netdev - Re: [PATCH 1/2] freezer: add unsafe versions of freezable helpers

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Date:	Mon, 6 May 2013 00:12:34 +0200
From:	Pavel Machek <pavel@....cz>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Colin Cross <ccross@...roid.com>, linux-kernel@...r.kernel.org,
	Trond Myklebust <Trond.Myklebust@...app.com>,
	Len Brown <len.brown@...el.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Paul Walmsley <paul@...an.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-nfs@...r.kernel.org,
	linux-pm@...r.kernel.org, netdev@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Ben Chan <benchan@...omium.org>
Subject: Re: [PATCH 1/2] freezer: add unsafe versions of freezable helpers

Hi!

> > NFS calls the freezable helpers with locks held, which is unsafe
> > and caused lockdep warnings when 6aa9707 "lockdep: check that no
> > locks held at freeze time" was applied (reverted in dbf520a).
> > Add new *_unsafe versions of the helpers that will not run the
> > lockdep test when 6aa9707 is reapplied, and call them from NFS.
> > 
> > Signed-off-by: Colin Cross <ccross@...roid.com>
> > ---
> >  fs/nfs/inode.c          |  2 +-
> >  fs/nfs/nfs3proc.c       |  2 +-
> >  fs/nfs/nfs4proc.c       |  4 ++--
> >  include/linux/freezer.h | 42 +++++++++++++++++++++++++++++++++++++++++-
> >  net/sunrpc/sched.c      |  2 +-
> >  5 files changed, 46 insertions(+), 6 deletions(-)
> > 
> > diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
> > index 1f94167..53cbee5 100644
> > --- a/fs/nfs/inode.c
> > +++ b/fs/nfs/inode.c
> > @@ -79,7 +79,7 @@ int nfs_wait_bit_killable(void *word)
> >  {
> >  	if (fatal_signal_pending(current))
> >  		return -ERESTARTSYS;
> > -	freezable_schedule();
> > +	freezable_schedule_unsafe();
> 
> I'd suggest naming such variants _unkillable() instead of _unsafe().
> 
> There's nothing inherently 'unsafe' about it: the user asked for a hard 
> NFS mount and is getting it: with the side effect that it exposes the 
> machine to network delays in a 'hard' way as well. Which means suspend may 
> block indefinitely as well on network failure.

You only want to use _unsafe() variants when you enter refrigerator
with locks held.

And entering refrigerator with locks is tricky... and unsafe :-). It
is not directly related to killability.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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