Re: cred_guard_mutex vs seq_file::lock [was: Re: 3.14.0+/x86: lockdep and mutexes not getting along]

From: Al Viro <viro@ZenIV.linux.org.uk>
To: David Howells <dhowells@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	"Michael L. Semon" <mlsemon35@gmail.com>,
	Ingo Molnar <mingo@kernel.org>,
	jason.low2@hp.com, linux-kernel@vger.kernel.org
Subject: Re: cred_guard_mutex vs seq_file::lock [was: Re: 3.14.0+/x86: lockdep and mutexes not getting along]
Date: Fri, 11 Apr 2014 16:07:25 +0100	[thread overview]
Message-ID: <20140411150724.GI18016@ZenIV.linux.org.uk> (raw)
In-Reply-To: <26259.1397227827@warthog.procyon.org.uk>

On Fri, Apr 11, 2014 at 03:50:27PM +0100, David Howells wrote:
> Peter Zijlstra <peterz@infradead.org> wrote:
> 
> > Al, David, any bright ideas on how to best fix this?
> 
> Have the seq_xxx() code throw an error if current->in_execve is true.  I can't
> think of any circumstance where execve() should be reading anything that uses
> seq_xxx().

*cringe*

I don't like it.  That really should be a responsiblity of specific ->show();
"I'm going to take that mutex, bugger off if we are in execve()" makes a lot
more sense than having e.g. seq_read() care of that.  IOW, I would very
much prefer the patch you've sent last week.

And yes, it might leave lockdep false positives, but that's better dealt with
by annotating the sucker ("this guy has a separate lockdep class for its
->lock").  E.g. by splitting proc_single_file_operations in two and having
the one used for those files do lockdep_set_class() in its ->open().