On Thu, 7 May 2015 18:28:34 +0100
Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, May 07, 2015 at 12:52:41PM -0400, Steven Rostedt wrote:
> >
> > Commit 698934df8b45 "VFS: Combine inode checks with d_is_negative() and
> > d_is_positive() in pathwalk" removed a check for inode being NULL in
> > walk_component() where the type is tested. Stressing my tracefs create
> > and remove instances while reading the files now triggers this:
>
> So you get NULL ->d_inode with stale flags? The thing is, ->d_inode
> becoming NULL should happen via d_delete(), which goes throug this:
But it's not the delete, it's the creation of a new d_entry. Pehaps it
gets linked premature? The tracing I had happened in
tracefs_create_file().
> unsigned flags = READ_ONCE(dentry->d_flags);
>
> flags &= ~(DCACHE_ENTRY_TYPE | DCACHE_FALLTHRU);
> WRITE_ONCE(dentry->d_flags, flags);
> smp_wmb();
> dentry->d_inode = NULL;
>
> and after that assignment to ->d_flags you'll see d_is_negative() being
> true. OTOH, we have
> *inode = dentry->d_inode;
> if (read_seqcount_retry(&dentry->d_seq, seq))
> in lookup_fast(), and read_seqcount_retry() is
> {
> smp_rmb();
> return __read_seqcount_retry(s, start);
> }
>
> IOW, we have smp_rmb() between fetching ->d_inode and checking ->d_flags.
>
> If you can reproduce that at will, could you make it dump nd->flags along with
> dentry involved?
I had them printed in my previous traces. The flags were 0x200088, and
they were 0 just before the call.
ftrace-test-mki-3201 [000] 85.306761: bprint: walk_component: inode=(nil) dentry=0xee3c0240 neg:1
ftrace-test-mki-3201 [000] 85.306761: bprint: walk_component: inode=(nil) dentry=0xee3c0240 neg:0 flags:200088
That was with this:
trace_printk("inode=%p dentry=%p neg:%d\n", inode, path->dentry,
d_is_negative(path->dentry));
err = -ENOENT;
if (d_is_negative(path->dentry))
goto out_path_put;
trace_printk("inode=%p dentry=%p neg:%d flags:%lx\n", inode, path->dentry,
d_is_negative(path->dentry), (long)path->dentry->d_flags);
-- Steve
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