[PATCH] VMCI: Fixup atomic64_t abuse

Peter Zijlstra
2019-06-06T09:34:37Z

The VMCI driver is abusing atomic64_t and atomic_t, there is no actual
atomic RmW operations around.

Rewrite the code to use a regular u64 with READ_ONCE() and
WRITE_ONCE() and a cast to 'unsigned long'. This fully preserves
whatever broken there was (it's not endian-safe for starters, and also
looks to be missing ordering).

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/vmw_vmci_defs.h | 30 ++++++++++--------------------
1 file changed, 10 insertions(+), 20 deletions(-)

--- a/include/linux/vmw_vmci_defs.h
+++ b/include/linux/vmw_vmci_defs.h
@@ -438,8 +438,8 @@ enum {
struct vmci_queue_header {
/* All fields are 64bit and aligned. */
struct vmci_handle handle; /* Identifier. */
- atomic64_t producer_tail; /* Offset in this queue. */
- atomic64_t consumer_head; /* Offset in peer queue. */
+ u64 producer_tail; /* Offset in this queue. */
+ u64 consumer_head; /* Offset in peer queue. */
};

/*
@@ -740,13 +740,9 @@ static inline void *vmci_event_data_payl
* prefix will be used, so correctness isn't an issue, but using a
* 64bit operation still adds unnecessary overhead.
*/
-static inline u64 vmci_q_read_pointer(atomic64_t *var)
+static inline u64 vmci_q_read_pointer(u64 *var)
{
-#if defined(CONFIG_X86_32)
- return atomic_read((atomic_t *)var);
-#else
- return atomic64_read(var);
-#endif
+ return READ_ONCE(*(unsigned long *)var);
}

/*
@@ -755,23 +751,17 @@ static inline u64 vmci_q_read_pointer(at
* never exceeds a 32bit value in this case. On 32bit SMP, using a
* locked cmpxchg8b adds unnecessary overhead.
*/
-static inline void vmci_q_set_pointer(atomic64_t *var,
- u64 new_val)
+static inline void vmci_q_set_pointer(u64 *var, u64 new_val)
{
-#if defined(CONFIG_X86_32)
- return atomic_set((atomic_t *)var, (u32)new_val);
-#else
- return atomic64_set(var, new_val);
-#endif
+ /* XXX buggered on big-endian */
+ WRITE_ONCE(*(unsigned long *)var, (unsigned long)new_val);
}

/*
* Helper to add a given offset to a head or tail pointer. Wraps the
* value of the pointer around the max size of the queue.
*/
-static inline void vmci_qp_add_pointer(atomic64_t *var,
- size_t add,
- u64 size)
+static inline void vmci_qp_add_pointer(u64 *var, size_t add, u64 size)
{
u64 new_val = vmci_q_read_pointer(var);

@@ -848,8 +838,8 @@ static inline void vmci_q_header_init(st
const struct vmci_handle handle)
{
q_header->handle = handle;
- atomic64_set(&q_header->producer_tail, 0);
- atomic64_set(&q_header->consumer_head, 0);
+ q_header->producer_tail = 0;
+ q_header->consumer_head = 0;
}

/*