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Swap taskq_ent_t with taskqid_t in taskq_thread_t

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The taskq_t's active thread list is sorted based on its
tqt_ent->tqent_id field. The list is kept sorted solely by inserting
new taskq_thread_t's in their correct sorted location; no other
means is used. This means that once inserted, if a taskq_thread_t's
tqt_ent->tqent_id field changes, the list runs the risk of no
longer being sorted.

Prior to the introduction of the taskq_dispatch_prealloc() interface,
this was not a problem as a taskq_ent_t actively being serviced under
the old interface should always have a static tqent_id field. Thus,
once the taskq_thread_t is added to the taskq_t's active thread list,
the taskq_thread_t's tqt_ent->tqent_id field would remain constant.

Now, this is no longer the case. Currently, if using the
taskq_dispatch_prealloc() interface, any given taskq_ent_t actively
being serviced _may_ have its tqent_id value incremented. This happens
when the preallocated taskq_ent_t structure is recursively dispatched.
Thus, a taskq_thread_t could potentially have its tqt_ent->tqent_id
field silently modified from under its feet. If this were to happen
to a taskq_thread_t on a taskq_t's active thread list, this would
compromise the integrity of the order of the list (as the list
_may_ no longer be sorted).

To get around this, the taskq_thread_t's taskq_ent_t pointer was
replaced with its own static copy of the tqent_id. So, as a taskq_ent_t
is pulled off of the taskq_t's pending list, a static copy of its
tqent_id is made and this copy is used to sort the active thread
list. Using a static copy is key in ensuring the integrity of the
order of the active thread list. Even if the underlying taskq_ent_t
is recursively dispatched (as has its tqent_id modified), this
static copy stored inside the taskq_thread_t will remain constant.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #71
This commit is contained in:
Prakash Surya 2011-12-16 09:44:31 -08:00 committed by Brian Behlendorf
parent c2dceb5cd5
commit e7e5f78e7b
2 changed files with 9 additions and 11 deletions
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2
include/sys/taskq.h
View File

@ -96,7 +96,7 @@ typedef struct taskq_thread {
struct list_head tqt_active_list;
struct task_struct *tqt_thread;
taskq_t *tqt_tq;
taskq_ent_t *tqt_ent;
taskqid_t tqt_id;
} taskq_thread_t;
/* Global system-wide dynamic task queue available for all consumers */

18
module/spl/spl-taskq.c
View File

@ -393,8 +393,8 @@ taskq_lowest_id(taskq_t *tq)
if (!list_empty(&tq->tq_active_list)) {
tqt = list_entry(tq->tq_active_list.next, taskq_thread_t,
tqt_active_list);
ASSERT(tqt->tqt_ent != NULL);
lowest_id = MIN(lowest_id, tqt->tqt_ent->tqent_id);
ASSERT(tqt->tqt_id != 0);
lowest_id = MIN(lowest_id, tqt->tqt_id);
}
SRETURN(lowest_id);
@ -417,7 +417,7 @@ taskq_insert_in_order(taskq_t *tq, taskq_thread_t *tqt)
list_for_each_prev(l, &tq->tq_active_list) {
w = list_entry(l, taskq_thread_t, tqt_active_list);
if (w->tqt_ent->tqent_id < tqt->tqt_ent->tqent_id) {
if (w->tqt_id < tqt->tqt_id) {
list_add(&tqt->tqt_active_list, l);
break;
}
@ -433,7 +433,6 @@ taskq_thread(void *args)
{
DECLARE_WAITQUEUE(wait, current);
sigset_t blocked;
taskqid_t id;
taskq_thread_t *tqt = args;
taskq_t *tq;
taskq_ent_t *t;
@ -484,8 +483,7 @@ taskq_thread(void *args)
/* In order to support recursively dispatching a
* preallocated taskq_ent_t, tqent_id must be
* stored prior to executing tqent_func. */
id = t->tqent_id;
tqt->tqt_ent = t;
tqt->tqt_id = t->tqent_id;
taskq_insert_in_order(tq, tqt);
tq->tq_nactive++;
spin_unlock_irqrestore(&tq->tq_lock, tq->tq_lock_flags);
@ -496,16 +494,16 @@ taskq_thread(void *args)
spin_lock_irqsave(&tq->tq_lock, tq->tq_lock_flags);
tq->tq_nactive--;
list_del_init(&tqt->tqt_active_list);
tqt->tqt_ent = NULL;
task_done(tq, t);
/* When the current lowest outstanding taskqid is
* done calculate the new lowest outstanding id */
if (tq->tq_lowest_id == id) {
if (tq->tq_lowest_id == tqt->tqt_id) {
tq->tq_lowest_id = taskq_lowest_id(tq);
ASSERT(tq->tq_lowest_id > id);
ASSERT3S(tq->tq_lowest_id, >, tqt->tqt_id);
}
tqt->tqt_id = 0;
wake_up_all(&tq->tq_wait_waitq);
}
@ -582,7 +580,7 @@ __taskq_create(const char *name, int nthreads, pri_t pri,
INIT_LIST_HEAD(&tqt->tqt_thread_list);
INIT_LIST_HEAD(&tqt->tqt_active_list);
tqt->tqt_tq = tq;
tqt->tqt_ent = NULL;
tqt->tqt_id = 0;
tqt->tqt_thread = kthread_create(taskq_thread, tqt,
"%s/%d", name, i);