rcu: Documentation update for CONFIG_PROVE_RCU
Adds a lockdep.txt file and updates checklist.txt and whatisRCU.txt to reflect the new lockdep-enabled capabilities of RCU. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: mathieu.desnoyers@polymtl.ca Cc: josh@joshtriplett.org Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org Cc: rostedt@goodmis.org Cc: Valdis.Kletnieks@vt.edu Cc: dhowells@redhat.com LKML-Reference: <1266887105-1528-13-git-send-email-paulmck@linux.vnet.ibm.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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@ -6,6 +6,8 @@ checklist.txt
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- Review Checklist for RCU Patches
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listRCU.txt
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- Using RCU to Protect Read-Mostly Linked Lists
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lockdep.txt
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- RCU and lockdep checking
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NMI-RCU.txt
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- Using RCU to Protect Dynamic NMI Handlers
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rcubarrier.txt
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@ -127,10 +127,14 @@ over a rather long period of time, but improvements are always welcome!
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perfectly legal (if redundant) for update-side code to
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use rcu_dereference() and the "_rcu()" list-traversal
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primitives. This is particularly useful in code that
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is common to readers and updaters. However, neither
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rcu_dereference() nor the "_rcu()" list-traversal
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primitives can substitute for a good concurrency design
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coordinating among multiple updaters.
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is common to readers and updaters. However, lockdep
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will complain if you access rcu_dereference() outside
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of an RCU read-side critical section. See lockdep.txt
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to learn what to do about this.
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Of course, neither rcu_dereference() nor the "_rcu()"
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list-traversal primitives can substitute for a good
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concurrency design coordinating among multiple updaters.
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b. If the list macros are being used, the list_add_tail_rcu()
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and list_add_rcu() primitives must be used in order
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@ -249,7 +253,9 @@ over a rather long period of time, but improvements are always welcome!
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must be protected by appropriate update-side locks. RCU
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read-side critical sections are delimited by rcu_read_lock()
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and rcu_read_unlock(), or by similar primitives such as
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rcu_read_lock_bh() and rcu_read_unlock_bh().
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rcu_read_lock_bh() and rcu_read_unlock_bh(), in which case
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the matching rcu_dereference() primitive must be used in order
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to keep lockdep happy, in this case, rcu_dereference_bh().
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The reason that it is permissible to use RCU list-traversal
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primitives when the update-side lock is held is that doing so
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@ -302,15 +308,15 @@ over a rather long period of time, but improvements are always welcome!
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not the case, a self-spawning RCU callback would prevent the
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victim CPU from ever going offline.)
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14. SRCU (srcu_read_lock(), srcu_read_unlock(), synchronize_srcu(),
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and synchronize_srcu_expedited()) may only be invoked from
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process context. Unlike other forms of RCU, it -is- permissible
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to block in an SRCU read-side critical section (demarked by
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srcu_read_lock() and srcu_read_unlock()), hence the "SRCU":
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"sleepable RCU". Please note that if you don't need to sleep
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in read-side critical sections, you should be using RCU rather
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than SRCU, because RCU is almost always faster and easier to
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use than is SRCU.
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14. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
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synchronize_srcu(), and synchronize_srcu_expedited()) may only
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be invoked from process context. Unlike other forms of RCU, it
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-is- permissible to block in an SRCU read-side critical section
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(demarked by srcu_read_lock() and srcu_read_unlock()), hence the
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"SRCU": "sleepable RCU". Please note that if you don't need
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to sleep in read-side critical sections, you should be using
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RCU rather than SRCU, because RCU is almost always faster and
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easier to use than is SRCU.
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Also unlike other forms of RCU, explicit initialization
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and cleanup is required via init_srcu_struct() and
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@ -0,0 +1,67 @@
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RCU and lockdep checking
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All flavors of RCU have lockdep checking available, so that lockdep is
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aware of when each task enters and leaves any flavor of RCU read-side
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critical section. Each flavor of RCU is tracked separately (but note
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that this is not the case in 2.6.32 and earlier). This allows lockdep's
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tracking to include RCU state, which can sometimes help when debugging
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deadlocks and the like.
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In addition, RCU provides the following primitives that check lockdep's
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state:
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rcu_read_lock_held() for normal RCU.
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rcu_read_lock_bh_held() for RCU-bh.
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rcu_read_lock_sched_held() for RCU-sched.
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srcu_read_lock_held() for SRCU.
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These functions are conservative, and will therefore return 1 if they
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aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set).
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This prevents things like WARN_ON(!rcu_read_lock_held()) from giving false
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positives when lockdep is disabled.
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In addition, a separate kernel config parameter CONFIG_PROVE_RCU enables
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checking of rcu_dereference() primitives:
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rcu_dereference(p):
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Check for RCU read-side critical section.
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rcu_dereference_bh(p):
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Check for RCU-bh read-side critical section.
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rcu_dereference_sched(p):
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Check for RCU-sched read-side critical section.
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srcu_dereference(p, sp):
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Check for SRCU read-side critical section.
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rcu_dereference_check(p, c):
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Use explicit check expression "c".
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rcu_dereference_raw(p)
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Don't check. (Use sparingly, if at all.)
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The rcu_dereference_check() check expression can be any boolean
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expression, but would normally include one of the rcu_read_lock_held()
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family of functions and a lockdep expression. However, any boolean
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expression can be used. For a moderately ornate example, consider
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the following:
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file = rcu_dereference_check(fdt->fd[fd],
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rcu_read_lock_held() ||
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lockdep_is_held(&files->file_lock) ||
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atomic_read(&files->count) == 1);
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This expression picks up the pointer "fdt->fd[fd]" in an RCU-safe manner,
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and, if CONFIG_PROVE_RCU is configured, verifies that this expression
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is used in:
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1. An RCU read-side critical section, or
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2. with files->file_lock held, or
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3. on an unshared files_struct.
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In case (1), the pointer is picked up in an RCU-safe manner for vanilla
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RCU read-side critical sections, in case (2) the ->file_lock prevents
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any change from taking place, and finally, in case (3) the current task
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is the only task accessing the file_struct, again preventing any change
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from taking place.
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There are currently only "universal" versions of the rcu_assign_pointer()
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and RCU list-/tree-traversal primitives, which do not (yet) check for
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being in an RCU read-side critical section. In the future, separate
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versions of these primitives might be created.
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@ -323,15 +323,17 @@ used as follows:
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Defer Protect
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a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock()
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call_rcu()
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call_rcu() rcu_dereference()
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b. call_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh()
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rcu_dereference_bh()
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c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched()
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preempt_disable() / preempt_enable()
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local_irq_save() / local_irq_restore()
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hardirq enter / hardirq exit
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NMI enter / NMI exit
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rcu_dereference_sched()
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These three mechanisms are used as follows:
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@ -781,9 +783,8 @@ Linux-kernel source code, but it helps to have a full list of the
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APIs, since there does not appear to be a way to categorize them
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in docbook. Here is the list, by category.
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RCU pointer/list traversal:
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RCU list traversal:
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rcu_dereference
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list_for_each_entry_rcu
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hlist_for_each_entry_rcu
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hlist_nulls_for_each_entry_rcu
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@ -809,7 +810,7 @@ RCU: Critical sections Grace period Barrier
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rcu_read_lock synchronize_net rcu_barrier
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rcu_read_unlock synchronize_rcu
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synchronize_rcu_expedited
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rcu_dereference synchronize_rcu_expedited
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call_rcu
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@ -817,7 +818,7 @@ bh: Critical sections Grace period Barrier
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rcu_read_lock_bh call_rcu_bh rcu_barrier_bh
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rcu_read_unlock_bh synchronize_rcu_bh
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synchronize_rcu_bh_expedited
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rcu_dereference_bh synchronize_rcu_bh_expedited
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sched: Critical sections Grace period Barrier
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@ -826,12 +827,14 @@ sched: Critical sections Grace period Barrier
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rcu_read_unlock_sched call_rcu_sched
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[preempt_disable] synchronize_sched_expedited
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[and friends]
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rcu_dereference_sched
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SRCU: Critical sections Grace period Barrier
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srcu_read_lock synchronize_srcu N/A
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srcu_read_unlock synchronize_srcu_expedited
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srcu_dereference
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SRCU: Initialization/cleanup
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init_srcu_struct
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