From: Nick Piggin <npiggin@kernel.dk>
Subject: Re: [patch] fs: fix deadlocks in writeback_if_idle
Date: Tue, 23 Nov 2010 21:54:32 +1100
Message-ID: <20101123105432.GA4483@amd>
References: <20101123100239.GA4232@amd>
 <4CEB96D7.80105@panasas.com>
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Cc: Nick Piggin <npiggin@kernel.dk>, linux-fsdevel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Al Viro <viro@ZenIV.linux.org.uk>, linux-ext4@vger.kernel.org,
	linux-btrfs@vger.kernel.org, Jan Kara <jack@suse.cz>,
	Eric Sandeen <sandeen@redhat.com>,
	Theodore Ts'o <tytso@mit.edu>
To: Boaz Harrosh <bharrosh@panasas.com>
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On Tue, Nov 23, 2010 at 12:26:31PM +0200, Boaz Harrosh wrote:
> >   *
> >   * Invoke writeback_inodes_sb if no writeback is currently underway.
> >   * Returns 1 if writeback was started, 0 if not.
> > + *
> > + * Even if 1 is returned, writeback may not be started if memory allocation
> > + * fails. This function makes no guarantees about anything.
> >   */
> >  int writeback_inodes_sb_if_idle(struct super_block *sb)
> >  {
> >  	if (!writeback_in_progress(sb->s_bdi)) {
> > -		down_read(&sb->s_umount);
> > -		writeback_inodes_sb(sb);
> > -		up_read(&sb->s_umount);
> > +		bdi_start_writeback(sb->s_bdi, get_nr_dirty_pages());
> >  		return 1;
> > -	} else
> > -		return 0;
> > +	}
> > +	return 0;
> >  }
> >  EXPORT_SYMBOL(writeback_inodes_sb_if_idle);
> >  
> > @@ -1172,17 +1173,18 @@ EXPORT_SYMBOL(writeback_inodes_sb_if_idl
> >   *
> >   * Invoke writeback_inodes_sb if no writeback is currently underway.
> >   * Returns 1 if writeback was started, 0 if not.
> > + *
> > + * Even if 1 is returned, writeback may not be started if memory allocation
> > + * fails. This function makes no guarantees about anything.
> >   */
> >  int writeback_inodes_sb_nr_if_idle(struct super_block *sb,
> >  				   unsigned long nr)
> >  {
> >  	if (!writeback_in_progress(sb->s_bdi)) {
> > -		down_read(&sb->s_umount);
> > -		writeback_inodes_sb_nr(sb, nr);
> > -		up_read(&sb->s_umount);
> > +		bdi_start_writeback(sb->s_bdi, nr);
> >  		return 1;
> > -	} else
> > -		return 0;
> > +	}
> > +	return 0;
> >  }
> >  EXPORT_SYMBOL(writeback_inodes_sb_nr_if_idle);
> >  
> 
> static inline int writeback_inodes_sb_if_idle(struct super_block *sb)
> {
> 	return writeback_inodes_sb_nr_if_idle(sb, get_nr_dirty_pages());
> }
> 
> In writeback.h, No?

I didn't care enough to move it :P I don't know if it matters.


> But it has a single user so please just kill it.
> 
> Also writeback_inodes_sb_nr_if_idle() has a single user. Combined with above,
> two users. Why not open code it in the two sites. It should be much
> clearer to understand what the magic is all about?

The filesystem shouldn't be aware of the details (the "magic") of how to
kick writeback, so I think the abstraction is right as is.

Thanks,
Nick