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    SubjectRe: [RFC PATCH 1/2] Fix PF_NOFREEZE and freezeable race
    On Fri, Apr 20, 2007 at 10:02:08PM +0400, Oleg Nesterov wrote:
    > On 04/19, Rafael J. Wysocki wrote:
    > >
    > > On Thursday, 19 April 2007 14:02, Gautham R Shenoy wrote:
    > > > This patch fixes the race pointed out by Oleg Nesterov.
    > > >
    > > > * Freezer marks a thread as freezeable.
    > > > * The thread now marks itself PF_NOFREEZE causing it to
    > > > freeze on calling try_to_freeze(). Thus the task is frozen, even though
    > > > it doesn't want to.
    > > > * Subsequent thaw_processes() will also fail to thaw the task since it is
    > > > marked PF_NOFREEZE.
    > > >
    > > > Avoid this problem by checking the current task's PF_NOFREEZE status in the
    > > > refrigerator before marking current as frozen.
    > > >
    > > > Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
    > >
    > > Looks good, although I'm not sure if we don't need to call recalc_sigpending()
    > > for tasks that turn out to be PF_NOFREEZE.
    >
    > I agree, we should clear TIF_SIGPENDING. It is not so critical for user-space
    > tasks, but for the kernel thread it may remain pending forever, causing subtle
    > failures.
    >
    > Gautham, isn't it possible to make a more simpler patch ? Just add PF_NOFREEZE
    > check to frozen_process,
    >
    > static inline void frozen_process(struct task_struct *p)
    > {
    > if (!unlikely(current->flags & PF_NOFREEZE)) {
    > p->flags |= PF_FROZEN;
    > wmb();
    > }
    > clear_tsk_thread_flag(p, TIF_FREEZE);
    > }
    >
    > No?

    Actually yes. The idea anyway was to check one last time before declaring
    ourselves as frozen. So I thought the best place was inside refrigerator since
    we are already holding the task_lock there.
    I wasn't too sure about calling recalc_sigpending(), but now that you
    mention it, I agree, this would be a nicer way to do it.

    Btw, since frozen_process is currently being called only from
    refrigerator, I am wondering if we still need the struct task_struct *p
    parameter there. It's very unlikely that some other task would mark a
    particular task as frozen. No?

    Anyways, Andrew, Could you please replace the earlier sent patch titled
    "fix_pf_nofreeze_and_freezeable_race.patch" with the following one?

    Thanks and Regards
    gautham.

    -->
    This patch fixes the race pointed out by Oleg Nesterov.

    * Freezer marks a thread as freezeable.
    * The thread now marks itself PF_NOFREEZE causing it to
    freeze on calling try_to_freeze(). Thus the task is frozen, even though
    it doesn't want to.
    * Subsequent thaw_processes() will also fail to thaw the task since it is
    marked PF_NOFREEZE.

    Avoid this problem by checking the task's PF_NOFREEZE status in
    frozen_processes() before marking the task as frozen.

    Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
    ---
    include/linux/freezer.h | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    Index: linux-2.6.21-rc6/include/linux/freezer.h
    ===================================================================
    --- linux-2.6.21-rc6.orig/include/linux/freezer.h
    +++ linux-2.6.21-rc6/include/linux/freezer.h
    @@ -57,8 +57,10 @@ static inline int thaw_process(struct ta
    */
    static inline void frozen_process(struct task_struct *p)
    {
    - p->flags |= PF_FROZEN;
    - wmb();
    + if (!unlikely(p->flags & PF_NOFREEZE)) {
    + p->flags |= PF_FROZEN;
    + wmb();
    + }
    clear_tsk_thread_flag(p, TIF_FREEZE);
    }

    --
    Gautham R Shenoy
    Linux Technology Center
    IBM India.
    "Freedom comes with a price tag of responsibility, which is still a bargain,
    because Freedom is priceless!"
    -
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