linux-kernel - Re: [PATCH 2/9] vfs: export do_splice

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Date:	Sat, 23 Mar 2013 11:49:11 +0900
From:	"J. R. Okajima" <hooanon05@...oo.co.jp>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	David Howells <dhowells@...hat.com>,
	Miklos Szeredi <miklos@...redi.hu>, jack@...e.cz,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	akpm@...ux-foundation.org, apw@...onical.com, nbd@...nwrt.org,
	neilb@...e.de, jordipujolp@...il.com, ezk@....cs.sunysb.edu,
	sedat.dilek@...glemail.com, mszeredi@...e.cz
Subject: Re: [PATCH 2/9] vfs: export do_splice_direct() to modules


Al Viro:
> The scenario, BTW, looks so:
> process A does sb_start_write() (on your upper layer)
> process B tries to freeze said upper layer and blocks, waiting for A to finish
> process C grabs ->i_mutex in your upper layer
> process C does vfs_write(), which blocks, since there's a pending attempt to
> freeze
> process A tries to grab ->i_mutex held by C and blocks

According to latest mm/filemap.c:generic_file_aio_write(),
	sb_start_write(inode->i_sb);
	mutex_lock(&inode->i_mutex);
	ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
	mutex_unlock(&inode->i_mutex);
	:::
	sb_end_write(inode->i_sb);

Process C would block *BEFORE* i_mutex by sb_start_write()? No?
Honestly speaking I didn't pay attention about the freeze feature since
I don't use it. I am making aufs to support it now. But I don't know how
to test it...


J. R. Okajima
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