linux-kernel - Re: [PATCH 00/17] fs, btrfs refcount conversions

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Date:   Thu, 9 Mar 2017 16:29:29 +0100
From:   David Sterba <dsterba@...e.cz>
To:     Qu Wenruo <quwenruo@...fujitsu.com>
Cc:     "Reshetova, Elena" <elena.reshetova@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jbacik@...com" <jbacik@...com>, "clm@...com" <clm@...com>,
        "dsterba@...e.com" <dsterba@...e.com>
Subject: Re: [PATCH 00/17] fs, btrfs refcount conversions

On Tue, Mar 07, 2017 at 03:49:52PM +0800, Qu Wenruo wrote:
> >>> If the patches pass all tests on your side, could you please take them in and
> >> propagate further?
> >>> I will continue with other kernel subsystems.
> >>
> >> The patchset itself looks like a common cleanup, while I did encounter
> >> several cases (almost all scrub tests) causing kernel warning due to
> >> underflow.
> >
> > Oh, could you please send me the warning outputs? I can hopefully analyze and fix them.
> 
> Attached. Which is the generated by running btrfs/070 test case.
> And I canceled the case almost instantly, so output is not much, but 
> still contains enough info.
> 
> Both refcount_inc() and refcount_sub_and_test() are causing warning.
> 
> So now I'm not sure which is the cause, btrfs or bad use of refcount?

We we do atomic_inc to get the first reference after initialization in
scrub_pages, instead of atomic_set (or an equivalent):

2266                 spage = kzalloc(sizeof(*spage), GFP_KERNEL);
2267                 if (!spage) {
...
2274                 }
...
2276                 scrub_page_get(spage);

so the references are 0 and refcount_inc will catch that, the fix is simple.

The refcount_sub_and_test reports seem to catch a bug in refcounting, I'm
analyzing it right now.