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Date:   Thu, 6 Dec 2018 15:22:01 +0100
From:   Alexander Lochmann <>
To:     Andreas Dilger <>
Cc:     Ext4 Developers List <>,
        Jan Kara <>,
        Horst Schirmeier <>,
        Al Viro <>
Subject: Re: RFC: LockDoc - Detecting Locking Bugs in the Linux Kernel

Am 28.11.18 um 21:34 schrieb Andreas Dilger:
> On Nov 27, 2018, at 3:19 PM, Alexander Lochmann <> wrote:
>> Hi folks,
>> during the past months we've been developing LockDoc, a trace-based
>> approach of Lock Analysis in the Linux Kernel.
>> LockDoc uses execution traces of an instrumented Linux Kernel to
>> automatically deduce
>> locking rules for all members of arbitrary kernel data structures.
>> The traces are gathered running a manually selected fs-specific subset
>> of the Linux Test Project in a virtual machine.
>> These locking rules can be used to generate a comprehensive locking
>> documentation and to reveal potential bugs.
> This is quite interesting, and looks useful provided that there is a
> workload that exercises the various codepaths.  How long does such an
> analysis take to run, and is there a plan to make this functionality
> available to developers (e.g. either as a web portal, or to be able
> to download the code and run it locally)?
It takes ~34 min to gather the trace, and another 4,5h to process it.
Yes, we intend to release our project. However, we do not have a
schedule yet.
>> LockDoc generates rules for each tuple of (data structure, member, {r,w}).
>> It completely ignores any element of type atomic{,64,long}_t as well as
>> atomic_*() functions.
>> Accesses during initialization and destruction of objects are also ignored.
>> The output of LockDoc looks like this:
>> inode member: i_flags [w] (15 lock combinations)
>>  hypotheses: 96
>>     15.8% (88 out of 558 mem accesses): EMBOTHER(inode.i_rwsem[w]) ->
>> EMBSAME(inode.i_rwsem[w])
>> inode w:i_flags CEX SEQ
>> 'EMBOTHER(inode.i_rwsem[w])' 'EMBSAME(inode.i_rwsem[w])'
>>     15.8% (88 out of 558 mem accesses): EMBOTHER(inode.i_rwsem[w])
>> inode w:i_flags CEX SEQ
>> 'EMBOTHER(inode.i_rwsem[w])'
>> !    99.8% (557 out of 558 mem accesses): EMBSAME(inode.i_rwsem[w])
>> ! inode w:i_flags CEX SEQ
>> 'EMBSAME(inode.i_rwsem[w])'
>>      100% (558 out of 558 mem accesses): (no locks held)
>>    (no counterexamples to be expected, this hypothesis has 100% support
>> in the observation set)
>> In this example LockDoc concludes that the lock
>> "EMBSAME(inode.i_rwsem[w])" is necessary for writing inode.i_flags.
>> EMBSAME stands for the lock embedded in the inode being accessed. In
>> this case it is the i_rwsem.
>> To be more precise, the write lock (--> "[w]") of i_rwsem is needed.
>> Based on this methodology, we can determine code locations that do not
>> adhere to the deduced locking rules.
>> The reports on rule-violating code include the stack trace and the
>> actual locks held.
> Looking at the page, it isn't very clear where some of the callpaths go.
> For example, in the "writing inode:ext4.i_nlink-__i_nlink" case, it
> shows a callpath from vfs_symlink() calling drop_nlink(), but this is not
> called directly from vfs_symlink().  It is actually going through
> dir->i_op->symlink() (ext4_symlink() in our case), so it would be best
> to show that dependency.
Thanks for pointing us to a GCC bug. :) Since drop_nlink() is a leaf
function, GCC 8.2 does not generate the frame pointer code - even with
CONFIG_FRAME_POINTER set. Using GCC 7.3 "fixes" this.
(Disable -regparm=3 would also "fix" this issue with GCC 8.2. Since this
concerns the kernel abi, this is not an option...)
> One minor bug in ext4_symlink() is that it should probably be calling
> clear_nlink() to make it clear the link count should be zero, instead
> of drop_nlink(), because we don't really want to trigger "remove_count"
> and because the later part of this function is using set_nlink(inode, 1)
> instead of inc_nlink() that decrements "remove_count" again.  This does
> not solve the reported warning, however.
> In ext4_orphan_add(), called immediately after drop_nlink(), it checks:
> 	WARN_ON_ONCE(!(inode->i_state & (I_NEW | I_FREEING)) &&
> 		     !inode_is_locked(inode));
> so this is the case where i_state has I_NEW set.  The question is whether
> it is worthwhile to grab inode_lock() in this code, just to keep the lock
> checker happy, or whether the code can be annotated to tell the checker
> that I_NEW means i_rwsem is not needed.
Who can help us here?
But it basically is a bug?

> Cheers, Andreas
>> We've now created a series of bug reports for the following data types:
>> struct inode (used by ext4), journal_t, and transaction_t.
>> We present the counterexamples for each tuple of (data structure,
>> member, {r,w}).
>> Depending on the complexity of the callgraph, the counterexamples are
>> either embedded in the callgraph or the callgraph is shown below them.
>> In the latter case, zooming can be enabled via a button in the heading.
>> We kindly ask you to have a look at our findings and send us some
>> comments back:
>> Our approach has already revealed one real bug and one suspicious
>> situation. Both have been confirmed by Jan.
>> Best regards,
>> Alex and Horst
>> --
>> Technische Universit├Ąt Dortmund
>> Alexander Lochmann                PGP key: 0xBC3EF6FD
>> Otto-Hahn-Str. 16                 phone:  +49.231.7556141
>> D-44227 Dortmund                  fax:    +49.231.7556116
> Cheers, Andreas

Technische Universit├Ąt Dortmund
Alexander Lochmann                PGP key: 0xBC3EF6FD
Otto-Hahn-Str. 16                 phone:  +49.231.7556141
D-44227 Dortmund                  fax:    +49.231.7556116

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