linux-ext4 - Re: jbd: a question on __log_wait_for

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Date:	Fri, 21 Aug 2009 15:53:05 +0200
From:	Jan Kara <jack@...e.cz>
To:	홍신 shin hong <hongshin@...il.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: jbd: a question on __log_wait_for_space()

  Hello,

> Hi. I have a question on __log_wait_for_space() in jbd/checkpoint.c
> while I read jbd code of Linux 2.6.30.4
> 
> In __log_wait_for_space(), it releases and re-takes journal->j_state_lock
> in while loop (at line 139).
> 
> After the retaking the lock, it checks 'space_left < nblocks'. It seems that
> it considers that the journal might be written during the lock
> releasing and re-taking.
> 
> Then, is it necessary to check 'journal->j_flags & JFS_ABORT' again too?
> I found that journal_t's j_flags is synchronized via j_state_lock. So, is there
> any possiblity that j_flags value can be changed during the lock
> releasing and re-taking?
  Well, it may happen that JFS_ABORT has been set while we didn't hold
j_state_lock but I don't see a problem in it. We just call a few other
functions and then reiterate the while loop and we exit the loop either
because there is enough space in the journal or becauce we notice the
JFS_ABORT flag.
  The point of JFS_ABORT check is that we didn't loop indefinitely when
the journal gets aborted and no blocks can be freed.

								Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
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