From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4C6A73D0.3040401@domain.hid> Date: Tue, 17 Aug 2010 13:34:40 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <4C6A6FE7.6030508@domain.hid> <4C6A70D7.4040306@domain.hid> In-Reply-To: <4C6A70D7.4040306@domain.hid> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Subject: Re: [Adeos-main] Deadlock-prone ipipe_critical_enter List-Id: General discussion about Adeos List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: adeos-main Gilles Chanteperdrix wrote: > Jan Kiszka wrote: >> Hi, >> >> it turned out ipipe_critical_enter is broken on SMP > 2 CPUs: On one >> CPU, Linux may have acquired an rwlock for reading when being preempted >> by the critical IPI. On some other CPU, Linux may have entered >> write_lock_irq[save] before the IPI arrived. The reader will be stuck in >> __ipipe_do_critical_sync, the writer in __write_lock_failed - forever. >> First seen on real silicon (once per "few" hundreds of boots), finally >> caught under KVM and nailed down. >> >> Two approaches to resolve this issue come to my mind so far. The first >> one is to restart the whole ipipe_critical_enter after some (how many?) >> cycles of futile waiting. The other is to accept the critical IPI even >> if the top-most domain is stalled (as it sits in write_lock_irq), but >> I'm not 100% that our optimistic IRQ mask will always allow this when >> Linux is on the top (I assume we can safely require other domains to >> avoid such deadlocks by design). >> >> Comments? Better ideas? > > I guess, the rwlocks are ipipe rwlocks, right? Nope, plain Linux tasklist_lock. No Xenomai domain active at this point, just Linux. > I am not sure it is different from your second idea, but what about > spinning in write_lock_irq/save with irqs on? Hard-IRQs on (which is what my second idea would rely on) or Linux IRQs on (which would involve patching Linux spinlock arch code and may have side effects)? Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux