From: Jeff Layton Date: Fri, 3 Apr 2015 13:04:04 +0000 (-0400) Subject: locks: use cmpxchg to assign i_flctx pointer X-Git-Tag: Ubuntu-snapdragon-4.4.0-1029.32~6791^2 X-Git-Url: https://git.proxmox.com/?a=commitdiff_plain;h=0429c2b5c1c4c8ba6cd563c1964baf3ed238df26;p=mirror_ubuntu-zesty-kernel.git locks: use cmpxchg to assign i_flctx pointer 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 with trinity that made me believe that that wasn't safe. At the time, changing the code to protect the i_flctx pointer seemed to fix the issue, but I now think that was just coincidence. 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 --- diff --git a/fs/locks.c b/fs/locks.c index 16cae1a00851..52b780fb5258 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;