Messages in this thread Patch in this message | | | Subject | Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM | From | Peter Zijlstra <> | Date | Thu, 26 May 2011 14:21:51 +0200 |
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On Thu, 2011-05-26 at 13:32 +0200, Peter Zijlstra wrote: > > The bad news is of course that I've got a little more head-scratching to > do, will keep you informed.
OK, that wasn't too hard.. (/me crosses fingers and prays Marc doesn't find more funnies ;-).
Does the below cure all woes?
--- Subject: sched: Fix ttwu() for __ARCH_WANT_INTERRUPTS_ON_CTXSW From: Peter Zijlstra <a.p.zijlstra@chello.nl> Date: Thu May 26 14:21:33 CEST 2011
Marc reported that e4a52bcb9 (sched: Remove rq->lock from the first half of ttwu()) broke his ARM-SMP machine. Now ARM is one of the few __ARCH_WANT_INTERRUPTS_ON_CTXSW users, so that exception in the ttwu() code was suspect.
Yong found that the interrupt could hit hits after context_switch() changes current but before it clears p->on_cpu, if that interrupt were to attempt a wake-up of p we would indeed find ourselves spinning in IRQ context.
Sort this by reverting to the old behaviour for this situation and perform a full remote wake-up.
Cc: Frank Rowand <frank.rowand@am.sony.com> Cc: Yong Zhang <yong.zhang0@gmail.com> Cc: Oleg Nesterov <oleg@redhat.com> Reported-by: Marc Zyngier <Marc.Zyngier@arm.com> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> --- kernel/sched.c | 37 ++++++++++++++++++++++++++++--------- 1 file changed, 28 insertions(+), 9 deletions(-)
Index: linux-2.6/kernel/sched.c =================================================================== --- linux-2.6.orig/kernel/sched.c +++ linux-2.6/kernel/sched.c @@ -2573,7 +2573,26 @@ static void ttwu_queue_remote(struct tas if (!next) smp_send_reschedule(cpu); } -#endif + +#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW +static int ttwu_activate_remote(struct task_struct *p, int wake_flags) +{ + struct rq *rq; + int ret = 0; + + rq = __task_rq_lock(p); + if (p->on_cpu) { + ttwu_activate(rq, p, ENQUEUE_WAKEUP); + ttwu_do_wakeup(rq, p, wake_flags); + ret = 1; + } + __task_rq_unlock(rq); + + return ret; + +} +#endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */ +#endif /* CONFIG_SMP */ static void ttwu_queue(struct task_struct *p, int cpu) { @@ -2631,17 +2650,17 @@ try_to_wake_up(struct task_struct *p, un while (p->on_cpu) { #ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW /* - * If called from interrupt context we could have landed in the - * middle of schedule(), in this case we should take care not - * to spin on ->on_cpu if p is current, since that would - * deadlock. + * In case the architecture enables interrupts in + * context_switch(), we cannot busy wait, since that + * would lead to live-locks when an interrupt hits and + * tries to wake up @prev. So bail and do a complete + * remote wakeup. */ - if (p == current) { - ttwu_queue(p, cpu); + if (ttwu_activate_remote(p, wake_flags)) goto stat; - } -#endif +#else cpu_relax(); +#endif } /* * Pairs with the smp_wmb() in finish_lock_switch().
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