Messages in this thread | | | Date | Mon, 16 Aug 2010 14:24:45 -0400 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH tip/core/rcu 09/10] rcu: update obsolete rcu_read_lock() comment. |
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* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > 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?
Yep, thanks!
Mathieu
> > 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
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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