Re: [RFC][PATCH] Fix a race between rwsem and the scheduler

[Oleg Nesterov]

On 08/30, Peter Zijlstra wrote:
>
>       /*
>        * Ensure we load p->on_rq _after_ p->state, otherwise it would
>        * be possible to, falsely, observe p->on_rq == 0 and get stuck
>        * in smp_cond_load_acquire() below.
>        *
>        * sched_ttwu_pending()                 try_to_wake_up()
>        *   [S] p->on_rq = 1;                  [L] P->state
>        *       UNLOCK rq->lock
>        *
>        * schedule()                           RMB
>        *       LOCK rq->lock
>        *       UNLOCK rq->lock
>        *
>        * [task p]
>        *   [S] p->state = UNINTERRUPTIBLE     [L] p->on_rq
>        *
>        * Pairs with the UNLOCK+LOCK on rq->lock from the
>        * last wakeup of our task and the schedule that got our task
>        * current.
>        */

Confused... how this connects to UNLOCK+LOCK on rq->lock? A LOAD can
leak into the critical section.

But context switch should imply mb() we can rely on?

Oleg.