lkml.org 
[lkml]   [2015]   [May]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RFC][PATCHSET v3] non-recursive pathname resolution & RCU symlinks
On Fri, May 15, 2015 at 08:37:20PM -0700, Linus Torvalds wrote:
> On May 15, 2015 8:17 PM, "Al Viro" <viro@zeniv.linux.org.uk> wrote:
> >
> > What for? All we need is a flag, waitqueue and being woken
> > up when the flag gets cleared.
>
> You need to have the flag somewhere.
>
> The child dentry doesn't exist yet.
>
> That's the point of the hashed entry. It approximates the not-yet-existing
> child dentry that we have *not* added to the parent until after lookup.

Point, but... A lot of our problems comes from the fact that ->i_mutex
doubles as protection against the addition to the list of children, on
top of protection of directory itself. What if we do the following:
have the normal case of __lookup_hash() (and other callers of lookup_real())
* allocate dentry, marked "in-lookup"
* do dcache lookup, likely to come up empty, _without_ touching
potential matches' d_lock, i.e. based on __d_lookup_rcu() (under
rcu_read_lock(), with rename_lock loop around it). Hold parent's ->d_lock
while walking the chain, grab refcount in the unlikely case the match had
been found. If nothing's found *and* rename_lock hadn't been touched, insert
the new dentry into hash and list of children before dropping ->d_lock.
* call ->lookup() (still under ->i_mutex, shared)
* clear "in-lookup" bit on _original_ dentry (we might very well
have returned a different one)
* kick the wait queue of parent's ->i_mutex

I'll need to think about that after I get some sleep, but it smells like
that could be feasible. Of course, that assumes we'll be able to cope
with hashed-but-currently-in-lookup dentries, but I think it might be
doable with some massage...


\
 
 \ /
  Last update: 2015-05-16 06:41    [W:0.152 / U:0.928 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site