Re: [RFC][PATCH] make sure that lock_for_kill() callers drop the locks in safe order

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Apr 10, 2026 at 09:24:04PM +0100, Al Viro wrote:
> On Fri, Apr 10, 2026 at 12:30:13PM -0700, Linus Torvalds wrote:
> 
> > The reason it exists is because lock_for_kill() can drop d_lock(), but
> > that's in the unlikely case that we cn't just immediately get the
> > inode lock.
> > 
> > So honestly, I think that rcu_read_lock() should be inside
> > lock_for_kill(), rather than in the caller as a "just in case things
> > go down".
> 
> Yup, in the cascade of followups I've mentioned...

FWIW, see #work.dcache-cleanups (on top of #work.dcache-busy-wait).  That's
obviously next cycle fodder, and it needs review and testing (at the moment
I've only build-tested that).

Branch is in git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git #work.dcache-cleanups
individual patches in followups.

I think the eviction machinery got easier to read, if nothing else, and
rcu_read_lock() scopes are saner now.  OTOH, if we *do* have RCU bugs wrt
overlapping scopes, we'll probably see a lot of UAF on that - rcu_read_lock()
scopes got aggressively minimized...

I've split the rcu_read_lock() massage into a series of small steps after
getting confused while trying to do them at once; not sure if it needs
to be carved up so much, but...

Shortlog:
Al Viro (11):
      shrink_dentry_list(): start with removing from shrink list
      fold lock_for_kill() into shrink_kill()
      fold lock_for_kill() and __dentry_kill() into common helper
      reducing rcu_read_lock() scopes in dput and friends, step 1
      reducing rcu_read_lock() scopes in dput and friends, step 2
      reducing rcu_read_lock() scopes in dput and friends, step 3
      reducing rcu_read_lock() scopes in dput and friends, step 4
      reducing rcu_read_lock() scopes in dput and friends, step 5
      reducing rcu_read_lock() scopes in dput and friends, step 6
      adjust calling conventions of lock_for_kill(), fold __dentry_kill() into dentry_kill()
      document dentry_kill()

Diffstat:
 fs/dcache.c | 214 ++++++++++++++++++++++++++++++++++++------------------------
 1 file changed, 129 insertions(+), 85 deletions(-)
63 lines of comments added - outside of comments it's -20LoC...