lkml.org 
[lkml]   [2011]   [Dec]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 3/3] kref: Remove the memory barriers
    On Mon, Dec 12, 2011 at 11:56:47PM +0100, Oliver Neukum wrote:
    > Am Montag, 12. Dezember 2011, 20:30:40 schrieb Greg KH:
    > > On Mon, Dec 12, 2011 at 11:20:16AM +0100, Oliver Neukum wrote:
    > > > Am Montag, 12. Dezember 2011, 10:57:31 schrieb Ming Lei:
    > > > > On Mon, Dec 12, 2011 at 4:54 PM, Peter Zijlstra <peterz@infradead.org> wrote:
    > > > > > On Mon, 2011-12-12 at 11:48 +0800, Ming Lei wrote:
    > > > > >> For kref, maybe it is still multiple operations done on one cpu vs them
    > > > > >> being visible on another, but seems a bit implicit, see the common kref
    > > > > >> usage below:
    > > > > >>
    > > > > >> CPU0 CPU1
    > > > > >> A:kref_init(&obj->ref)
    > > > > >
    > > > > > how does CPU0 get a ref to obj?
    > > > >
    > > > > Suppose open/close/read/.. context is run on CPU0, and driver .probe/.release
    > > > > context(hotplug context) is run on CPU1. There are a few examples on
    > > > > usb driver(eg. usb-skeleton.c, ...)
    > > >
    > > > USB generally relies on an implied barrier just as:
    > > >
    > > > /* we can register the device now, as it is ready */
    > > > retval = usb_register_dev(interface, &skel_class);
    > > >
    > > > Generally reference counting cannot help you if kfree() is involved
    > >
    > > So, Oliver, you don't have any objection to this patch removing the
    > > barriers in kref, right? Originally you added them, I just wanted to
    > > make sure before I applied this.
    >
    > I do not remember any more why I introduced this.
    >
    > I guess I worried not about the increment, but the decrement.
    > Which makes me wonder what happens if you don't intend
    > to get the kref again, but need to make sure it is usually freed,
    > like:
    >
    > CPU A CPU B
    >
    > kref_get(p)
    > start_io(p)
    > [interrupt from IO]
    > kref_put(p)
    >
    > You need an ordering primitive between start_io() and kref_get()
    > or the counter could go negative.

    Really? On an atomic variable? I didn't think this was needed for
    atomics to ensure this type of thing couldn't happen.

    > I think I was worried about it missing.

    I'd be worried as well, but I don't think that can really happen, or am
    I totally wrong?

    greg k-h


    \
     
     \ /
      Last update: 2011-12-13 00:17    [W:0.024 / U:60.928 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site