From: NeilBrown <neilb@xxxxxxx>
Subject: seq_file: document how per-entry resources are managed.
Patch series "Fix some seq_file users that were recently broken".
A recent change to seq_file broke some users which were using seq_file
in a non-"standard" way ... though the "standard" isn't documented, so
they can be excused. The result is a possible leak - of memory in one
case, of references to a 'transport' in the other.
These three patches:
1/ document and explain the problem
2/ fix the problem user in x86
3/ fix the problem user in net/sctp
This patch (of 3):
Users of seq_file will sometimes find it convenient to take a resource,
such as a lock or memory allocation, in the ->start or ->next operations.
These are per-entry resources, distinct from per-session resources which
are taken in ->start and released in ->stop.
The preferred management of these is release the resource on the
subsequent call to ->next or ->stop.
However prior to Commit 1f4aace60b0e ("fs/seq_file.c: simplify seq_file
iteration code and interface") it happened that ->show would always be
called after ->start or ->next, and a few users chose to release the
resource in ->show.
This is no longer reliable. Since the mentioned commit, ->next will
always come after a successful ->show (to ensure m->index is updated
correctly), so the original ordering cannot be maintained.
This patch updates the documentation to clearly state the required
behaviour. Other patches will fix the few problematic users.
[akpm@xxxxxxxxxxxxxxxxxxxx: fix typo, per Willy]
Link: https://lkml.kernel.org/r/161248518659.21478.2484341937387294998.stgit@noble1
Link: https://lkml.kernel.org/r/161248539020.21478.3147971477400875336.stgit@noble1
Fixes: 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and interface")
Signed-off-by: NeilBrown <neilb@xxxxxxx>
Cc: Xin Long <lucien.xin@xxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Vlad Yasevich <vyasevich@xxxxxxxxx>
Cc: Neil Horman <nhorman@xxxxxxxxxxxxx>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@xxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/filesystems/seq_file.rst | 6 ++++++
1 file changed, 6 insertions(+)
--- a/Documentation/filesystems/seq_file.rst~seq_file-document-how-per-entry-resources-are-managed
+++ a/Documentation/filesystems/seq_file.rst
@@ -217,6 +217,12 @@ between the calls to start() and stop(),
is a reasonable thing to do. The seq_file code will also avoid taking any
other locks while the iterator is active.
+The iterater value returned by start() or next() is guaranteed to be
+passed to a subsequent next() or stop() call. This allows resources
+such as locks that were taken to be reliably released. There is *no*
+guarantee that the iterator will be passed to show(), though in practice
+it often will be.
+
Formatted output
================
_
