Messages in this thread | | | Date | Fri, 20 Feb 2009 07:41:09 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] mm: fix lazy vmap purging (use-after-free error) |
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On Fri, Feb 20, 2009 at 04:18:12PM +0200, Pekka Enberg wrote: > On Fri, 2009-02-20 at 15:01 +0100, Ingo Molnar wrote: > > * Ingo Molnar <mingo@elte.hu> wrote: > > > > > ah, indeed: > > > > > > list_del_rcu(&va->list); > > > > > > i suspect it could be hit big time in a workload that opens > > > more than 512 files, as expand_files() uses a > > > vmalloc()+vfree() pair in that case. > > > > hm, perhaps it's not a problem after all. The freeing is done > > via rcu, and list_del_rcu() leaves the forward pointer intact. > > > > So how did it happen that the entry got kfree()d before the loop > > was done? We are in a spinlocked section so the CPU should not > > have entered rcu processing. > > RCU. Lets CC Paul. > > Looking at it, Documentation/RCU/checklist.txt states that: > > 7. If the updater uses call_rcu(), then the corresponding readers > must use rcu_read_lock() and rcu_read_unlock(). > > which we don't do for this loop. I fail to see how it could be a > kmemcheck false positive, so it's probably a real bug.
And checklist.txt rule #7 is even more important in preemptable kernels, especially if CONFIG_PREEMPT_RT.
Thanx, Paul
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