Commit 74d874e7 authored by Paul E. McKenney's avatar Paul E. McKenney

rcu: Update documentation to cover call_srcu() and srcu_barrier().

The advent of call_srcu() and srcu_barrier() obsoleted some of the
documentation, so this commit brings that up to date.
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent cba6d0d6
......@@ -162,9 +162,9 @@ over a rather long period of time, but improvements are always welcome!
when publicizing a pointer to a structure that can
be traversed by an RCU read-side critical section.
5. If call_rcu(), or a related primitive such as call_rcu_bh() or
call_rcu_sched(), is used, the callback function must be
written to be called from softirq context. In particular,
5. If call_rcu(), or a related primitive such as call_rcu_bh(),
call_rcu_sched(), or call_srcu() is used, the callback function
must be written to be called from softirq context. In particular,
it cannot block.
6. Since synchronize_rcu() can block, it cannot be called from
......@@ -202,11 +202,12 @@ over a rather long period of time, but improvements are always welcome!
updater uses call_rcu_sched() or synchronize_sched(), then
the corresponding readers must disable preemption, possibly
by calling rcu_read_lock_sched() and rcu_read_unlock_sched().
If the updater uses synchronize_srcu(), the the corresponding
readers must use srcu_read_lock() and srcu_read_unlock(),
and with the same srcu_struct. The rules for the expedited
primitives are the same as for their non-expedited counterparts.
Mixing things up will result in confusion and broken kernels.
If the updater uses synchronize_srcu() or call_srcu(),
the the corresponding readers must use srcu_read_lock() and
srcu_read_unlock(), and with the same srcu_struct. The rules for
the expedited primitives are the same as for their non-expedited
counterparts. Mixing things up will result in confusion and
broken kernels.
One exception to this rule: rcu_read_lock() and rcu_read_unlock()
may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
......@@ -333,14 +334,14 @@ over a rather long period of time, but improvements are always welcome!
victim CPU from ever going offline.)
14. SRCU (srcu_read_lock(), srcu_read_unlock(), srcu_dereference(),
synchronize_srcu(), and synchronize_srcu_expedited()) may only
be invoked from process context. 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 sections, you should be using
RCU rather than SRCU, because RCU is almost always faster and
easier to use than is SRCU.
synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu())
may only be invoked from process context. 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 sections, you should be
using 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
......@@ -353,8 +354,8 @@ over a rather long period of time, but improvements are always welcome!
cleanup_srcu_struct(). These are passed a "struct srcu_struct"
that defines the scope of a given SRCU domain. Once initialized,
the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
synchronize_srcu(), and synchronize_srcu_expedited(). A given
synchronize_srcu() waits only for SRCU read-side critical
synchronize_srcu(), synchronize_srcu_expedited(), and call_srcu().
A given synchronize_srcu() waits only for SRCU read-side critical
sections governed by srcu_read_lock() and srcu_read_unlock()
calls that have been passed the same srcu_struct. This property
is what makes sleeping read-side critical sections tolerable --
......@@ -374,7 +375,7 @@ 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() relates to SRCU just as they do
Note that, rcu_assign_pointer() relates to SRCU just as it does
to other forms of RCU.
15. The whole point of call_rcu(), synchronize_rcu(), and friends
......
......@@ -79,8 +79,6 @@ complete. Pseudo-code using rcu_barrier() is as follows:
2. Execute rcu_barrier().
3. Allow the module to be unloaded.
Quick Quiz #1: Why is there no srcu_barrier()?
The rcutorture module makes use of rcu_barrier in its exit function
as follows:
......@@ -162,7 +160,7 @@ for any pre-existing callbacks to complete.
Then lines 55-62 print status and do operation-specific cleanup, and
then return, permitting the module-unload operation to be completed.
Quick Quiz #2: Is there any other situation where rcu_barrier() might
Quick Quiz #1: Is there any other situation where rcu_barrier() might
be required?
Your module might have additional complications. For example, if your
......@@ -242,7 +240,7 @@ reaches zero, as follows:
4 complete(&rcu_barrier_completion);
5 }
Quick Quiz #3: What happens if CPU 0's rcu_barrier_func() executes
Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
immediately (thus incrementing rcu_barrier_cpu_count to the
value one), but the other CPU's rcu_barrier_func() invocations
are delayed for a full grace period? Couldn't this result in
......@@ -259,12 +257,7 @@ so that your module may be safely unloaded.
Answers to Quick Quizzes
Quick Quiz #1: Why is there no srcu_barrier()?
Answer: Since there is no call_srcu(), there can be no outstanding SRCU
callbacks. Therefore, there is no need to wait for them.
Quick Quiz #2: Is there any other situation where rcu_barrier() might
Quick Quiz #1: Is there any other situation where rcu_barrier() might
be required?
Answer: Interestingly enough, rcu_barrier() was not originally
......@@ -278,7 +271,7 @@ Answer: Interestingly enough, rcu_barrier() was not originally
implementing rcutorture, and found that rcu_barrier() solves
this problem as well.
Quick Quiz #3: What happens if CPU 0's rcu_barrier_func() executes
Quick Quiz #2: What happens if CPU 0's rcu_barrier_func() executes
immediately (thus incrementing rcu_barrier_cpu_count to the
value one), but the other CPU's rcu_barrier_func() invocations
are delayed for a full grace period? Couldn't this result in
......
......@@ -174,11 +174,20 @@ torture_type The type of RCU to test, with string values as follows:
and synchronize_rcu_bh_expedited().
"srcu": srcu_read_lock(), srcu_read_unlock() and
call_srcu().
"srcu_sync": srcu_read_lock(), srcu_read_unlock() and
synchronize_srcu().
"srcu_expedited": srcu_read_lock(), srcu_read_unlock() and
synchronize_srcu_expedited().
"srcu_raw": srcu_read_lock_raw(), srcu_read_unlock_raw(),
and call_srcu().
"srcu_raw_sync": srcu_read_lock_raw(), srcu_read_unlock_raw(),
and synchronize_srcu().
"sched": preempt_disable(), preempt_enable(), and
call_rcu_sched().
......
......@@ -833,9 +833,9 @@ sched: Critical sections Grace period Barrier
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_lock synchronize_srcu srcu_barrier
srcu_read_unlock call_srcu
srcu_read_lock_raw synchronize_srcu_expedited
srcu_read_unlock_raw
srcu_dereference
......
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