Messages in this thread | | | Date | Wed, 9 Dec 2009 18:28:25 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [patch 0/9] Fix various __task_cred related invalid RCU assumptions |
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On Thu, Dec 10, 2009 at 12:52:46AM -0000, Thomas Gleixner wrote: > While auditing the read_lock(&tasklist_lock) sites for a possible > conversion to rcu-read_lock() I stumbled over an unprotected user of > __task_cred in kernel/sys.c > > That caused me to audit all the __task_cred usage sites except in > kernel/exit.c. > > Most of the usage sites are correct, but some of them trip over > invalid assumptions about the protection which is given by RCU. > > - spinlocked/preempt_disabled regions are equivalent to rcu_read_lock(): > > That's wrong. RCU does not guarantee that. > > It has been that way due to implementation details and it still is > valid for CONFIG_TREE_PREEMPT_RCU=n, but there is no guarantee that > this will be the case forever.
To back this up, item #2 from Documentation/RCU/checklist.txt says:
2. Do the RCU read-side critical sections make proper use of rcu_read_lock() and friends? These primitives are needed to prevent grace periods from ending prematurely, which could result in data being unceremoniously freed out from under your read-side code, which can greatly increase the actuarial risk of your kernel.
As a rough rule of thumb, any dereference of an RCU-protected pointer must be covered by rcu_read_lock() or rcu_read_lock_bh() or by the appropriate update-side lock.
> - interrupt disabled regions are equivalent to rcu_read_lock(): > > Wrong again. RCU does not guarantee that. > > It's true for current mainline, but again this is an implementation > detail and there is no guarantee by the RCU semantics. > > Indeed we want to get rid of that to avoid scalability issues on > large systems and preempt-rt got already rid of it to a certain > extent.
Same item #2 above covers this.
The only exception is when you use synchronize_sched(), as described in the "Defer"/"Protect" list near line 323 of the 2.6.32 version of Documentation/RCU/whatisRCU.txt:
Defer Protect
a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock() call_rcu()
b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
c. synchronize_sched() preempt_disable() / preempt_enable() local_irq_save() / local_irq_restore() hardirq enter / hardirq exit NMI enter / NMI exit
And yes, I need to update this based on the addition of rcu_read_lock_sched() and friends. I will be doing another documentation update soon.
> I'm sure we are lucky that CONFIG_TREE_PREEMPT_RCU=y is not yet wide > spread and the code pathes are esoteric enough not to trigger that > subtle races (some of them might just error out silently). > > Nevertheless we need to fix all invalid assumptions about RCU > protection.
Agreed!!!
> The following patch series fixes all yet known affected __task_cred() > sites, but there is more auditing of all other rcu users necessary.
Thank you very much for putting this series together -- I will take a quick look at them.
Thanx, Paul
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