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From: Al Viro <viro@ZenIV.linux.org.uk>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 00/16 v3] tracing: Add new file system tracefs
Date: Tue, 27 Jan 2015 04:34:21 +0000	[thread overview]
Message-ID: <20150127043421.GV29656@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20150126193909.19967bd0@grimm.local.home>

On Mon, Jan 26, 2015 at 07:39:09PM -0500, Steven Rostedt wrote:
> On Mon, 26 Jan 2015 18:49:16 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > On Mon, 26 Jan 2015 18:43:14 -0500
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> >  
> > > That is, we can not hold i_mutex and take trace_types_lock.
> > > 
> > > trace_types_lock needs to be held with the creation or destruction
> > > of events, which is what mkdir an rmdir do.
> > 
> > Although, I can not remember how this happened, but lockdep blew up on
> > me with this. I'll look again.
> 
> OK, found it. When events are created (module is loaded), the
> trace_types_lock is taken, and the event directories are created (these
> are in /sys/kernel/debug/tracing/events). We need to grab the i_mutex
> in order to create these files.
> 
> This means that we can not take the trace_types_lock within the mkdir
> or rmdir calls.
> 
> The directories are not static even outside the mkdir and rmdir calls.
> As events can be created from several sources, which will create new
> files and directories.

Frankly, the first impression is that you have trace_types_lock way too
high in the hierarchy - you are taking it well outside of the level where
you start walking through ftrace_trace_arrays...

What else is it used for?  You seem to use it protect trace_array refcounts,
but you use them in a very odd way...  What happens if lookup for a file
in that tree loses CPU just as it enters trace_array_get() (e.g. on the
same trace_types_lock), at which point rm .../instances/<whatever> comes,
sees that tr->ref is still zero, kicks the sucker out of the list and
frees it, immediately followed by mkdir creating a new instace?  If the
allocation of the new trace_array happens to reuse the just-freed one,
your trace_array_get() will actually succeed, won't it?

It's not fatal (after all, everything will work as if you opened the similar
file in new instance in the first place), but it looks rather bogus...

  reply	other threads:[~2015-01-27  4:34 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-01-26 15:09 [PATCH 00/16 v3] tracing: Add new file system tracefs Steven Rostedt
2015-01-26 15:09 ` [PATCH 01/16 v3] debugfs_{mkdir,create,link}(): get rid of redundant argument Steven Rostedt
2015-01-26 15:09 ` [PATCH 02/16 v3] debugfs: split the beginning and the end of __create_file() off Steven Rostedt
2015-01-26 15:09 ` [PATCH 03/16 v3] debugfs: kill __create_file() Steven Rostedt
2015-01-26 15:09 ` [PATCH 04/16 v3] fold debugfs_link() into caller Steven Rostedt
2015-01-26 15:09 ` [PATCH 05/16 v3] debugfs_mknod(): get rid useless arguments Steven Rostedt
2015-01-26 15:09 ` [PATCH 06/16 v3] fold debugfs_mkdir() into caller Steven Rostedt
2015-01-26 15:09 ` [PATCH 07/16 v3] fold debugfs_create() " Steven Rostedt
2015-01-26 15:09 ` [PATCH 08/16 v3] fold debugfs_mknod() into callers Steven Rostedt
2015-01-26 15:09 ` [PATCH 09/16 v3] debugfs: take mode-dependent parts of debugfs_get_inode() " Steven Rostedt
2015-01-26 15:09 ` [PATCH 10/16 v3] debugfs: split end_creating() into success and failure cases Steven Rostedt
2015-01-26 15:09 ` [PATCH 11/16 v3] new primitive: debugfs_create_automount() Steven Rostedt
2015-01-26 15:09 ` [PATCH 12/16 v3] tracefs: Add new tracefs file system Steven Rostedt
2015-01-26 15:09 ` [PATCH 13/16 v3] tracing: Convert the tracing facility over to use tracefs Steven Rostedt
2015-01-26 15:09 ` [PATCH 14/16 v3] tracing: Automatically mount tracefs on debugfs/tracing Steven Rostedt
2015-01-26 15:09 ` [PATCH 15/16 v3] tracefs: Add directory /sys/kernel/tracing Steven Rostedt
2015-01-26 15:09 ` [PATCH 16/16 v3] tracing: Have mkdir and rmdir be part of tracefs Steven Rostedt
2015-01-26 19:30 ` [PATCH 00/16 v3] tracing: Add new file system tracefs Al Viro
2015-01-26 20:42   ` Steven Rostedt
2015-01-26 20:44     ` Steven Rostedt
2015-01-26 20:58       ` Steven Rostedt
2015-01-26 21:44         ` Al Viro
2015-01-26 21:46     ` Al Viro
2015-01-26 23:02       ` Al Viro
2015-01-26 23:39         ` Steven Rostedt
2015-01-26 23:43       ` Steven Rostedt
2015-01-26 23:49         ` Steven Rostedt
2015-01-27  0:39           ` Steven Rostedt
2015-01-27  4:34             ` Al Viro [this message]
2015-01-27 15:15               ` Steven Rostedt
2015-01-27  0:37         ` Al Viro
2015-01-27  1:02           ` Steven Rostedt
2015-01-27  1:03             ` Steven Rostedt
2015-01-27  4:38               ` Al Viro
2015-01-27 15:19                 ` Steven Rostedt
2015-01-27 15:44                   ` Steven Rostedt
2015-01-26 21:40   ` Al Viro
2015-01-26 23:37     ` Steven Rostedt
2015-01-27  2:34       ` Steven Rostedt
2015-01-27  4:46         ` Al Viro

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