Anything that walks all inodes on sb->s_inodes list without rescheduling
risks softlockups.

Previous efforts were made in 2 functions, see:

c27d82f fs/drop_caches.c: avoid softlockups in drop_pagecache_sb()
ac05fbb inode: don't softlockup when evicting inodes

but there hasn't been an audit of all walkers, so do that now.  This
also consistently moves the cond_resched() calls to the bottom of each
loop in cases where it already exists.

One loop remains: remove_dquot_ref(), because I'm not quite sure how
to deal with that one w/o taking the i_lock.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Pull vfs d_inode/d_flags memory ordering fixes from Al Viro:
 "Fallout from tree-wide audit for ->d_inode/->d_flags barriers use.
  Basically, the problem is that negative pinned dentries require
  careful treatment - unless ->d_lock is locked or parent is held at
  least shared, another thread can make them positive right under us.

  Most of the uses turned out to be safe - the main surprises as far as
  filesystems are concerned were

   - race in dget_parent() fastpath, that might end up with the caller
     observing the returned dentry _negative_, due to insufficient
     barriers. It is positive in memory, but we could end up seeing the
     wrong value of ->d_inode in CPU cache. Fixed.

   - manual checks that result of lookup_one_len_unlocked() is positive
     (and rejection of negatives). Again, insufficient barriers (we
     might end up with inconsistent observed values of ->d_inode and
     ->d_flags). Fixed by switching to a new primitive that does the
     checks itself and returns ERR_PTR(-ENOENT) instead of a negative
     dentry. That way we get rid of boilerplate converting negatives
     into ERR_PTR(-ENOENT) in the callers and have a single place to
     deal with the barrier-related mess - inside fs/namei.c rather than
     in every caller out there.

  The guts of pathname resolution *do* need to be careful - the race
  found by Ritesh is real, as well as several similar races.
  Fortunately, it turns out that we can take care of that with fairly
  local changes in there.

  The tree-wide audit had not been fun, and I hate the idea of repeating
  it. I think the right approach would be to annotate the places where
  we are _not_ guaranteed ->d_inode/->d_flags stability and have sparse
  catch regressions. But I'm still not sure what would be the least
  invasive way of doing that and it's clearly the next cycle fodder"

* 'fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs:
  fs/namei.c: fix missing barriers when checking positivity
  fix dget_parent() fastpath race
  new helper: lookup_positive_unlocked()
  fs/namei.c: pull positivity check into follow_managed()

Most of the callers of lookup_one_len_unlocked() treat negatives are
ERR_PTR(-ENOENT).  Provide a helper that would do just that.  Note
that a pinned positive dentry remains positive - it's ->d_inode is
stable, etc.; a pinned _negative_ dentry can become positive at any
point as long as you are not holding its parent at least shared.
So using lookup_one_len_unlocked() needs to be careful;
lookup_positive_unlocked() is safer and that's what the callers
end up open-coding anyway.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Quota statistics counted as 64-bit per-cpu counter. Reading sums per-cpu
fractions as signed 64-bit int, filters negative values and then reports
lower half as signed 32-bit int.

Result may looks like:

fs.quota.allocated_dquots = 22327
fs.quota.cache_hits = -489852115
fs.quota.drops = -487288718
fs.quota.free_dquots = 22083
fs.quota.lookups = -486883485
fs.quota.reads = 22327
fs.quota.syncs = 335064
fs.quota.writes = 3088689

Values bigger than 2^31-1 reported as negative.

All counters except "allocated_dquots" and "free_dquots" are monotonic,
thus they should be reported as is without filtering negative values.

Kernel doesn't have generic helper for 64-bit sysctl yet,
let's use at least unsigned long.

Link: https://lore.kernel.org/r/157337934693.2078.9842146413181153727.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Jan Kara <jack@suse.cz>

Make dquot_get_state() gracefully handle a situation when there are no
quota files present even though quotas are enabled.

Signed-off-by: Jan Kara <jack@suse.cz>

Quota on and quota off are protected by s_umount semaphore held in
exclusive mode since commit 7d6cd73d33b6 "quota: Hold s_umount in
exclusive mode when enabling / disabling quotas". This makes it
impossible for dquot_disable() to race with other enabling or disabling
of quotas. Simplify the cleanup done by dquot_disable() based on this
fact and also remove some stale comments. As a bonus this cleanup makes
dquot_disable() properly handle a case when there are no quota inodes.

Signed-off-by: Jan Kara <jack@suse.cz>

Now dquot_enable() has only two internal callers and both of them just
need to update quota flags and don't need most of checks. Just drop
dquot_enable() and fold necessary functionality into the two calling
places.

Signed-off-by: Jan Kara <jack@suse.cz>

Rename vfs_load_quota_inode() to dquot_load_quota_inode() to be
consistent with naming of other functions used for enabling quota
accounting from filesystems. Also export the function and add some
sanity checks to assure filesystems are calling the function properly.

Signed-off-by: Jan Kara <jack@suse.cz>

We already have quota inode loaded when resuming quotas. Use
vfs_load_quota() to avoid some pointless churn with the quota inode.

Signed-off-by: Jan Kara <jack@suse.cz>