/*
* RCU segmented callback lists, function definitions
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, you can access it online at
* http://www.gnu.org/licenses/gpl-2.0.html.
*
* Copyright IBM Corporation, 2017
*
* Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
*/
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/interrupt.h>
#include "rcu_segcblist.h"
/* Initialize simple callback list. */
void rcu_cblist_init(struct rcu_cblist *rclp)
{
rclp->head = NULL;
rclp->tail = &rclp->head;
rclp->len = 0;
rclp->len_lazy = 0;
}
/*
* Dequeue the oldest rcu_head structure from the specified callback
* list. This function assumes that the callback is non-lazy, but
* the caller can later invoke rcu_cblist_dequeued_lazy() if it
* finds otherwise (and if it cares about laziness). This allows
* different users to have different ways of determining laziness.
*/
struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp)
{
struct rcu_head *rhp;
rhp = rclp->head;
if (!rhp)
return NULL;
rclp->len--;
rclp->head = rhp->next;
if (!rclp->head)
rclp->tail = &rclp->head;
return rhp;
}
/*
* Initialize an rcu_segcblist structure.
*/
void rcu_segcblist_init(struct rcu_segcblist *rsclp)
{
int i;
BUILD_BUG_ON(RCU_NEXT_TAIL + 1 != ARRAY_SIZE(rsclp->gp_seq));
BUILD_BUG_ON(ARRAY_SIZE(rsclp->tails) != ARRAY_SIZE(rsclp->gp_seq));
rsclp->head = NULL;
for (i = 0; i < RCU_CBLIST_NSEGS; i++)
rsclp->tails[i] = &rsclp->head;
rsclp->len = 0;
rsclp->len_lazy = 0;
}
/*
* Disable the specified rcu_segcblist structure, so that callbacks can
* no longer be posted to it. This structure must be empty.
*/
void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
{
WARN_ON_ONCE(!rcu_segcblist_empty(rsclp));
WARN_ON_ONCE(rcu_segcblist_n_cbs(rsclp));
WARN_ON_ONCE(rcu_segcblist_n_lazy_cbs(rsclp));
rsclp->tails[RCU_NEXT_TAIL] = NULL;
}
/*
* Does the specified rcu_segcblist structure contain callbacks that
* are ready to be invoked?
*/
bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp)
{
return rcu_segcblist_is_enabled(rsclp) &&
&rsclp->head != rsclp->tails[RCU_DONE_TAIL];
}
/*
* Does the specified rcu_segcblist structure contain callbacks that
* are still pending, that is, not yet ready to be invoked?
*/
bool rcu_segcblist_pend_cbs(struct rcu_segcblist *rsclp)
{
return rcu_segcblist_is_enabled(rsclp) &&
!rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL);
}
/*
* Return a pointer to the first callback in the specified rcu_segcblist
* structure. This is useful for diagnostics.
*/
struct rcu_head *rcu_segcblist_first_cb(struct rcu_segcblist *rsclp)
{
if (rcu_segcblist_is_enabled(rsclp))
return rsclp->head;
return NULL;
}
/*
* Return a pointer to the first pending callback in the specified
* rcu_segcblist structure. This is useful just after posting a given
* callback -- if that callback is the first pending callback, then
* you cannot rely on someone else having already started up the required
* grace period.
*/
struct rcu_head *rcu_segcblist_first_pend_cb(struct rcu_segcblist *rsclp)
{
if (rcu_segcblist_is_enabled(rsclp))
return *rsclp->tails[RCU_DONE_TAIL];
return NULL;
}
/*
* Enqueue the specified callback onto the specified rcu_segcblist
* structure, updating accounting as needed. Note that the ->len
* field may be accessed locklessly, hence the WRITE_ONCE().
* The ->len field is used by rcu_barrier() and friends to determine
* if it must post a callback on this structure, and it is OK
* for rcu_barrier() to sometimes post callbacks needlessly, but
* absolutely not OK for it to ever miss posting a callback.
*/
void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
struct rcu_head *rhp, bool lazy)
{
WRITE_ONCE(rsclp->len, rsclp->len + 1); /* ->len sampled locklessly. */
if (lazy)
rsclp->len_lazy++;
smp_mb(); /* Ensure counts are updated before callback is enqueued. */
rhp->next = NULL;
*rsclp->tails[RCU_NEXT_TAIL] = rhp;
rsclp->tails[RCU_NEXT_TAIL] = &rhp->next;
}
/*
* Entrain the specified callback onto the specified rcu_segcblist at
* the end of the last non-empty segment. If the entire rcu_segcblist
* is empty, make no change, but return false.
*
* This is intended for use by rcu_barrier()-like primitives, -not-
* for normal grace-period use. IMPORTANT: The callback you enqueue
* will wait for all prior callbacks, NOT necessarily for a grace
* period. You have been warned.
*/
bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
struct rcu_head *rhp, bool lazy)
{
int i;
if (rcu_segcblist_n_cbs(rsclp) == 0)
return false;
WRITE_ONCE(rsclp->len, rsclp->len + 1);
if (lazy)
rsclp->len_lazy++;
smp_mb(); /* Ensure counts are updated before callback is entrained. */
rhp->next = NULL;
for (i = RCU_NEXT_TAIL; i > RCU_DONE_TAIL; i--)
if (rsclp->tails[i] != rsclp->tails[i - 1])
break;
*rsclp->tails[i] = rhp;
for (; i <= RCU_NEXT_TAIL; i++)
rsclp->tails[i] = &rhp->next;
return true;
}
/*
* Extract only the counts from the specified rcu_segcblist structure,
* and place them in the specified rcu_cblist structure. This function
* supports both callback orphaning and invocation, hence the separation
* of counts and callbacks. (Callbacks ready for invocation must be
* orphaned and adopted separately from pending callbacks, but counts
* apply to all callbacks. Locking must be used to make sure that
* both orphaned-callbacks lists are consistent.)
*/
void rcu_segcblist_extract_count(struct rcu_segcblist *rsclp,
struct rcu_cblist *rclp)
{
rclp->len_lazy += rsclp->len_lazy;
rclp->len += rsclp->len;
rsclp->len_lazy = 0;
WRITE_ONCE(rsclp->len, 0); /* ->len sampled locklessly. */
}
/*
* Extract only those callbacks ready to be invoked from the specified
* rcu_segcblist structure and place them in the specified rcu_cblist
* structure.
*/
void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
struct rcu_cblist *rclp)
{
int i;
if (!rcu_segcblist_ready_cbs(rsclp))
return; /* Nothing to do. */
*rclp->tail = rsclp->head;
rsclp->head = *rsclp->tails[RCU_DONE_TAIL];
*rsclp->tails[RCU_DONE_TAIL] = NULL;
rclp->tail = rsclp->tails[RCU_DONE_TAIL];
for (i = RCU_CBLIST_NSEGS - 1; i >= RCU_DONE_TAIL; i--)
if (rsclp->tails[i] == rsclp->tails[RCU_DONE_TAIL])
rsclp->tails[i] = &rsclp->head;
}
/*
* Extract only those callbacks still pending (not yet ready to be
* invoked) from the specified rcu_segcblist structure and place them in
* the specified rcu_cblist structure. Note that this loses information
* about any callbacks that might have been partway done waiting for
* their grace period. Too bad! They will have to start over.
*/
void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
struct rcu_cblist *rclp)
{
int i;
if (!rcu_segcblist_pend_cbs(rsclp))
return; /* Nothing to do. */
*rclp->tail = *rsclp->tails[RCU_DONE_TAIL];
rclp->tail = rsclp->tails[RCU_NEXT_TAIL];
*rsclp->tails[RCU_DONE_TAIL] = NULL;
for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++)
rsclp->tails[i] = rsclp->tails[RCU_DONE_TAIL];
}
/*
* Insert counts from the specified rcu_cblist structure in the
* specified rcu_segcblist structure.
*/
void rcu_segcblist_insert_count(struct rcu_segcblist *rsclp,
struct rcu_cblist *rclp)
{
rsclp->len_lazy += rclp->len_lazy;
/* ->len sampled locklessly. */
WRITE_ONCE(rsclp->len, rsclp->len + rclp->len);
rclp->len_lazy = 0;
rclp->len = 0;
}
/*
* Move callbacks from the specified rcu_cblist to the beginning of the
* done-callbacks segment of the specified rcu_segcblist.
*/
void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp,
struct rcu_cblist *rclp)
{
int i;
if (!rclp->head)
return; /* No callbacks to move. */
*rclp->tail = rsclp->head;
rsclp->head = rclp->head;
for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
if (&rsclp->head == rsclp->tails[i])
rsclp->tails[i] = rclp->tail;
else
break;
rclp->head = NULL;
rclp->tail = &rclp->head;
}
/*
* Move callbacks from the specified rcu_cblist to the end of the
* new-callbacks segment of the specified rcu_segcblist.
*/
void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
struct rcu_cblist *rclp)
{
if (!rclp->head)
return; /* Nothing to do. */
*rsclp->tails[RCU_NEXT_TAIL] = rclp->head;
rsclp->tails[RCU_NEXT_TAIL] = rclp->tail;
rclp->head = NULL;
rclp->tail = &rclp->head;
}
/*
* Advance the callbacks in the specified rcu_segcblist structure based
* on the current value passed in for the grace-period counter.
*/
void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
{
int i, j;
WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
return;
/*
* Find all callbacks whose ->gp_seq numbers indicate that they
* are ready to invoke, and put them into the RCU_DONE_TAIL segment.
*/
for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++) {
if (ULONG_CMP_LT(seq, rsclp->gp_seq[i]))
break;
rsclp->tails[RCU_DONE_TAIL] = rsclp->tails[i];
}
/* If no callbacks moved, nothing more need be done. */
if (i == RCU_WAIT_TAIL)
return;
/* Clean up tail pointers that might have been misordered above. */
for (j = RCU_WAIT_TAIL; j < i; j++)
rsclp->tails[j] = rsclp->tails[RCU_DONE_TAIL];
/*
* Callbacks moved, so clean up the misordered ->tails[] pointers
* that now point into the middle of the list of ready-to-invoke
* callbacks. The overall effect is to copy down the later pointers
* into the gap that was created by the now-ready segments.
*/
for (j = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++, j++) {
if (rsclp->tails[j] == rsclp->tails[RCU_NEXT_TAIL])
break; /* No more callbacks. */
rsclp->tails[j] = rsclp->tails[i];
rsclp->gp_seq[j] = rsclp->gp_seq[i];
}
}
/*
* "Accelerate" callbacks based on more-accurate grace-period information.
* The reason for this is that RCU does not synchronize the beginnings and
* ends of grace periods, and that callbacks are posted locally. This in
* turn means that the callbacks must be labelled conservatively early
* on, as getting exact information would degrade both performance and
* scalability. When more accurate grace-period information becomes
* available, previously posted callbacks can be "accelerated", marking
* them to complete at the end of the earlier grace period.
*
* This function operates on an rcu_segcblist structure, and also the
* grace-period sequence number seq at which new callbacks would become
* ready to invoke. Returns true if there are callbacks that won't be
* ready to invoke until seq, false otherwise.
*/
bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
{
int i;
WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
return false;
/*
* Find the segment preceding the oldest segment of callbacks
* whose ->gp_seq[] completion is at or after that passed in via
* "seq", skipping any empty segments. This oldest segment, along
* with any later segments, can be merged in with any newly arrived
* callbacks in the RCU_NEXT_TAIL segment, and assigned "seq"
* as their ->gp_seq[] grace-period completion sequence number.
*/
for (i = RCU_NEXT_READY_TAIL; i > RCU_DONE_TAIL; i--)
if (rsclp->tails[i] != rsclp->tails[i - 1] &&
ULONG_CMP_LT(rsclp->gp_seq[i], seq))
break;
/*
* If all the segments contain callbacks that correspond to
* earlier grace-period sequence numbers than "seq", leave.
* Assuming that the rcu_segcblist structure has enough
* segments in its arrays, this can only happen if some of
* the non-done segments contain callbacks that really are
* ready to invoke. This situation will get straightened
* out by the next call to rcu_segcblist_advance().
*
* Also advance to the oldest segment of callbacks whose
* ->gp_seq[] completion is at or after that passed in via "seq",
* skipping any empty segments.
*/
if (++i >= RCU_NEXT_TAIL)
return false;
/*
* Merge all later callbacks, including newly arrived callbacks,
* into the segment located by the for-loop above. Assign "seq"
* as the ->gp_seq[] value in order to correctly handle the case
* where there were no pending callbacks in the rcu_segcblist
* structure other than in the RCU_NEXT_TAIL segment.
*/
for (; i < RCU_NEXT_TAIL; i++) {
rsclp->tails[i] = rsclp->tails[RCU_NEXT_TAIL];
rsclp->gp_seq[i] = seq;
}
return true;
}
/*
* Scan the specified rcu_segcblist structure for callbacks that need
* a grace period later than the one specified by "seq". We don't look
* at the RCU_DONE_TAIL or RCU_NEXT_TAIL segments because they don't
* have a grace-period sequence number.
*/
bool rcu_segcblist_future_gp_needed(struct rcu_segcblist *rsclp,
unsigned long seq)
{
int i;
for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++)
if (rsclp->tails[i - 1] != rsclp->tails[i] &&
ULONG_CMP_LT(seq, rsclp->gp_seq[i]))
return true;
return false;
}
/*
* Merge the source rcu_segcblist structure into the destination
* rcu_segcblist structure, then initialize the source. Any pending
* callbacks from the source get to start over. It is best to
* advance and accelerate both the destination and the source
* before merging.
*/
void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
struct rcu_segcblist *src_rsclp)
{
struct rcu_cblist donecbs;
struct rcu_cblist pendcbs;
rcu_cblist_init(&donecbs);
rcu_cblist_init(&pendcbs);
rcu_segcblist_extract_count(src_rsclp, &donecbs);
rcu_segcblist_extract_done_cbs(src_rsclp, &donecbs);
rcu_segcblist_extract_pend_cbs(src_rsclp, &pendcbs);
rcu_segcblist_insert_count(dst_rsclp, &donecbs);
rcu_segcblist_insert_done_cbs(dst_rsclp, &donecbs);
rcu_segcblist_insert_pend_cbs(dst_rsclp, &pendcbs);
rcu_segcblist_init(src_rsclp);
}
