linux-ext4 - Re: [RFCv1 01/72] e2fsck: Fix unbalanced mutex unlock for BOUNCE

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Message-Id: <1BEDD834-2D4A-4E8E-936C-90DB5E322F9C@dilger.ca>
Date:   Fri, 18 Nov 2022 07:20:55 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     "Ritesh Harjani (IBM)" <ritesh.list@...il.com>
Cc:     Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        Harshad Shirwadkar <harshadshirwadkar@...il.com>,
        Wang Shilong <wshilong@....com>,
        Andreas Dilger <adilger.kernel@...ger.ca>, Li Xi <lixi@....com>
Subject: Re: [RFCv1 01/72] e2fsck: Fix unbalanced mutex unlock for BOUNCE_MTX

On Nov 18, 2022, at 05:37, Ritesh Harjani (IBM) <ritesh.list@...il.com> wrote:
> 
> On 22/11/18 04:34AM, Andreas Dilger wrote:
>>> On Nov 7, 2022, at 06:22, Ritesh Harjani (IBM) <ritesh.list@...il.com> wrote:
>>> 
>>> f_crashdisk test failed with UNIX_IO_FORCE_BOUNCE=yes due to unbalanced
>>> mutex unlock in below path.
>>> 
>>> This patch fixes it.
>>> 
>>> Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
>>> ---
>>> lib/ext2fs/unix_io.c | 1 -
>>> 1 file changed, 1 deletion(-)
>>> 
>>> diff --git a/lib/ext2fs/unix_io.c b/lib/ext2fs/unix_io.c
>>> index e53db333..5b894826 100644
>>> --- a/lib/ext2fs/unix_io.c
>>> +++ b/lib/ext2fs/unix_io.c
>>> @@ -305,7 +305,6 @@ bounce_read:
>>>   while (size > 0) {
>>>       actual = read(data->dev, data->bounce, align_size);
>>>       if (actual != align_size) {
>>> -            mutex_unlock(data, BOUNCE_MTX);
>> 
>> This patch doesn't show enough context, but AFAIK this is jumping before mutex_down()
>> is called, so this *should* be correct as is?
> 
> Thanks for the review, Andreas.
> 
> Yeah, the patch diff above is not sufficient since it doesn't share enuf
> context.
> But essentially when "actual" is not equal to "align_size", then in this if
> condition it goes to label "short_read:", which always goto error_unlock,
> where we anyways call mutex_unlock()
> 
> Looking at a lot of labels in this function, this definitely looks like 
> something which can be cleaned up ("raw_read_blk()"). 
> I will add that to my list of todos. 

You are correct, and it means this code is just not very clear to the reader. I think it
would make more sense to move the "short_read:" label to the end of the code:

                  actual = read(...);
                  if (actual != size)
                          goto error_short_read;
                  goto success_unlock;
        :
                 actual = read(...);
                 if (actual != align_size) {
                           actual = really_read;
                           buf -= really_read;
                           size += really_read;
                           goto error_short_read;
                 }
        :
success_unlock:
        mutex_unlock(...);
        return 0;

error_short_read:
        if (actual < 0) {
                 retval = errno;
                 actual = 0;
        } else {
                 retval = EXT2_ET_SHORT_READ;
        }
error_unlock:
        mutex_unlock(...);

That way the code follows the normal error handling convention and is less likely to be
surprising to the reader. 

Cheers, Andreas