[PATCH v3 0/8] debugfs: per-file removal protection

Nicolai Stange
2017-10-30T23:15:46Z

Hi,

this is v3 of the per-file removal protection with the main change to v2
being that it's sent in non-RFC mode now.

For this, I dropped the questionable former [9/9] ("debugfs: free
debugfs_fsdata instances") for now. Perhaps I'll resend it later on
its own as a RFC again.

The problem this series attempts to address is that any indefinitely
blocking fops blocks _unrelated_ debugfs_remove()ers. This will be resolved
by introducing a per-file removal protection mechanism in place of the
former global debugfs_srcu shared among all debugfs files.

This problem has first been spotted and debugged by Johannes Berg [1]
and Tyler Hall is now facing the same issue again [2].

There's one patch to non-debugfs code,
[5/8] ("IB/hfi1: convert to debugfs_file_get() and -put()"),
which should be taken together with the rest of the series because
it depends on prior patches and later patches depend on it.

Applies to next-20171018.

I reviewed the patches once again and did an allmodconfig as well as an
allnoconfig build. I did more thorough testing on v2, including whether
the removal protection still works.

Thanks,

Nicolai

Changes to v2:
[9/9] ("debugfs: free debugfs_fsdata instances")
- dropped for now.

Changes to v1:
[2/9] ("debugfs: implement per-file removal protection")
- In an attempt to resolve the issue reported by the kernel test robot
for v1, restrict the "extended removal logic" to regular files in
__debugfs_remove().

[8/9] ("debugfs: defer debugfs_fsdata allocation to first usage")
- Following review from Johannes Berg, replace the WARN_ON in
debugfs_real_fops() by a WARN + 'return NULL'. The return NULL is
expected to crash current soon and serves as an alternative for a
BUG_ON here.
- Mention the change in debugfs_real_fops() in the commit message.

[9/9] ("debugfs: free debugfs_fsdata instances")
- Following advice from Paul E. McKenney, make debugfs_file_get()
release the RCU read section inbetween retry loop iterations.
- Fix a race in debugfs_file_get()'s path handling a concurrent
debugfs_file_put(): the former must not "help out resetting ->d_fsdata"
because this can wipe out another debugfs_file_get()'s achievements.

[1] http://lkml.kernel.org/r/1490280886.2766.4.camel-cdvu00un1VgdHxzADdlk8Q@public.gmane.org
[2] https://lkml.kernel.org/r/CAOjnSCYGprej+vEEsSXwr=wO+eWLe2d6sHQYTpp-DFpQ3pmguw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org

Nicolai Stange (8):
debugfs: add support for more elaborate ->d_fsdata
debugfs: implement per-file removal protection
debugfs: debugfs_real_fops(): drop __must_hold sparse annotation
debugfs: convert to debugfs_file_get() and -put()
IB/hfi1: convert to debugfs_file_get() and -put()
debugfs: purge obsolete SRCU based removal protection
debugfs: call debugfs_real_fops() only after debugfs_file_get()
debugfs: defer debugfs_fsdata allocation to first usage

drivers/infiniband/hw/hfi1/debugfs.c | 20 ++--
fs/debugfs/file.c | 210 +++++++++++++++++++++--------------
fs/debugfs/inode.c | 56 +++++++---
fs/debugfs/internal.h | 14 +++
include/linux/debugfs.h | 33 +-----
lib/Kconfig.debug | 1 -
6 files changed, 196 insertions(+), 138 deletions(-)

--
2.13.6

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