lkml.org 
[lkml]   [2013]   [Jun]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    SubjectRe: [PATCH RFC ticketlock] Auto-queued ticketlock
    From
    On Wed, Jun 12, 2013 at 5:20 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
    >
    > Actually, dget_parent() change might be broken; the thing is, the assumptions
    > are more subtle than "zero -> non-zero only happens under ->d_lock". It's
    > actually "new references are grabbed by somebody who's either already holding
    > one on the same dentry _or_ holding ->d_lock". That's what d_invalidate()
    > check for ->d_count needs for correctness - caller holds one reference, so
    > comparing ->d_count with 2 under ->d_lock means checking that there's no other
    > holders _and_ there won't be any new ones appearing.

    For the particular case of dget_parent() maybe dget_parent() should
    just double-check the original dentry->d_parent pointer after getting
    the refcount on it (and if the parent has changed, drop the refcount
    again and go to the locked version). That might be a good idea anyway,
    and should fix the possible race (which would be with another cpu
    having to first rename the child to some other parent, and the
    d_invalidate() the original parent)

    That said, the case we'd really want to fix isn't dget_parent(), but
    just the normal RCU lookup finishing touches (the__d_rcu_to_refcount()
    case you already mentioned) . *If* we could do that without ever
    taking the d_lock on the target, that would be lovely. But it would
    seem to have the exact same issue. Although maybe the
    dentry_rcuwalk_barrier() thing ends up solving it (ie if we had a
    lookup at a bad time, we know it will fail the sequence count test, so
    we're ok).

    Subtle, subtle.

    Linus


    \
     
     \ /
      Last update: 2013-06-13 02:41    [W:3.495 / U:0.700 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site