Messages in this thread Patch in this message | | | Date | Mon, 16 Aug 2010 10:55:07 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 09/10] rcu: update obsolete rcu_read_lock() comment. |
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On Mon, Aug 16, 2010 at 10:45:32AM -0400, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > The comment says that blocking is illegal in rcu_read_lock()-style > > RCU read-side critical sections, which is no longer entirely true > > given preemptible RCU. This commit provides a fix. > > > > Suggested-by: David Miller <davem@davemloft.net> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > --- > > include/linux/rcupdate.h | 15 ++++++++++++++- > > 1 files changed, 14 insertions(+), 1 deletions(-) > > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h > > index 24b8966..d7af96e 100644 > > --- a/include/linux/rcupdate.h > > +++ b/include/linux/rcupdate.h > > @@ -458,7 +458,20 @@ extern int rcu_my_thread_group_empty(void); > > * will be deferred until the outermost RCU read-side critical section > > * completes. > > * > > - * It is illegal to block while in an RCU read-side critical section. > > + * You can avoid reading and understanding the next paragraph by > > + * following this rule: don't put anything in an rcu_read_lock() RCU > > + * read-side critical section that would block in a !PREEMPT kernel. > > + * But if you want the full story, read on! > > + * > > + * In non-preemptible RCU implementations (TREE_RCU and TINY_RCU), it > > + * is illegal to block while in an RCU read-side critical section. In > > + * preemptible RCU implementations (TREE_PREEMPT_RCU and TINY_PREEMPT_RCU) > > + * in CONFIG_PREEMPT kernel builds, RCU read-side critical sections may > > + * be preempted, but explicit blocking is illegal. Finally, in preemptible > > + * RCU implementations in real-time (CONFIG_PREEMPT_RT) kernel builds, > > + * RCU read-side critical sections may be preempted and they may also > > + * block, but only when acquiring spinlocks that are subject to priority > > + * inheritance. > > It might be good to add a note about locking chain dependency that is > created in the RT case, e.g., the lock we are sharing with another > context in preempt RT is subject to the same rules as the RCU C.S.. It > should never call synchronize_rcu(); this would cause a RCU+lock-induced > deadlock. > > I must admit, however, that because calling synchronize_rcu() from > spinlocks is already forbidden, this is already implied.
Thank you for looking this over!
I am updating the srcu_read_lock() docbook comments to call out the potential for this problem, given that SRCU read-side critical sections can acquire mutexes, which can be held across both synchronize_srcu() and synchronize_srcu_expedited().
Seem reasonable?
Thanx, Paul
diff --git a/include/linux/srcu.h b/include/linux/srcu.h index 6f456a7..58971e8 100644 --- a/include/linux/srcu.h +++ b/include/linux/srcu.h @@ -139,7 +139,12 @@ static inline int srcu_read_lock_held(struct srcu_struct *sp) * @sp: srcu_struct in which to register the new reader. * * Enter an SRCU read-side critical section. Note that SRCU read-side - * critical sections may be nested. + * critical sections may be nested. However, it is illegal to + * call anything that waits on an SRCU grace period for the same + * srcu_struct, whether directly or indirectly. Please note that + * one way to indirectly wait on an SRCU grace period is to acquire + * a mutex that is held elsewhere while calling synchronize_srcu() or + * synchronize_srcu_expedited(). */ static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp) { > Thanks, > > Mathieu > > > */ > > static inline void rcu_read_lock(void) > > { > > -- > > 1.7.0.6 > > > > -- > Mathieu Desnoyers > Operating System Efficiency R&D Consultant > EfficiOS Inc. > http://www.efficios.com
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