rcu: Add documentation for raw SRCU read-side primitives

Update various files in Documentation/RCU to reflect srcu_read_lock_raw()
and srcu_read_unlock_raw().  Credit to Peter Zijlstra for suggesting
use of the existing _raw suffix instead of the earlier bulkref names.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2011-11-03 13:43:24 -07:00
parent 0c53dd8b31
commit 9ceae0e248
4 changed files with 29 additions and 16 deletions

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@ -328,6 +328,12 @@ over a rather long period of time, but improvements are always welcome!
RCU rather than SRCU, because RCU is almost always faster and
easier to use than is SRCU.
If you need to enter your read-side critical section in a
hardirq or exception handler, and then exit that same read-side
critical section in the task that was interrupted, then you need
to srcu_read_lock_raw() and srcu_read_unlock_raw(), which avoid
the lockdep checking that would otherwise this practice illegal.
Also unlike other forms of RCU, explicit initialization
and cleanup is required via init_srcu_struct() and
cleanup_srcu_struct(). These are passed a "struct srcu_struct"

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@ -38,11 +38,11 @@ o How can the updater tell when a grace period has completed
Preemptible variants of RCU (CONFIG_TREE_PREEMPT_RCU) get the
same effect, but require that the readers manipulate CPU-local
counters. These counters allow limited types of blocking
within RCU read-side critical sections. SRCU also uses
CPU-local counters, and permits general blocking within
RCU read-side critical sections. These two variants of
RCU detect grace periods by sampling these counters.
counters. These counters allow limited types of blocking within
RCU read-side critical sections. SRCU also uses CPU-local
counters, and permits general blocking within RCU read-side
critical sections. These variants of RCU detect grace periods
by sampling these counters.
o If I am running on a uniprocessor kernel, which can only do one
thing at a time, why should I wait for a grace period?

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@ -114,12 +114,11 @@ o A hardware failure. This is quite unlikely, but has occurred
This resulted in a series of RCU CPU stall warnings, eventually
leading the realization that the CPU had failed.
The RCU, RCU-sched, and RCU-bh implementations have CPU stall
warning. SRCU does not have its own CPU stall warnings, but its
calls to synchronize_sched() will result in RCU-sched detecting
RCU-sched-related CPU stalls. Please note that RCU only detects
CPU stalls when there is a grace period in progress. No grace period,
no CPU stall warnings.
The RCU, RCU-sched, and RCU-bh implementations have CPU stall warning.
SRCU does not have its own CPU stall warnings, but its calls to
synchronize_sched() will result in RCU-sched detecting RCU-sched-related
CPU stalls. Please note that RCU only detects CPU stalls when there is
a grace period in progress. No grace period, no CPU stall warnings.
To diagnose the cause of the stall, inspect the stack traces.
The offending function will usually be near the top of the stack.

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@ -834,6 +834,8 @@ SRCU: Critical sections Grace period Barrier
srcu_read_lock synchronize_srcu N/A
srcu_read_unlock synchronize_srcu_expedited
srcu_read_lock_raw
srcu_read_unlock_raw
srcu_dereference
SRCU: Initialization/cleanup
@ -855,27 +857,33 @@ list can be helpful:
a. Will readers need to block? If so, you need SRCU.
b. What about the -rt patchset? If readers would need to block
b. Is it necessary to start a read-side critical section in a
hardirq handler or exception handler, and then to complete
this read-side critical section in the task that was
interrupted? If so, you need SRCU's srcu_read_lock_raw() and
srcu_read_unlock_raw() primitives.
c. What about the -rt patchset? If readers would need to block
in an non-rt kernel, you need SRCU. If readers would block
in a -rt kernel, but not in a non-rt kernel, SRCU is not
necessary.
c. Do you need to treat NMI handlers, hardirq handlers,
d. Do you need to treat NMI handlers, hardirq handlers,
and code segments with preemption disabled (whether
via preempt_disable(), local_irq_save(), local_bh_disable(),
or some other mechanism) as if they were explicit RCU readers?
If so, you need RCU-sched.
d. Do you need RCU grace periods to complete even in the face
e. Do you need RCU grace periods to complete even in the face
of softirq monopolization of one or more of the CPUs? For
example, is your code subject to network-based denial-of-service
attacks? If so, you need RCU-bh.
e. Is your workload too update-intensive for normal use of
f. Is your workload too update-intensive for normal use of
RCU, but inappropriate for other synchronization mechanisms?
If so, consider SLAB_DESTROY_BY_RCU. But please be careful!
f. Otherwise, use RCU.
g. Otherwise, use RCU.
Of course, this all assumes that you have determined that RCU is in fact
the right tool for your job.