106e2d3 100644
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -170,9 +170,9 @@ void inline _spin_lock_cb(spinlock_t *lock, void (*cb)(void 
*), void *data)
             cb(data);
         arch_lock_relax();
     }
+    arch_lock_acquire_barrier();
     got_lock(&lock->debug);
     LOCK_PROFILE_GOT;
-    arch_lock_acquire_barrier();
 }
 
 void _spin_lock(spinlock_t *lock)
@@ -198,9 +198,9 @@ unsigned long _spin_lock_irqsave(spinlock_t *lock)
 
 void _spin_unlock(spinlock_t *lock)
 {
-    arch_lock_release_barrier();
     LOCK_PROFILE_REL;
     rel_lock(&lock->debug);
+    arch_lock_release_barrier();
     add_sized(&lock->tickets.head, 1);
     arch_lock_signal();
     preempt_enable();
@@ -249,15 +249,15 @@ int _spin_trylock(spinlock_t *lock)
         preempt_enable();
         return 0;
     }
+    /*
+     * cmpxchg() is a full barrier so no need for an
+     * arch_lock_acquire_barrier().
+     */
     got_lock(&lock->debug);
 #ifdef CONFIG_DEBUG_LOCK_PROFILE
     if (lock->profile)
         lock->profile->time_locked = NOW();
 #endif
-    /*
-     * cmpxchg() is a full barrier so no need for an
-     * arch_lock_acquire_barrier().
-     */
     return 1;
 }
 
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