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    Subject[PATCH 3/3] introduce task_rcu_dereference()
    task_struct is only protected by RCU if it was found on a RCU protected
    list (say, for_each_process() or find_task_by_vpid()).

    And as Kirill pointed out rq->curr isn't protected by RCU, the scheduler
    drops the (potentially) last reference without RCU gp, this means that we
    need to fix the code which uses foreign_rq->curr under rcu_read_lock().

    Add a new helper which can be used to dereferene rq->curr or any other
    pointer to task_struct assuming that it should be cleared or updated
    before the final put_task_struct(). It returns non-NULL only if this
    task can't go away before rcu_read_unlock().

    Suggested-by: Kirill Tkhai <ktkhai@parallels.com>
    Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    ---
    include/linux/sched.h | 1 +
    kernel/exit.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
    2 files changed, 50 insertions(+), 0 deletions(-)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 857ba40..0ba420e 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -2300,6 +2300,7 @@ extern void block_all_signals(int (*notifier)(void *priv), void *priv,
    sigset_t *mask);
    extern void unblock_all_signals(void);
    extern void release_task(struct task_struct * p);
    +extern struct task_struct *task_rcu_dereference(struct task_struct **ptask);
    extern int send_sig_info(int, struct siginfo *, struct task_struct *);
    extern int force_sigsegv(int, struct task_struct *);
    extern int force_sig_info(int, struct siginfo *, struct task_struct *);
    diff --git a/kernel/exit.c b/kernel/exit.c
    index 32c58f7..d8b95c2 100644
    --- a/kernel/exit.c
    +++ b/kernel/exit.c
    @@ -213,6 +213,55 @@ repeat:
    goto repeat;
    }

    +struct task_struct *task_rcu_dereference(struct task_struct **ptask)
    +{
    + struct task_struct *task;
    + struct sighand_struct *sighand;
    +
    + /*
    + * We need to verify that release_task() was not called and thus
    + * delayed_put_task_struct() can't run and drop the last reference
    + * before rcu_read_unlock(). We check task->sighand != NULL, but
    + * we can read the already freed and reused memory.
    + */
    + retry:
    + task = rcu_dereference(*ptask);
    + if (!task)
    + return NULL;
    +
    + probe_slab_address(&task->sighand, sighand);
    + /*
    + * Pairs with atomic_dec_and_test(usage) in put_task_struct(task).
    + * If this task was already freed we can not miss the preceding
    + * update of this pointer.
    + */
    + smp_rmb();
    + if (unlikely(task != ACCESS_ONCE(*ptask)))
    + goto retry;
    +
    + /*
    + * Either this is the same task and we can trust sighand != NULL, or
    + * its memory was re-instantiated as another instance of task_struct.
    + * In the latter case the new task can not go away until another rcu
    + * gp pass, so the only problem is that sighand == NULL can be false
    + * positive but we can pretend we got this NULL before it was freed.
    + */
    + if (sighand)
    + return task;
    +
    + /*
    + * We could even eliminate the false positive mentioned above:
    + *
    + * probe_slab_address(&task->sighand, sighand);
    + * if (sighand)
    + * goto retry;
    + *
    + * if sighand != NULL because we read the freed memory we should
    + * see the new pointer, otherwise we will likely return this task.
    + */
    + return NULL;
    +}
    +
    /*
    * This checks not only the pgrp, but falls back on the pid if no
    * satisfactory pgrp is found. I dunno - gdb doesn't work correctly
    --
    1.5.5.1



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