During the v3.20/v4.0 cycle, I had originally had the code manage the
inode->i_flctx pointer using a compare-and-swap operation instead of
the i_lock.
Sasha Levin though hit a problem while testing trinity that made me
believe that that wasn't safe. I now think though that I completely
misread the problem, even though it seemed like it went away when
we started using the i_lock to protect this pointer.
The issue was likely the same race that Kirill Shutemov hit while
testing the pre-rc1 v4.0 kernel and that Linus spotted. Due to the
way that the spinlock was dropped in the middle of flock_lock_file,
you could end up with multiple flock locks for the same struct file
on the inode.
Reinstate the use of a CAS operation to assign this pointer since it's
likely to be more efficient and gets the i_lock completely out of the
file locking business.
Signed-off-by: Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx>
---
fs/locks.c | 9 +--------
1 file changed, 1 insertion(+), 8 deletions(-)
diff --git a/fs/locks.c b/fs/locks.c
index 4347f3dc17cc..22c0785c00ed 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -223,14 +223,7 @@ locks_get_lock_context(struct inode *inode, int type)
* Assign the pointer if it's not already assigned. If it is, then
* free the context we just allocated.
*/
- spin_lock(&inode->i_lock);
- if (likely(!inode->i_flctx)) {
- inode->i_flctx = new;
- new = NULL;
- }
- spin_unlock(&inode->i_lock);
-
- if (new)
+ if (cmpxchg(&inode->i_flctx, NULL, new))
kmem_cache_free(flctx_cache, new);
out:
return inode->i_flctx;
--
2.1.0
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