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sched/wait, RCU: Introduce rcuwait machinery

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rcuwait provides support for (single) RCU-safe task wait/wake functionality,
with the caveat that it must not be called after exit_notify(), such that
we avoid racing with rcu delayed_put_task_struct callbacks, task_struct
being rcu unaware in this context -- for which we similarly have
task_rcu_dereference() magic, but with different return semantics, which
can conflict with the wakeup side.

The interfaces are quite straightforward:

  rcuwait_wait_event()
  rcuwait_wake_up()

More details are in the comments, but it's perhaps worth mentioning at least,
that users must provide proper serialization when waiting on a condition, and
avoid corrupting a concurrent waiter. Also care must be taken between the task
and the condition for when calling the wakeup -- we cannot miss wakeups. When
porting users, this is for example, a given when using waitqueues in that
everything is done under the q->lock. As such, it can remove sources of non
preemptable unbounded work for realtime.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave@stgolabs.net
Link: http://lkml.kernel.org/r/1484148146-14210-2-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Davidlohr Bueso
2017-01-11 07:22:25 -08:00
committed by Ingo Molnar
parent 642fa448ae
commit 8f95c90ceb
2 changed files with 93 additions and 0 deletions
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30
kernel/exit.c
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@@ -55,6 +55,7 @@
#include <linux/shm.h>
#include <linux/kcov.h>
#include <linux/random.h>
#include <linux/rcuwait.h>
#include <linux/uaccess.h>
#include <asm/unistd.h>
@@ -282,6 +283,35 @@ retry:
return task;
}
void rcuwait_wake_up(struct rcuwait *w)
{
struct task_struct *task;
rcu_read_lock();
/*
* Order condition vs @task, such that everything prior to the load
* of @task is visible. This is the condition as to why the user called
* rcuwait_trywake() in the first place. Pairs with set_current_state()
* barrier (A) in rcuwait_wait_event().
*
* WAIT WAKE
* [S] tsk = current [S] cond = true
* MB (A) MB (B)
* [L] cond [L] tsk
*/
smp_rmb(); /* (B) */
/*
* Avoid using task_rcu_dereference() magic as long as we are careful,
* see comment in rcuwait_wait_event() regarding ->exit_state.
*/
task = rcu_dereference(w->task);
if (task)
wake_up_process(task);
rcu_read_unlock();
}
struct task_struct *try_get_task_struct(struct task_struct **ptask)
{
struct task_struct *task;