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rcu: Implement a variant of Peter's SRCU algorithm

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This commit implements a variant of Peter's algorithm, which may be found
at https://lkml.org/lkml/2012/2/1/119.

o	Make the checking lock-free to enable parallel checking.
	Parallel checking is required when (1) the original checking
	task is preempted for a long time, (2) sychronize_srcu_expedited()
	starts during an ongoing SRCU grace period, or (3) we wish to
	avoid acquiring a lock.

o	Since the checking is lock-free, we avoid a mutex in state machine
	for call_srcu().

o	Remove the SRCU_REF_MASK and remove the coupling with the flipping.
	This might allow us to remove the preempt_disable() in future
	versions, though such removal will need great care because it
	rescinds the one-old-reader-per-CPU guarantee.

o	Remove a smp_mb(), simplify the comments and make the smp_mb() pairs
	more intuitive.

Inspired-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Lai Jiangshan 2012-02-27 09:29:09 -08:00 • committed by Paul E. McKenney
parent 18108ebfeb
commit b52ce066c5
2 changed files with 70 additions and 86 deletions
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7
include/linux/srcu.h
View file

@ -32,18 +32,13 @@
struct srcu_struct_array {
unsigned long c[2];
unsigned long seq[2];
};
/* Bit definitions for field ->c above and ->snap below. */
#define SRCU_USAGE_BITS 1
#define SRCU_REF_MASK (ULONG_MAX >> SRCU_USAGE_BITS)
#define SRCU_USAGE_COUNT (SRCU_REF_MASK + 1)
struct srcu_struct {
unsigned completed;
struct srcu_struct_array __percpu *per_cpu_ref;
struct mutex mutex;
unsigned long snap[NR_CPUS];
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */