From: Oleg Nesterov <oleg@redhat.com>
To: Kirill Tkhai <ktkhai@parallels.com>,
Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
Vladimir Davydov <vdavydov@parallels.com>,
Kirill Tkhai <tkhai@yandex.ru>, Christoph Lameter <cl@linux.com>
Subject: [PATCH 3/3] introduce task_rcu_dereference()
Date: Mon, 27 Oct 2014 20:54:46 +0100 [thread overview]
Message-ID: <20141027195446.GD11736@redhat.com> (raw)
In-Reply-To: <20141027195339.GA11736@redhat.com>
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
next prev parent reply other threads:[~2014-10-27 18:58 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-10-22 7:17 [PATCH v4] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Kirill Tkhai
2014-10-22 21:30 ` introduce task_rcu_dereference? Oleg Nesterov
2014-10-22 22:23 ` Oleg Nesterov
2014-10-23 18:15 ` Oleg Nesterov
2014-10-23 8:10 ` Kirill Tkhai
2014-10-23 18:18 ` Oleg Nesterov
2014-10-24 7:51 ` Kirill Tkhai
2014-10-27 19:53 ` [PATCH 0/3] introduce task_rcu_dereference() Oleg Nesterov
2014-10-27 19:54 ` [PATCH 1/3] probe_kernel_address() can use __probe_kernel_read() Oleg Nesterov
2014-10-27 19:54 ` [PATCH 2/3] introduce probe_slab_address() Oleg Nesterov
2014-10-27 19:21 ` Christoph Lameter
2014-10-28 5:44 ` Kirill Tkhai
2014-10-28 5:48 ` Kirill Tkhai
2014-10-28 15:01 ` Peter Zijlstra
2014-10-28 17:56 ` Kirill Tkhai
2014-10-28 18:00 ` Kirill Tkhai
2014-10-28 19:55 ` Oleg Nesterov
2014-10-28 20:12 ` Oleg Nesterov
2014-10-29 5:10 ` Kirill Tkhai
2014-10-27 19:54 ` Oleg Nesterov [this message]
2014-10-28 6:22 ` [PATCH 3/3] introduce task_rcu_dereference() Kirill Tkhai
2016-05-18 17:02 ` Peter Zijlstra
2016-05-18 18:23 ` Oleg Nesterov
2016-05-18 19:10 ` Peter Zijlstra
2016-05-18 19:57 ` Oleg Nesterov
2016-05-26 11:34 ` Peter Zijlstra
2016-06-03 10:49 ` [tip:sched/core] sched/fair: Use task_rcu_dereference() tip-bot for Oleg Nesterov
2016-06-03 10:48 ` [tip:sched/core] sched/api: Introduce task_rcu_dereference() and try_get_task_struct() tip-bot for Oleg Nesterov
2014-10-28 11:02 ` [tip:sched/core] sched/numa: Fix unsafe get_task_struct() in task_numa_assign() tip-bot for Kirill Tkhai
2014-11-08 3:48 ` [PATCH v4] sched/numa: fix " Sasha Levin
2014-11-09 14:07 ` Kirill Tkhai
2014-11-10 10:03 ` Peter Zijlstra
2014-11-10 15:48 ` Sasha Levin
2014-11-10 16:01 ` Peter Zijlstra
2014-11-16 9:50 ` [tip:sched/urgent] sched/numa: Avoid selecting oneself as swap target tip-bot for Peter Zijlstra
2014-11-10 16:03 ` [PATCH v4] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Peter Zijlstra
2014-11-10 16:09 ` Sasha Levin
2014-11-10 16:16 ` Peter Zijlstra
2014-11-10 16:10 ` Kirill Tkhai
2014-11-10 16:36 ` Kirill Tkhai
2014-11-10 16:44 ` Sasha Levin
2014-11-10 20:01 ` Kirill Tkhai
2014-11-12 9:49 ` Kirill Tkhai
2014-11-15 2:38 ` Sasha Levin
2014-11-18 17:30 ` Sasha Levin
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