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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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