Messages in this thread | | | Date | Sun, 25 Mar 2007 07:58:16 +0530 | From | Srivatsa Vaddagiri <> | Subject | Re: [ckrm-tech] [PATCH 1/7] containers (V7): Generic container system abstracted from cpusets code |
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On Sat, Mar 24, 2007 at 06:41:28PM -0700, Paul Jackson wrote: > > the following code becomes racy with cpuset_exit() ... > > > > atomic_inc(&cs->count); > > rcu_assign_pointer(tsk->cpuset, cs); > > task_unlock(tsk); > > eh ... so ... ? > > I don't know of any sequence where that causes any problem. > > Do you see one?
Let's say we had two cpusets CS1 amd CS2 (both different from top_cpuset). CS1 has just one task T1 in it (CS1->count = 0) while CS2 has no tasks in it (CS2->count = 0).
Now consider:
-------------------------------------------------------------------- CPU0 (attach_task T1 to CS2) CPU1 (T1 is exiting) --------------------------------------------------------------------
task_lock(T1);
oldcs = tsk->cpuset; [oldcs = CS1]
T1->flags & PF_EXITING? (No)
T1->flags = PF_EXITING;
atomic_inc(&CS2->count);
cpuset_exit() cs = tsk->cpuset; (cs = CS1) T1->cpuset = CS2;
T1->cpuset = &top_cpuset;
task_unlock(T1);
CS2 has one bogus count now (with no tasks in it), which may prevent it from being removed/freed forever.
Not just this, continuing further we have more trouble:
-------------------------------------------------------------------- CPU0 (attach_task T1 to CS2) CPU1 (T1 is exiting) --------------------------------------------------------------------
synchronize_rcu() atomic_dec(&CS1->count); [CS1->count = 0]
if atomic_dec_and_test(&oldcs->count)) [CS1->count = -1]
We now have CS1->count negative. Is that good? I am uncomfortable ..
We need a task_lock() in cpuset_exit to avoid this race.
-- Regards, vatsa - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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