Sleepable Read-Copy Update

Read-copy update (RCU) is a synchronization API that is sometimes used in place of reader-writer locks. RCU’s read-side primitives offer extremely low overhead and deterministic execution time. These properties imply that RCU updaters cannot block RCU readers, which means that RCU updaters can be expensive, as they must leave old versions of the data structure in place to accommodate pre-existing readers. Furthermore, these old versions must be reclaimed after all pre-existing readers complete. The Linux kernel offers a number of RCU implementations, the first such implementation being called “Classic RCU”. Classic RCU requires that read-side critical sections obey the same rules obeyed by the critical sections of pure spinlocks: blocking or sleeping of any sort is strictly prohibited. This has frequently been an obstacle to the use of RCU, and Paul has received numerous requests for a “sleepable RCU” (SRCU) that permits arbitrary sleeping (or blocking) within RCU read-side critical sections. Paul had previously rejected all such requests as unworkable, since arbitrary sleeping in RCU read-side could indefinitely extend grace periods, which in turn could result in arbitrarily large amounts of memory awaiting the end of a grace period, which finally would result in disaster, as fancifully depicted in Figure 1, with the most likely disaster being hangs due to memory exhaustion. After all, any concurrency-control primitive that could result in system hangs — even when used correctly – does not deserve to exist. However, the realtime kernels that require spinlock Figure 1: Sleeping While RCU Reading Considered Harmful