Re: finish_task_switch && prev_state (Was: sched, timers: use after free in __lock_task_sighand when exiting a process)
From: Peter Zijlstra
Date: Tue Jul 29 2014 - 05:22:48 EST
On Tue, Jul 29, 2014 at 11:10:18AM +0200, Peter Zijlstra wrote:
> On Tue, Jul 15, 2014 at 04:25:25PM +0200, Oleg Nesterov wrote:
> > And probably I missed something again, but it seems that this logic is broken
> > with __ARCH_WANT_UNLOCKED_CTXSW.
> > Of course, even if I am right this is pure theoretical, but smp_wmb() before
> > "->on_cpu = 0" is not enough and we need a full barrier ?
> (long delay there, forgot about this thread, sorry)
> Yes, I think I see that.. but now I think the comment is further wrong.
> Its not rq->lock that is important, remember, a concurrent wakeup onto
> another CPU does not require our rq->lock at all.
> It is the ->on_cpu = 0 store that is important (for both the
> UNLOCKED_CTXSW cases). As soon as that store comes through the task can
> start running on the remote cpu.
> Now the below patch 'fixes' this but at the cost of adding a full
> barrier which is somewhat unfortunate to say the least.
> wmb's are free on x86 and generally cheaper than mbs, so it would to
> find another solution to this problem...
Something like so then?
kernel/sched/core.c | 36 ++++++++++++++++++++----------------
1 file changed, 20 insertions(+), 16 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2676866b4394..179390f7380d 100644
@@ -2190,6 +2190,7 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,
* finish_task_switch - clean up after a task-switch
* @rq: runqueue associated with task-switch
* @prev: the thread we just switched away from.
+ * @prev_state: the state of @prev before we switched away from it.
* finish_task_switch must be called after the context switch, paired
* with a prepare_task_switch call before the context switch.
@@ -2201,26 +2202,14 @@ prepare_task_switch(struct rq *rq, struct task_struct *prev,
* with the lock held can cause deadlocks; see schedule() for
-static void finish_task_switch(struct rq *rq, struct task_struct *prev)
+finish_task_switch(struct rq *rq, struct task_struct *prev, long prev_state)
struct mm_struct *mm = rq->prev_mm;
- long prev_state;
rq->prev_mm = NULL;
- * A task struct has one reference for the use as "current".
- * If a task dies, then it sets TASK_DEAD in tsk->state and calls
- * schedule one last time. The schedule call will never return, and
- * the scheduled task must drop that reference.
- * The test for TASK_DEAD must occur while the runqueue locks are
- * still held, otherwise prev could be scheduled on another cpu, die
- * there before we look at prev->state, and then the reference would
- * be dropped twice.
- * Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
- prev_state = prev->state;
@@ -2279,7 +2268,7 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
struct rq *rq = this_rq();
- finish_task_switch(rq, prev);
+ finish_task_switch(rq, prev, 0);
* FIXME: do we need to worry about rq being invalidated by the
@@ -2304,6 +2293,21 @@ context_switch(struct rq *rq, struct task_struct *prev,
struct task_struct *next)
struct mm_struct *mm, *oldmm;
+ * A task struct has one reference for the use as "current".
+ * If a task dies, then it sets TASK_DEAD in tsk->state and calls
+ * schedule one last time. The schedule call will never return, and
+ * the scheduled task must drop that reference.
+ * We must observe prev->state before clearing prev->on_cpu (in
+ * finish_lock_switch), otherwise a concurrent wakeup can get prev
+ * running on another CPU and we could race with its RUNNING -> DEAD
+ * transition, and then the reference would be dropped twice.
+ * We avoid the race by observing prev->state while it is still
+ * current.
+ long prev_state = prev->state;
prepare_task_switch(rq, prev, next);
@@ -2347,7 +2351,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
* CPUs since it called schedule(), thus the 'rq' on its stack
* frame will be invalid.
- finish_task_switch(this_rq(), prev);
+ finish_task_switch(this_rq(), prev, prev_state);
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