Messages in this thread | | | Date | Thu, 7 May 2015 13:39:35 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH] VFS: Add back check for !inode in walk_component() |
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On Thu, 7 May 2015 18:28:34 +0100 Al Viro <viro@ZenIV.linux.org.uk> 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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