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rcu: Update docs for rcu_access_pointer and rcu_dereference_protected

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Update examples and lists of APIs to include these new
primitives.

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
Cc: eric.dumazet@gmail.com
LKML-Reference: <1270852752-25278-3-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Paul E. McKenney 2010-04-09 15:39:12 -07:00 committed by Ingo Molnar
parent c08c68dd76
commit 50aec0024e
4 changed files with 57 additions and 21 deletions
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37
Documentation/RCU/NMI-RCU.txt
View file

@ -34,7 +34,7 @@ NMI handler.
cpu = smp_processor_id();
++nmi_count(cpu);
if (!rcu_dereference(nmi_callback)(regs, cpu))
if (!rcu_dereference_sched(nmi_callback)(regs, cpu))
default_do_nmi(regs);
nmi_exit();
@ -47,12 +47,13 @@ function pointer. If this handler returns zero, do_nmi() invokes the
default_do_nmi() function to handle a machine-specific NMI. Finally,
preemption is restored.
Strictly speaking, rcu_dereference() is not needed, since this code runs
only on i386, which does not need rcu_dereference() anyway. However,
it is a good documentation aid, particularly for anyone attempting to
do something similar on Alpha.
In theory, rcu_dereference_sched() is not needed, since this code runs
only on i386, which in theory does not need rcu_dereference_sched()
anyway. However, in practice it is a good documentation aid, particularly
for anyone attempting to do something similar on Alpha or on systems
with aggressive optimizing compilers.
Quick Quiz: Why might the rcu_dereference() be necessary on Alpha,
Quick Quiz: Why might the rcu_dereference_sched() be necessary on Alpha,
given that the code referenced by the pointer is read-only?
@ -99,17 +100,21 @@ invoke irq_enter() and irq_exit() on NMI entry and exit, respectively.
Answer to Quick Quiz
Why might the rcu_dereference() be necessary on Alpha, given
Why might the rcu_dereference_sched() be necessary on Alpha, given
that the code referenced by the pointer is read-only?
Answer: The caller to set_nmi_callback() might well have
initialized some data that is to be used by the
new NMI handler. In this case, the rcu_dereference()
would be needed, because otherwise a CPU that received
an NMI just after the new handler was set might see
the pointer to the new NMI handler, but the old
pre-initialized version of the handler's data.
initialized some data that is to be used by the new NMI
handler. In this case, the rcu_dereference_sched() would
be needed, because otherwise a CPU that received an NMI
just after the new handler was set might see the pointer
to the new NMI handler, but the old pre-initialized
version of the handler's data.
More important, the rcu_dereference() makes it clear
to someone reading the code that the pointer is being
protected by RCU.
This same sad story can happen on other CPUs when using
a compiler with aggressive pointer-value speculation
optimizations.
More important, the rcu_dereference_sched() makes it
clear to someone reading the code that the pointer is
being protected by RCU-sched.

7
Documentation/RCU/checklist.txt
View file

@ -260,7 +260,8 @@ over a rather long period of time, but improvements are always welcome!
The reason that it is permissible to use RCU list-traversal
primitives when the update-side lock is held is that doing so
can be quite helpful in reducing code bloat when common code is
shared between readers and updaters.
shared between readers and updaters. Additional primitives
are provided for this case, as discussed in lockdep.txt.
10. Conversely, if you are in an RCU read-side critical section,
and you don't hold the appropriate update-side lock, you -must-
@ -344,8 +345,8 @@ over a rather long period of time, but improvements are always welcome!
requiring SRCU's read-side deadlock immunity or low read-side
realtime latency.
Note that, rcu_assign_pointer() and rcu_dereference() relate to
SRCU just as they do to other forms of RCU.
Note that, rcu_assign_pointer() relates to SRCU just as they do
to other forms of RCU.
15. The whole point of call_rcu(), synchronize_rcu(), and friends
is to wait until all pre-existing readers have finished before

28
Documentation/RCU/lockdep.txt
View file

@ -32,9 +32,20 @@ checking of rcu_dereference() primitives:
srcu_dereference(p, sp):
Check for SRCU read-side critical section.
rcu_dereference_check(p, c):
Use explicit check expression "c".
Use explicit check expression "c". This is useful in
code that is invoked by both readers and updaters.
rcu_dereference_raw(p)
Don't check. (Use sparingly, if at all.)
rcu_dereference_protected(p, c):
Use explicit check expression "c", and omit all barriers
and compiler constraints. This is useful when the data
structure cannot change, for example, in code that is
invoked only by updaters.
rcu_access_pointer(p):
Return the value of the pointer and omit all barriers,
but retain the compiler constraints that prevent duplicating
or coalescsing. This is useful when when testing the
value of the pointer itself, for example, against NULL.
The rcu_dereference_check() check expression can be any boolean
expression, but would normally include one of the rcu_read_lock_held()
@ -59,7 +70,20 @@ In case (1), the pointer is picked up in an RCU-safe manner for vanilla
RCU read-side critical sections, in case (2) the ->file_lock prevents
any change from taking place, and finally, in case (3) the current task
is the only task accessing the file_struct, again preventing any change
from taking place.
from taking place. If the above statement was invoked only from updater
code, it could instead be written as follows:
file = rcu_dereference_protected(fdt->fd[fd],
lockdep_is_held(&files->file_lock) ||
atomic_read(&files->count) == 1);
This would verify cases #2 and #3 above, and furthermore lockdep would
complain if this was used in an RCU read-side critical section unless one
of these two cases held. Because rcu_dereference_protected() omits all
barriers and compiler constraints, it generates better code than do the
other flavors of rcu_dereference(). On the other hand, it is illegal
to use rcu_dereference_protected() if either the RCU-protected pointer
or the RCU-protected data that it points to can change concurrently.
There are currently only "universal" versions of the rcu_assign_pointer()
and RCU list-/tree-traversal primitives, which do not (yet) check for

6
Documentation/RCU/whatisRCU.txt
View file

@ -840,6 +840,12 @@ SRCU: Initialization/cleanup
init_srcu_struct
cleanup_srcu_struct
All: lockdep-checked RCU-protected pointer access
rcu_dereference_check
rcu_dereference_protected
rcu_access_pointer
See the comment headers in the source code (or the docbook generated
from them) for more information.