Re: [PATCH v2 3/3] sched/preempt: fix cond_resched_lock() and cond_resched_softirq()

On Wed, Jul 15, 2015 at 03:52:34PM +0300, Konstantin Khlebnikov wrote:

On 15.07.2015 15:16, Eric Dumazet wrote:

quoted

On Wed, 2015-07-15 at 12:52 +0300, Konstantin Khlebnikov wrote:

quoted

These functions check should_resched() before unlocking spinlock/bh-enable:
preempt_count always non-zero => should_resched() always returns false.
cond_resched_lock() worked iff spin_needbreak is set.

Interesting, this definitely used to work (linux-3.11)

Any idea of which commit broke things ?

Searching... done

This one: bdb43806589096ac4272fe1307e789846ac08d7c in v3.13

before

-static inline int should_resched(void)
-{
-       return need_resched() && !(preempt_count() & PREEMPT_ACTIVE);
-}

after

+static __always_inline bool should_resched(void)
+{
+       return unlikely(!*preempt_count_ptr());
+}

Argh, indeed!

So,

Fixes: bdb438065890 ("sched: Extract the basic add/sub preempt_count
modifiers")

Thanks!