Re: [BUG] fs: ext4: possible ABBA deadlock in ext4_inline_data_truncate() and ext4_punch_hole()

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On 2021/12/11 2:05, Theodore Y. Ts'o wrote:
On Fri, Dec 10, 2021 at 10:03:37AM +0800, Jia-Ju Bai wrote:
Thank you very much for the detailed explanation!
I will improve my static analysis tool for this point.
I'm not sure it will be possible to programatically detect why the
ABBA deadlock isn't possible without having the static analyzer having
a semantic understanding how the code works (so it can understand that
that code path which leads to the ABBA deadlock won't get executed).

It may very well be that being able to understand why the ABBA
deadlock can't happen in that case is equivalent to solving the
halting problem.  But if you do come up with a clever way of improving
your static analysis tool, I'll be excited to see it!

Hi Ted,

Thanks a lot for your advice!
According to your last message, ext4_punch_hole() and ext4_inline_data_truncate() both call ext4_has_inline_data() to check whether the inode has inline data. In ext4_inline_data_truncate(), when ext4_has_inline_data() returns zero, the function returns. In ext4_punch_hole(), when ext4_has_inline_data() returns zero, the function continues. Thus, I think I can add such "concurrency" path conditions in my tool to filter out false positives, by assuming that the same function calls or data structure fields should return/store the same value in concurrency code paths without race conditions.

In fact, my tool can validate path conditions of each sequential code path. I can find ways to validate "concurrency" path conditions in my tool :)


Best wishes,
Jia-Ju Bai



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