From: William Lee Irwin III <wli@holomorphy.com>
Document the requirement to use a memory barrier prior to wake_up_bit().
Signed-off-by: Andrew Morton <akpm@osdl.org>
---
25-akpm/kernel/wait.c | 7 +++++++
1 files changed, 7 insertions(+)
diff -puN kernel/wait.c~document-wake_up_bits-requirement-for-preceding-memory-barriers kernel/wait.c
--- 25/kernel/wait.c~document-wake_up_bits-requirement-for-preceding-memory-barriers Tue Aug 31 14:49:26 2004
+++ 25-akpm/kernel/wait.c Tue Aug 31 14:49:26 2004
@@ -219,6 +219,13 @@ EXPORT_SYMBOL(__wake_up_bit);
* is the part of the hashtable's accessor API that wakes up waiters
* on a bit. For instance, if one were to have waiters on a bitflag,
* one would call wake_up_bit() after clearing the bit.
+ *
+ * In order for this to function properly, as it uses waitqueue_active()
+ * internally, some kind of memory barrier must be done prior to calling
+ * this. Typically, this will be smp_mb__after_clear_bit(), but in some
+ * cases where bitflags are manipulated non-atomically under a lock, one
+ * may need to use a less regular barrier, such fs/inode.c's smp_mb(),
+ * because spin_unlock() does not guarantee a memory barrier.
*/
void fastcall wake_up_bit(void *word, int bit)
{
_