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authorNeilBrown <neilb@suse.de>2021-02-25 17:22:25 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2021-02-26 09:41:05 -0800
commitb3656d8227f4c45812c6b40815d8f4e446ed372a (patch)
treec3b2726e6af82b6b14da81bcfaf5ca58efad84f0 /Documentation/filesystems/seq_file.rst
parent3159ed57792be7453793bda27297a423e1c63d6c (diff)
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@linux-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@suse.de> Cc: Xin Long <lucien.xin@gmail.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Ingo Molnar <mingo@redhat.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Andy Lutomirski <luto@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Vlad Yasevich <vyasevich@gmail.com> Cc: Neil Horman <nhorman@tuxdriver.com> Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com> Cc: "David S. Miller" <davem@davemloft.net> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/filesystems/seq_file.rst')
-rw-r--r--Documentation/filesystems/seq_file.rst6
1 files changed, 6 insertions, 0 deletions
diff --git a/Documentation/filesystems/seq_file.rst b/Documentation/filesystems/seq_file.rst
index 56856481dc8d..a6726082a7c2 100644
--- a/Documentation/filesystems/seq_file.rst
+++ b/Documentation/filesystems/seq_file.rst
@@ -217,6 +217,12 @@ between the calls to start() and stop(), so holding a lock during that time
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.
+
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