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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 */

149
kernel/srcu.c
View File

@ -72,11 +72,26 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
/*
* Returns approximate total of the readers' ->seq[] values for the
* rank of per-CPU counters specified by idx.
*/
static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
{
int cpu;
unsigned long sum = 0;
unsigned long t;
for_each_possible_cpu(cpu) {
t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
sum += t;
}
return sum;
}
/*
* Returns approximate number of readers active on the specified rank
* of per-CPU counters. Also snapshots each counter's value in the
* corresponding element of sp->snap[] for later use validating
* the sum.
* of the per-CPU ->c[] counters.
*/
static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
{
@ -87,26 +102,45 @@ static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
for_each_possible_cpu(cpu) {
t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
sum += t;
sp->snap[cpu] = t;
}
return sum & SRCU_REF_MASK;
return sum;
}
/*
* To be called from the update side after an index flip. Returns true
* if the modulo sum of the counters is stably zero, false if there is
* some possibility of non-zero.
* Return true if the number of pre-existing readers is determined to
* be stably zero. An example unstable zero can occur if the call
* to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
* but due to task migration, sees the corresponding __srcu_read_unlock()
* decrement. This can happen because srcu_readers_active_idx() takes
* time to sum the array, and might in fact be interrupted or preempted
* partway through the summation.
*/
static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
{
int cpu;
unsigned long seq;
seq = srcu_readers_seq_idx(sp, idx);
/*
* The following smp_mb() A pairs with the smp_mb() B located in
* __srcu_read_lock(). This pairing ensures that if an
* __srcu_read_lock() increments its counter after the summation
* in srcu_readers_active_idx(), then the corresponding SRCU read-side
* critical section will see any changes made prior to the start
* of the current SRCU grace period.
*
* Also, if the above call to srcu_readers_seq_idx() saw the
* increment of ->seq[], then the call to srcu_readers_active_idx()
* must see the increment of ->c[].
*/
smp_mb(); /* A */
/*
* Note that srcu_readers_active_idx() can incorrectly return
* zero even though there is a pre-existing reader throughout.
* To see this, suppose that task A is in a very long SRCU
* read-side critical section that started on CPU 0, and that
* no other reader exists, so that the modulo sum of the counters
* no other reader exists, so that the sum of the counters
* is equal to one. Then suppose that task B starts executing
* srcu_readers_active_idx(), summing up to CPU 1, and then that
* task C starts reading on CPU 0, so that its increment is not
@ -122,53 +156,31 @@ static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
return false;
/*
* Since the caller recently flipped ->completed, we can see at
* most one increment of each CPU's counter from this point
* forward. The reason for this is that the reader CPU must have
* fetched the index before srcu_readers_active_idx checked
* that CPU's counter, but not yet incremented its counter.
* Its eventual counter increment will follow the read in
* srcu_readers_active_idx(), and that increment is immediately
* followed by smp_mb() B. Because smp_mb() D is between
* the ->completed flip and srcu_readers_active_idx()'s read,
* that CPU's subsequent load of ->completed must see the new
* value, and therefore increment the counter in the other rank.
* The remainder of this function is the validation step.
* The following smp_mb() D pairs with the smp_mb() C in
* __srcu_read_unlock(). If the __srcu_read_unlock() was seen
* by srcu_readers_active_idx() above, then any destructive
* operation performed after the grace period will happen after
* the corresponding SRCU read-side critical section.
*
* Note that there can be at most NR_CPUS worth of readers using
* the old index, which is not enough to overflow even a 32-bit
* integer. (Yes, this does mean that systems having more than
* a billion or so CPUs need to be 64-bit systems.) Therefore,
* the sum of the ->seq[] counters cannot possibly overflow.
* Therefore, the only way that the return values of the two
* calls to srcu_readers_seq_idx() can be equal is if there were
* no increments of the corresponding rank of ->seq[] counts
* in the interim. But the missed-increment scenario laid out
* above includes an increment of the ->seq[] counter by
* the corresponding __srcu_read_lock(). Therefore, if this
* scenario occurs, the return values from the two calls to
* srcu_readers_seq_idx() will differ, and thus the validation
* step below suffices.
*/
smp_mb(); /* A */
smp_mb(); /* D */
/*
* Now, we check the ->snap array that srcu_readers_active_idx()
* filled in from the per-CPU counter values. Since
* __srcu_read_lock() increments the upper bits of the per-CPU
* counter, an increment/decrement pair will change the value
* of the counter. Since there is only one possible increment,
* the only way to wrap the counter is to have a huge number of
* counter decrements, which requires a huge number of tasks and
* huge SRCU read-side critical-section nesting levels, even on
* 32-bit systems.
*
* All of the ways of confusing the readings require that the scan
* in srcu_readers_active_idx() see the read-side task's decrement,
* but not its increment. However, between that decrement and
* increment are smb_mb() B and C. Either or both of these pair
* with smp_mb() A above to ensure that the scan below will see
* the read-side tasks's increment, thus noting a difference in
* the counter values between the two passes.
*
* Therefore, if srcu_readers_active_idx() returned zero, and
* none of the counters changed, we know that the zero was the
* correct sum.
*
* Of course, it is possible that a task might be delayed
* for a very long time in __srcu_read_lock() after fetching
* the index but before incrementing its counter. This
* possibility will be dealt with in __synchronize_srcu().
*/
for_each_possible_cpu(cpu)
if (sp->snap[cpu] !=
ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]))
return false; /* False zero reading! */
return true;
return srcu_readers_seq_idx(sp, idx) == seq;
}
/**
@ -216,9 +228,9 @@ int __srcu_read_lock(struct srcu_struct *sp)
preempt_disable();
idx = rcu_dereference_index_check(sp->completed,
rcu_read_lock_sched_held()) & 0x1;
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) +=
SRCU_USAGE_COUNT + 1;
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
smp_mb(); /* B */ /* Avoid leaking the critical section. */
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
preempt_enable();
return idx;
}
@ -257,17 +269,6 @@ static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
{
int trycount = 0;
/*
* If a reader fetches the index before the ->completed increment,
* but increments its counter after srcu_readers_active_idx_check()
* sums it, then smp_mb() D will pair with __srcu_read_lock()'s
* smp_mb() B to ensure that the SRCU read-side critical section
* will see any updates that the current task performed before its
* call to synchronize_srcu(), or to synchronize_srcu_expedited(),
* as the case may be.
*/
smp_mb(); /* D */
/*
* SRCU read-side critical sections are normally short, so wait
* a small amount of time before possibly blocking.
@ -281,18 +282,6 @@ static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
schedule_timeout_interruptible(1);
}
}
/*
* The following smp_mb() E pairs with srcu_read_unlock()'s
* smp_mb C to ensure that if srcu_readers_active_idx_check()
* sees srcu_read_unlock()'s counter decrement, then any
* of the current task's subsequent code will happen after
* that SRCU read-side critical section.
*
* It also ensures the order between the above waiting and
* the next flipping.
*/
smp_mb(); /* E */
}
static void srcu_flip(struct srcu_struct *sp)