Commit 150593bf authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Ingo Molnar

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: default avatarKirill Tkhai <ktkhai@parallels.com>
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
[ Updated comments; added try_get_task_struct()]
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Link: http://lkml.kernel.org/r/20160518170218.GY3192@twins.programming.kicks-ass.netSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent df55f462
......@@ -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);
......
......@@ -210,6 +210,82 @@ void release_task(struct task_struct *p)
goto 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
......
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