Re: [Bisected] commit 71574865 (vfs: do_last(): common slow lookup) breaks CUPS printing

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Jul 30, 2012 at 09:10:23AM +0200, Markus Trippelsdorf wrote:
> 
> Looks like you're right. The first warning happens during startup. The last one
> when I print a test page (which now succeeds).  
> Thanks Al.

> WTF: open("/dev/input/mice", 34946)
> WTF: open("/dev/usblp0", 32898)
> WTF: open("/dev/usb/lp0", 32898)

Ahhh...  OK, yes - it's the case we had missed (and where the manpage
needs correction, BTW).  O_EXCL for *devices* has additional semantics;
it's not "fail if exists", it's "fail if already opened by somebody".
No need to pester CUPS folks (except that I really hope that this
open of /dev/input/mice does *not* come from them)...

All right, the proper fix is *not* removing O_EXCL from flags; we want
it to reach ->f_flags, so that device open would work correctly.  I think
we need this, but I wonder if that's all; ->atomic_open() instances
might need to be corrected as well.  I've pushed this into for-next/for-linus
for now:

commit f8310c59201b183ebee2e3fe0c7242f5729be0af
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Mon Jul 30 11:50:30 2012 +0400

    fix O_EXCL handling for devices
    
    O_EXCL without O_CREAT has different semantics; it's "fail if already opened",
    not "fail if already exists".  commit 71574865 broke that...
    
    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namei.c b/fs/namei.c
index 618d353..e133bf3 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2418,7 +2418,7 @@ static int atomic_open(struct nameidata *nd, struct dentry *dentry,
 	if ((open_flag & O_CREAT) && !IS_POSIXACL(dir))
 		mode &= ~current_umask();
 
-	if (open_flag & O_EXCL) {
+	if ((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT)) {
 		open_flag &= ~O_TRUNC;
 		*opened |= FILE_CREATED;
 	}
@@ -2742,7 +2742,7 @@ retry_lookup:
 	}
 
 	error = -EEXIST;
-	if (open_flag & O_EXCL)
+	if ((open_flag & (O_EXCL | O_CREAT)) == (O_EXCL | O_CREAT))
 		goto exit_dput;
 
 	error = follow_managed(path, nd->flags);