Messages in this thread | | | Subject | Re: [patch 0/9] Fix various __task_cred related invalid RCU assumptions | From | Peter Zijlstra <> | Date | Thu, 10 Dec 2009 06:13:51 +0100 |
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On Wed, 2009-12-09 at 19:15 -0800, Linus Torvalds wrote: > > On Wed, 9 Dec 2009, Paul E. McKenney wrote: > > > 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: > > Hmm. This seems to be a difference that the tree-RCU things introduced, > no? I wonder if we have other areas where we just knew that a spinlock > would make an rcu read-lock unnecessary (which used to be true..)
That failed being true when we merged PREEMPT_RCU,.. which was a long time ago.
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