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doc: rcu: Remove obsolete checklist item about synchronize_rcu usage

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Since the RCU mechanisms have been consolidated, the checklist item
warning that synchronize_rcu() waits only for RCU readers is obsolete.
This commit therefore removes this checklist item.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Joel Fernandes (Google) 2018-10-05 16:18:13 -07:00 committed by Paul E. McKenney
parent bc2072c9ad
commit e060a03a1c
1 changed files with 7 additions and 30 deletions
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37
Documentation/RCU/checklist.txt
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@ -320,37 +320,14 @@ over a rather long period of time, but improvements are always welcome!
will break Alpha, cause aggressive compilers to generate bad code,
and confuse people trying to read your code.
11. Note that synchronize_rcu() -only- guarantees to wait until
all currently executing rcu_read_lock()-protected RCU read-side
critical sections complete. It does -not- necessarily guarantee
that all currently running interrupts, NMIs, preempt_disable()
code, or idle loops will complete. Therefore, if your
read-side critical sections are protected by something other
than rcu_read_lock(), do -not- use synchronize_rcu().
Similarly, disabling preemption is not an acceptable substitute
for rcu_read_lock(). Code that attempts to use preemption
disabling where it should be using rcu_read_lock() will break
in CONFIG_PREEMPT=y kernel builds.
If you want to wait for interrupt handlers, NMI handlers, and
code under the influence of preempt_disable(), you instead
need to use synchronize_irq() or synchronize_sched().
This same limitation also applies to synchronize_rcu_bh()
and synchronize_srcu(), as well as to the asynchronous and
expedited forms of the three primitives, namely call_rcu(),
call_rcu_bh(), call_srcu(), synchronize_rcu_expedited(),
synchronize_rcu_bh_expedited(), and synchronize_srcu_expedited().
12. Any lock acquired by an RCU callback must be acquired elsewhere
11. Any lock acquired by an RCU callback must be acquired elsewhere
with softirq disabled, e.g., via spin_lock_irqsave(),
spin_lock_bh(), etc. Failing to disable irq on a given
acquisition of that lock will result in deadlock as soon as
the RCU softirq handler happens to run your RCU callback while
interrupting that acquisition's critical section.
13. RCU callbacks can be and are executed in parallel. In many cases,
12. RCU callbacks can be and are executed in parallel. In many cases,
the callback code simply wrappers around kfree(), so that this
is not an issue (or, more accurately, to the extent that it is
an issue, the memory-allocator locking handles it). However,
@ -366,7 +343,7 @@ over a rather long period of time, but improvements are always welcome!
not the case, a self-spawning RCU callback would prevent the
victim CPU from ever going offline.)
14. Unlike other forms of RCU, it -is- permissible to block in an
13. Unlike other forms of RCU, it -is- permissible to block in an
SRCU read-side critical section (demarked by srcu_read_lock()
and srcu_read_unlock()), hence the "SRCU": "sleepable RCU".
Please note that if you don't need to sleep in read-side critical
@ -410,7 +387,7 @@ over a rather long period of time, but improvements are always welcome!
Note that rcu_dereference() and rcu_assign_pointer() relate to
SRCU just as they do to other forms of RCU.
15. The whole point of call_rcu(), synchronize_rcu(), and friends
14. The whole point of call_rcu(), synchronize_rcu(), and friends
is to wait until all pre-existing readers have finished before
carrying out some otherwise-destructive operation. It is
therefore critically important to -first- remove any path
@ -422,13 +399,13 @@ over a rather long period of time, but improvements are always welcome!
is the caller's responsibility to guarantee that any subsequent
readers will execute safely.
16. The various RCU read-side primitives do -not- necessarily contain
15. The various RCU read-side primitives do -not- necessarily contain
memory barriers. You should therefore plan for the CPU
and the compiler to freely reorder code into and out of RCU
read-side critical sections. It is the responsibility of the
RCU update-side primitives to deal with this.
17. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
16. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the
__rcu sparse checks to validate your RCU code. These can help
find problems as follows:
@ -451,7 +428,7 @@ over a rather long period of time, but improvements are always welcome!
These debugging aids can help you find problems that are
otherwise extremely difficult to spot.
18. If you register a callback using call_rcu(), call_rcu_bh(),
17. If you register a callback using call_rcu(), call_rcu_bh(),
call_rcu_sched(), or call_srcu(), and pass in a function defined
within a loadable module, then it in necessary to wait for
all pending callbacks to be invoked after the last invocation