User: Password:
Subscribe / Log in / New account

dynamic_debug: multiple, pending queries in boot-arg

From:  Jim Cromie <>
Subject:  [patch 00/25] dynamic_debug: multiple, pending queries in boot-arg
Date:  Mon, 25 Jul 2011 15:42:25 -0600
Message-ID:  <>
Archive-link:  Article

This patchset extends dynamic-debug facility to allow
use of pr_debug() within a loadable module's module_init()
function.  Query/rules can be given on the boot-line,
and are saved to a pending list if they cannot be applied
immediately.  Later, when the module is being loaded, the
pending list is scanned, and matching rules are applied.
Thus pr_debug() calls in the module's initialization function
are active when it is invoked.

Changes since rev1:

- rebased on top of Jasons & Joes patchset
- fixed accidental unescape removal, noted by Bart
- trim src-path patch checks for matching prefix before trimming
 should now work for out-of-tree modules.
- undid verbose newline-strip in exec-queries - Bart
- verbose param 644, not 744 - Bart, Greg
- whitespace - Bart
- added 'a' flag - Jason, Bart
- drop pending_max - Bart

Changes since rev2:

1. lock around all list-work, pending-ct

In response to Bart's locking-bug observation, I hoisted locks up to
callers, so theyd protect all list, pending-ct manipulation.  This
means longer hold-times, but less locking/unlocking.  Left as separate
patch for now, partly cuz having pr_info's under lock gave me some
heartburn.  That said, lockdep didnt complain.  I think I got the
interim patches correct too.

2. filter-flags

Ive extended flags spec to have <match-flags>* <op> <new-flags>*
IE, matching /[pmflta]*[+-=][pmflta]*/

match-flags (optional) allows a query/rule to be more selective, which
increases the utility of otherwise unconstrained rules.  So the
following query matches all call-sites that are already enabled,
adding the TID flag.

$> echo " p+t " > <dbgfs>/dynamic_debug/control

3. 'a' pending query modification, removal

Real purpose of 2 was to allow modification or deletion of currently
pending queries:

$> echo "module foo +ap" > <debugfs>/dynamic_debug/control
$> echo "module foo +apt" > <debugfs>/dynamic_debug/control
$> echo "module foo ap=_" > <debugfs>/dynamic_debug/control

1st command adds a pending query on module foo
2nd command modifies it by adding a TID flag
3rd command deletes the pending query by setting flags to 0

Note that 2,3 have exact match on the query-spec, the match-flags in 3
specify flags required to match against the pending query; the 't'
flag added in 2 is not required, but allowed.

With explicit deletion of pending rules, I removed the tacit
delete-on-apply behavior.

4. $> cat <dbfs>/dynamic_debug/pending

This displays currently pending queries, simplifying their deletion.
Its not correct right now; it iterates over set, but repeats

5.  Fuller multi-command input

writes to control-file are parsed on '\n' as well as ';',
and '#' are recognized as comments.  
With this, the following works.

root@voyage:~# cat debugfs-file

  # blank lines ok
  module dynamic_debug +p              ; # turn on self-debugging
  # these are quite noisy when grepping
  # $DEBUGFS/dynamic_debug/{control,pending}
    # silence them (also, leading spaces allowed in comments)
  func ddebug_proc_show -p
  func ddebug_proc_next -p     ; # trailing comments need cmd terminator
  func pending_proc_show -p    ;
  func pending_proc_next -p

root@voyage:~# cat debugfs-file > /dbgfs/dynamic_debug/control
split into words: "module" "dynamic_debug" "+p"
changed $srcroot/lib/dynamic_debug.c:223 [dynamic_debug]ddebug_change =p
changed $srcroot/lib/dynamic_debug.c:576 [dynamic_debug]ddebug_save_pending =p

Here-docs work too, but shell interferes with comments.


1. echo " +p " > /dbg/dynamic_debug/control

The above felt a little radical, but it isnt really; it works on
mainline.  Therefore one part of the Doc is slightly misleading (last

 The match-spec's are used to choose a subset of the known dprintk()
 callsites to which to apply the flags-spec.  Think of them as a query
 with implicit ANDs between each pair.  Note that an empty list of
 match-specs is possible, but is not very useful because it will not
 match any debug statement callsites.

2. this runs 2 separate writes.

printf "module nsc_gpio +p\n module pc8736x_gpio +p\npc8736x_gpio +p\n" \
      > /dbg/dynamic_debug/control

This form of command is seen by kernel as 2 separate writes, so the
form is not usable in boot-line in mainline; it breaks the debug
facility (see patch 6).  Using ';' to separate multiple commands does
work on boot-line.

Jim Cromie

1-11 are straightforward, I think ready to go.
12-25 are the core feature addition, and have bigger implications.  
I think theyre mostly solid, but they probably warrant more scrutiny.
25 needs some help.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

Copyright © 2011, Eklektix, Inc.
Comments and public postings are copyrighted by their creators.
Linux is a registered trademark of Linus Torvalds