lkml.org 
[lkml]   [2007]   [Feb]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRe: freezer problems
    Date
    On Monday, 19 February 2007 23:41, Oleg Nesterov wrote:
    > On 02/19, Rafael J. Wysocki wrote:
    > >
    > > On Monday, 19 February 2007 21:23, Oleg Nesterov wrote:
    > >
    > > > > @@ -199,6 +189,10 @@ static void thaw_tasks(int thaw_user_spa
    > > > >
    > > > > do_each_thread(g, p) {
    > > > > + if (freezer_should_skip(p))
    > > > > + cancel_freezing(p);
    > > > > + } while_each_thread(g, p);
    > > > > + do_each_thread(g, p) {
    > > > > if (!freezeable(p))
    > > > > continue;
    > > >
    > > > Any reason for 2 separate do_each_thread() loops ?
    > >
    > > Yes. If there is a "freeze" request pending for the vfork parent (TIF_FREEZE
    > > set), we have to cancel it before the child is unfrozen, since otherwise the
    > > parent may go freezing after we try to reset PF_FROZEN for it.
    >
    > I see, thanks... thaw_process() doesn't take TIF_FREEZE into account.
    >
    > But doesn't this mean we have a race?
    >
    > Suppose that try_to_freeze_tasks() failed. It does cancel_freezing() for each
    > process before return, but what if some thread already checked TIF_FREEZE and
    > (for simplicity) it is preempted before frozen_process() in refrigerator().
    >
    > thaw_tasks() runs, ignores this task (P), returns. P gets CPU, and becomes
    > frozen, but nobody will thaw it.
    >
    > No?

    Well, I think this is highly theoretical. Namely, try_to_freeze_tasks() only
    fails after the timeout that's currently set to 20 sec., and it yields the CPU
    in each iteration of the main loop. The task in question would have to refuse
    being frozen for 20 sec. and then suddenly decide to freeze itself right before
    try_to_freeze_tasks() checks the timeout for the very last time. Then, it
    would have to get preempted at this very moment and stay unfrozen at least
    until thaw_tasks() starts running and in fact even longer.

    I think we may avoid this by making try_to_freeze_tasks() sleep for some time
    after it has reset TIF_FREEZE for all tasks in the error path, if anyone is
    ever able to trigger it.

    Greetings,
    Rafael
    -
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2007-02-20 00:45    [W:4.107 / U:0.688 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site