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Diffstat (limited to 'Documentation/RCU/checklist.txt')
| -rw-r--r-- | Documentation/RCU/checklist.txt | 38 |
1 files changed, 38 insertions, 0 deletions
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index 1d50cf0c905..f4dffadbcb0 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -221,3 +221,41 @@ over a rather long period of time, but improvements are always welcome! disable irq on a given acquisition of that lock will result in deadlock as soon as the RCU callback happens to interrupt that acquisition's critical section. + +13. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_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. + + 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" + that defines the scope of a given SRCU domain. Once initialized, + the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock() + and synchronize_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 passd the same + srcu_struct. This property is what makes sleeping read-side + critical sections tolerable -- a given subsystem delays only + its own updates, not those of other subsystems using SRCU. + Therefore, SRCU is less prone to OOM the system than RCU would + be if RCU's read-side critical sections were permitted to + sleep. + + The ability to sleep in read-side critical sections does not + come for free. First, corresponding srcu_read_lock() and + srcu_read_unlock() calls must be passed the same srcu_struct. + Second, grace-period-detection overhead is amortized only + over those updates sharing a given srcu_struct, rather than + being globally amortized as they are for other forms of RCU. + Therefore, SRCU should be used in preference to rw_semaphore + only in extremely read-intensive situations, or in situations + 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. |