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Message-ID: <tip-150593bf869393d10a79f6bd3df2585ecc20a9bb@git.kernel.org>
Date: Fri, 3 Jun 2016 03:48:32 -0700
From: tip-bot for Oleg Nesterov <tipbot@...or.com>
To: linux-tip-commits@...r.kernel.org
Cc: tglx@...utronix.de, cmetcalf@...hip.com,
linux-kernel@...r.kernel.org, oleg@...hat.com, efault@....de,
ktkhai@...allels.com, hpa@...or.com, tkhai@...dex.ru,
vdavydov@...allels.com, torvalds@...ux-foundation.org,
mingo@...nel.org, peterz@...radead.org, cl@...ux.com
Subject: [tip:sched/core] sched/api: Introduce task_rcu_dereference() and
try_get_task_struct()
Commit-ID: 150593bf869393d10a79f6bd3df2585ecc20a9bb
Gitweb: http://git.kernel.org/tip/150593bf869393d10a79f6bd3df2585ecc20a9bb
Author: Oleg Nesterov <oleg@...hat.com>
AuthorDate: Wed, 18 May 2016 19:02:18 +0200
Committer: Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 3 Jun 2016 09:18:57 +0200
sched/api: Introduce task_rcu_dereference() and try_get_task_struct()
Generally 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()).
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 dereference 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().
( Also add try_get_task_struct() to make it easier to use this API
correctly. )
Suggested-by: Kirill Tkhai <ktkhai@...allels.com>
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
[ Updated comments; added try_get_task_struct()]
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Chris Metcalf <cmetcalf@...hip.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Kirill Tkhai <tkhai@...dex.ru>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Vladimir Davydov <vdavydov@...allels.com>
Link: http://lkml.kernel.org/r/20160518170218.GY3192@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
include/linux/sched.h | 3 ++
kernel/exit.c | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 79 insertions(+)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 6e42ada..dee41bf 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2139,6 +2139,9 @@ static inline void put_task_struct(struct task_struct *t)
__put_task_struct(t);
}
+struct task_struct *task_rcu_dereference(struct task_struct **ptask);
+struct task_struct *try_get_task_struct(struct task_struct **ptask);
+
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
extern void task_cputime(struct task_struct *t,
cputime_t *utime, cputime_t *stime);
diff --git a/kernel/exit.c b/kernel/exit.c
index 9e6e135..2fb4d44 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -211,6 +211,82 @@ repeat:
}
/*
+ * Note that if this function returns a valid task_struct pointer (!NULL)
+ * task->usage must remain >0 for the duration of the RCU critical section.
+ */
+struct task_struct *task_rcu_dereference(struct task_struct **ptask)
+{
+ struct sighand_struct *sighand;
+ struct task_struct *task;
+
+ /*
+ * 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_kernel_address(&task->sighand, sighand);
+
+ /*
+ * Pairs with atomic_dec_and_test() in put_task_struct(). If this task
+ * was already freed we can not miss the preceding update of this
+ * pointer.
+ */
+ smp_rmb();
+ if (unlikely(task != READ_ONCE(*ptask)))
+ goto retry;
+
+ /*
+ * We've re-checked that "task == *ptask", now we have two different
+ * cases:
+ *
+ * 1. This is actually the same task/task_struct. In this case
+ * sighand != NULL tells us it is still alive.
+ *
+ * 2. This is another task which got the same memory for task_struct.
+ * We can't know this of course, and we can not trust
+ * sighand != NULL.
+ *
+ * In this case we actually return a random value, but this is
+ * correct.
+ *
+ * If we return NULL - we can pretend that we actually noticed that
+ * *ptask was updated when the previous task has exited. Or pretend
+ * that probe_slab_address(&sighand) reads NULL.
+ *
+ * If we return the new task (because sighand is not NULL for any
+ * reason) - this is fine too. This (new) task can't go away before
+ * another gp pass.
+ *
+ * And note: We could even eliminate the false positive if re-read
+ * task->sighand once again to avoid the falsely NULL. But this case
+ * is very unlikely so we don't care.
+ */
+ if (!sighand)
+ return NULL;
+
+ return task;
+}
+
+struct task_struct *try_get_task_struct(struct task_struct **ptask)
+{
+ struct task_struct *task;
+
+ rcu_read_lock();
+ task = task_rcu_dereference(ptask);
+ if (task)
+ get_task_struct(task);
+ rcu_read_unlock();
+
+ return task;
+}
+
+/*
* Determine if a process group is "orphaned", according to the POSIX
* definition in 2.2.2.52. Orphaned process groups are not to be affected
* by terminal-generated stop signals. Newly orphaned process groups are
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