On Mon, 5 Mar 2012, Steven Rostedt wrote: > Thomas, > > I was running my cpu hotplug stress test along with a kernel compile and > after about 40 minutes of running it locked up. It happened in the > read_seqcount_begin() that is called by d_lookup(). > > ksoftirqd was caught here: > > static __always_inline unsigned read_seqbegin(const seqlock_t *sl) > { > unsigned ret; > > repeat: > ret = ACCESS_ONCE(sl->sequence); > if (unlikely(ret & 1)) { > cpu_relax(); > goto repeat; > } > smp_rmb(); > > return ret; > } > > It preempted the holder of the seqlock that was held for write, and as > that holder had migrate disabled, it couldn't be scheduled. Then > ksoftirqd went into this infinite loop and the system locked up. > > This patch fixes the issue by grabbing and releasing the write lock when > it detects contention. It only works with seqlocks and not seqcounts > that have their own locking. But we could add an api to include those > too if needed. Errm. rt15 has /* * Starvation safe read side for RT */ static inline unsigned read_seqbegin(seqlock_t *sl) { unsigned ret; repeat: ret = sl->seqcount.sequence; if (unlikely(ret & 1)) { /* * Take the lock and let the writer proceed (i.e. evtl * boost it), otherwise we could loop here forever. */ spin_lock(&sl->lock); spin_unlock(&sl->lock); goto repeat; } return ret; } #endif > Because read_seqlocks are used in the VDSO area, a raw_read_seqcount_begin() > was created to allow userspace tasks to access read_seqcount(). > As the grabbing of the write_lock() is not allowed in VDSO, nor > is even referencing it. This is completely bogus. The VDSO update write side runs with interrupts disabled, so it cannot be preempted at all. > Note, a live lock can still happen if the userspace task that > does the read_seqlock is of higher priority than a user doing > the write_lock, so userspace needs to be careful. What the hell are you smoking? Thanks, tglx -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html