linux-kernel - [PATCH 5.4 301/340] seq_file: document how per-entry resources are managed.

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Date:   Mon,  1 Mar 2021 17:14:05 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, NeilBrown <neilb@...e.de>,
        Xin Long <lucien.xin@...il.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.4 301/340] seq_file: document how per-entry resources are managed.

From: NeilBrown <neilb@...e.de>

commit b3656d8227f4c45812c6b40815d8f4e446ed372a upstream.

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@...ux-foundation.org: 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@...e.de>
Cc: Xin Long <lucien.xin@...il.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Jonathan Corbet <corbet@....net>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Vlad Yasevich <vyasevich@...il.com>
Cc: Neil Horman <nhorman@...driver.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 Documentation/filesystems/seq_file.txt |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/Documentation/filesystems/seq_file.txt
+++ b/Documentation/filesystems/seq_file.txt
@@ -192,6 +192,12 @@ 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