Re: allow preemption in check_task_state
From: Steven Rostedt <rostedt@goodmis.org>
Date: 2014-02-10 16:11:17
Also in:
lkml
Subject is missing patch number. On Mon, 10 Feb 2014 16:38:56 +0100 Nicholas Mc Guire [off-list ref] wrote:
A lockfree approach to check_task_state This treates the state as an indicator variable and use it to probe saved_state lock free. There is actually no consistency demand on state/saved_state but rather a consistency demand on the transitions of the two variables but those transition, based on path inspection, are not independent. Its probably not faster than the lock/unlock case if uncontended - atleast it does not show up in benchmark results, but it would never be hit by a full pi-boost cycle as there is no contention. This also was tested against the test-case from Sebastian as well as rnning a few scripted gdb breakpoint debugging/single-stepping loops to trigger this.
To trigger what?
quoted hunk ↗ jump to hunk
Tested-by: Andreas Platschek <redacted> Tested-by: Carsten Emde <redacted> Signed-off-by: Nicholas Mc Guire <redacted> --- kernel/sched/core.c | 10 ++++++++-- 1 files changed, 8 insertions(+), 2 deletions(-)diff --git a/kernel/sched/core.c b/kernel/sched/core.c index bf93f63..5690ba3 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c@@ -1074,11 +1074,17 @@ static int migration_cpu_stop(void *data); static bool check_task_state(struct task_struct *p, long match_state) { bool match = false; + long state, saved_state; + + /* catch restored state */ + do { + state = p->state; + saved_state = p->saved_state; + rmb(); /* make sure we actually catch updates */
The problem I have with this is that there's no matching wmb(). Also, shouldn't that be a smp_rmb(), I don't think we can race with devices here.
+ } while (state != p->state); - raw_spin_lock_irq(&p->pi_lock); if (p->state == match_state || p->saved_state == match_state) match = true; - raw_spin_unlock_irq(&p->pi_lock); return match; }
In rtmutex.c we have: pi_lock(&self->pi_lock); __set_current_state(self->saved_state); self->saved_state = TASK_RUNNING; pi_unlock(&self->pi_lock); As there is no wmb() here, it can be very possible that another CPU will see saved_state as TASK_RUNNING, and current state as TASK_RUNNING, and miss the update completely. I would not want to add a wmb() unless there is a real bug with the check state, as the above is in a very fast path and the check state is in a slower path. -- Steve