[PATCHSET v4 driver-core-next] kernfs, sysfs, driver-core: implement synchronous self-removal

[Tejun Heo]

Hello,

This is v4 of kernfs self-removal patchset.  Unfortunately, this
series has a somewhat turbulent history.  After sending out the first
iteration[1], I realized that cgroup's use case would need more
flexibility so I called it off and after a while posted the second
version[2] which contained patches which were slightly stale by
mistake, so soon after v3[3] was posted, which got applied to the
driver-core-next for 3.14-rc1 inclusion; unfortunately, while working
on it later, I realized that the new more flexible scheme was
fundamentally broken and the first approach in v1 was actually the
correct one.  So, the whole series got reverted and this is the fourth
trial aiming for the 3.15-rc1 window.  This time, the whole cgroup
conversion is completed on top and verified to work properly, so
hopefully this take is the last for this series.

As described above, the approach used in the first iteration was the
correct one, so this patchset is an updated version of v1.  The only
necessary addition was exposing breaking out of s_active protection as
a separate API.

* Incorporate fixes from v3 of the patchset.

* kernfs_[un]break_active_protection() added.  "break" puts the active
  reference while "unbreak" undoes it so that the refcnt is balanced;
  however, unbreaking doesn't and can't restore active reference
  protection itself.  Once broken, active ref protection is gone for
  the full duration of the method invocation.  This allows more
  complex implementation of self-removal where self-removal attempts
  may be rejected by shifting the responsibility of ensuring object
  accessbility to the kernfs user.

  This API is a bit cumbersome to use but the simpler
  kernfs_remove_self() is still provided and built on top of the above
  API.

Original patch description follows.

kernfs / sysfs implement the "sever" semantic for userland accesses.
When a node is removed, no further userland operations are allowed and
the in-flight ones are drained before removal is finished.  This makes
policing post-mortem userland accesses trivial for its users;
unfortunately, this comes with a drawback - a node which tries to
delete oneself through one of its userland operations deadlocks.
Removal wants to drain the active access that the operation itself is
running on top of.

This currently is worked around in the sysfs layer using
sysfs_schedule_callback() which punts the actual removal to a work
item.  While making the operation asynchronous kinda works, it's a bit
cumbersome to use and its behavior isn't quite correct as the caller
has no way of telling when or even whether the operation is actually
complete.  If such self-removal is followed by another operation which
expects the removed name to be available, there's no way to make the
second operation reliable - e.g. something like "echo 1 > asdf/delete;
echo asdf > create_new_child" can't work properly.

This patchset improves kernfs removal path and implements
kernfs_remove_self() which is to be called from an on-going kernfs
operation and removes the self node.  The function can be called
concurrently and only one will return %true and all others will wait
until the winner's file operation is complete (not the
kernfs_remove_self() call itself but the enclosing file operation
which invoked the function).  This ensures that if there are multiple
concurrent "echo 1 > asdf/delete", all of them would finish only after
the whole store_delete() method is complete.

kernfs_remove_self() is exposed to upper layers through
sysfs_remove_file_self() and device_remove_file_self().  The existing
users of device_schedule_callback() are converted to use remove_self
and the unused async mechanism is removed.

This patchset contains the following eleven patches.

 0001-kernfs-replace-kernfs_node-u.completion-with-kernfs_.patch
 0002-kernfs-restructure-removal-path-to-fix-possible-prem.patch
 0003-kernfs-invoke-kernfs_unmap_bin_file-directly-from-ke.patch
 0004-kernfs-remove-kernfs_addrm_cxt.patch
 0005-kernfs-remove-KERNFS_ACTIVE_REF-and-add-kernfs_lockd.patch
 0006-kernfs-remove-KERNFS_REMOVED.patch
 0007-kernfs-sysfs-driver-core-implement-kernfs_remove_sel.patch
 0008-pci-use-device_remove_file_self-instead-of-device_sc.patch
 0009-scsi-use-device_remove_file_self-instead-of-device_s.patch
 0010-s390-use-device_remove_file_self-instead-of-device_s.patch
 0011-sysfs-driver-core-remove-unused-sysfs-device-_schedu.patch

0001 replaces kernfs_node->u.completion with a hierarchy-wide
wait_queue_head.  This will be used to fix concurrent removal
behavior.

0002 fixes premature completion of node removal when multiple removers
are competing.  This shouldn't matter for the existing sysfs users.

0003-0006 clean up removal path.  The size of kernfs_node is reduced
by one pointer in the process.

0007 implements kernfs_remove_self() and friends.

0008-0011 convert the existing users of device_schedule_callback() to
device_remove_file_self() and removes now unused async mechanism.

After the changes, kernfs_node is shrunken by a pointer and LOC goes
down a bit too.

The patchset is on top of the current linus#master 54c0a4b46150
("Merge branch 'akpm' (incoming from Andrew)").  While 3.14-rc1 hasn't
been tagged yet, it's unlikely that the affected code area would
receive any significant updates in the meantime.  If the patchset
doesn't apply cleanly on -rc1, I'll post the refreshed version.

The patchset is also available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git review-kernfs-suicide

diffstat follows.

 arch/s390/include/asm/ccwgroup.h |    1
 arch/s390/pci/pci_sysfs.c        |   18 -
 drivers/base/core.c              |   50 +---
 drivers/pci/pci-sysfs.c          |   17 -
 drivers/s390/block/dcssblk.c     |   14 -
 drivers/s390/cio/ccwgroup.c      |   26 +-
 drivers/scsi/scsi_sysfs.c        |   15 -
 fs/kernfs/dir.c                  |  447 +++++++++++++++++++++++----------------
 fs/kernfs/file.c                 |    6
 fs/kernfs/kernfs-internal.h      |   14 -
 fs/kernfs/symlink.c              |    6
 fs/sysfs/file.c                  |  115 ++--------
 include/linux/device.h           |   13 -
 include/linux/kernfs.h           |   18 -
 include/linux/sysfs.h            |   16 -
 15 files changed, 373 insertions(+), 403 deletions(-)

Thanks.

--
tejun

[1] http://lkml.kernel.org/g/<1389117590-25705-1-git-send-email-tj@xxxxxxxxxx>
[2] http://lkml.kernel.org/g/<1389361620-5086-1-git-send-email-tj@xxxxxxxxxx>
[3] http://lkml.kernel.org/g/<1389362251-8128-1-git-send-email-tj@xxxxxxxxxx>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/