blk-cgroup: Fix RCU correctness warning in cfq_init_queue()
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From: Li Zefan on 25 Apr 2010 21:40 >>>>>> With RCU correctness on, We see following warning. This patch fixes it. >>>>> This is in initialization code, so that there cannot be any concurrent >>>>> updates, correct? If so, looks good. >>>>> >>>> I think theoritically two instances of cfq_init_queue() can be running >>>> in parallel (for two different devices), and they both can call >>>> blkiocg_add_blkio_group(). But then we use a spin lock to protect >>>> blkio_cgroup. >>>> >>>> spin_lock_irqsave(&blkcg->lock, flags); >>>> >>>> So I guess two parallel updates should be fine. >>> OK, in that case, would it be possible add this spinlock to the condition >>> checked by css_id()'s rcu_dereference_check()? >> Hi Paul, >> >> I think adding these spinlock to condition checked might become little >> messy. And the reason being that this lock is subsystem (controller) >> specific and maintained by controller. Now if any controller implements >> a lock and we add that lock in css_id() rcu_dereference_check(), it will >> look ugly. >> >> So probably a better way is to make sure that css_id() is always called >> under rcu read lock so that we don't hit this warning? > > As long as holding rcu_read_lock() prevents css_id() from the usual > problems such as access memory that was concurrently freed, yes. > blkiocg_add_blkio_group() also calls cgroup_path(), which also needs to be called within rcu_read_lock, so I think Vivek's patch is better than the one you posted in another mail thread. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |