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make cancel_rearming_delayed_work() reliable

Browse source 
Thanks to Jarek Poplawski for the ideas and for spotting the bug in the
initial draft patch.

cancel_rearming_delayed_work() currently has many limitations, because it
requires that dwork always re-arms itself via queue_delayed_work().  So it
hangs forever if dwork doesn't do this, or cancel_rearming_delayed_work/
cancel_delayed_work was already called.  It uses flush_workqueue() in a
loop, so it can't be used if workqueue was freezed, and it is potentially
live- lockable on busy system if delay is small.

With this patch cancel_rearming_delayed_work() doesn't make any assumptions
about dwork, it can re-arm itself via queue_delayed_work(), or
queue_work(), or do nothing.

As a "side effect", cancel_work_sync() was changed to handle re-arming works
as well.

Disadvantages:

	- this patch adds wmb() to insert_work().

	- slowdowns the fast path (when del_timer() succeeds on entry) of
	  cancel_rearming_delayed_work(), because wait_on_work() is called
	  unconditionally. In that case, compared to the old version, we are
	  doing "unneeded" lock/unlock for each online CPU.

	  On the other hand, this means we don't need to use cancel_work_sync()
	  after cancel_rearming_delayed_work().

	- complicates the code (.text grows by 130 bytes).

[akpm@linux-foundation.org: fix speling]
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: David Chinner <dgc@sgi.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Gautham Shenoy <ego@in.ibm.com>
Acked-by: Jarek Poplawski <jarkao2@o2.pl>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Oleg Nesterov 2007-05-09 02:34:46 -07:00 committed by Linus Torvalds
parent 7b0834c26f
commit 6e84d644b5
1 changed files with 91 additions and 49 deletions
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140
kernel/workqueue.c
View file

@ -120,6 +120,11 @@ static void insert_work(struct cpu_workqueue_struct *cwq,
struct work_struct *work, int tail)
{
set_wq_data(work, cwq);
/*
* Ensure that we get the right work->data if we see the
* result of list_add() below, see try_to_grab_pending().
*/
smp_wmb();
if (tail)
list_add_tail(&work->entry, &cwq->worklist);
else
@ -383,7 +388,46 @@ void fastcall flush_workqueue(struct workqueue_struct *wq)
}
EXPORT_SYMBOL_GPL(flush_workqueue);
static void wait_on_work(struct cpu_workqueue_struct *cwq,
/*
* Upon a successful return, the caller "owns" WORK_STRUCT_PENDING bit,
* so this work can't be re-armed in any way.
*/
static int try_to_grab_pending(struct work_struct *work)
{
struct cpu_workqueue_struct *cwq;
int ret = 0;
if (!test_and_set_bit(WORK_STRUCT_PENDING, work_data_bits(work)))
return 1;
/*
* The queueing is in progress, or it is already queued. Try to
* steal it from ->worklist without clearing WORK_STRUCT_PENDING.
*/
cwq = get_wq_data(work);
if (!cwq)
return ret;
spin_lock_irq(&cwq->lock);
if (!list_empty(&work->entry)) {
/*
* This work is queued, but perhaps we locked the wrong cwq.
* In that case we must see the new value after rmb(), see
* insert_work()->wmb().
*/
smp_rmb();
if (cwq == get_wq_data(work)) {
list_del_init(&work->entry);
ret = 1;
}
}
spin_unlock_irq(&cwq->lock);
return ret;
}
static void wait_on_cpu_work(struct cpu_workqueue_struct *cwq,
struct work_struct *work)
{
struct wq_barrier barr;
@ -400,20 +444,7 @@ static void wait_on_work(struct cpu_workqueue_struct *cwq,
wait_for_completion(&barr.done);
}
/**
* cancel_work_sync - block until a work_struct's callback has terminated
* @work: the work which is to be flushed
*
* cancel_work_sync() will attempt to cancel the work if it is queued. If the
* work's callback appears to be running, cancel_work_sync() will block until
* it has completed.
*
* cancel_work_sync() is designed to be used when the caller is tearing down
* data structures which the callback function operates upon. It is expected
* that, prior to calling cancel_work_sync(), the caller has arranged for the
* work to not be requeued.
*/
void cancel_work_sync(struct work_struct *work)
static void wait_on_work(struct work_struct *work)
{
struct cpu_workqueue_struct *cwq;
struct workqueue_struct *wq;
@ -423,29 +454,62 @@ void cancel_work_sync(struct work_struct *work)
might_sleep();
cwq = get_wq_data(work);
/* Was it ever queued ? */
if (!cwq)
return;
/*
* This work can't be re-queued, no need to re-check that
* get_wq_data() is still the same when we take cwq->lock.
*/
spin_lock_irq(&cwq->lock);
list_del_init(&work->entry);
work_clear_pending(work);
spin_unlock_irq(&cwq->lock);
wq = cwq->wq;
cpu_map = wq_cpu_map(wq);
for_each_cpu_mask(cpu, *cpu_map)
wait_on_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
wait_on_cpu_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
}
/**
* cancel_work_sync - block until a work_struct's callback has terminated
* @work: the work which is to be flushed
*
* cancel_work_sync() will cancel the work if it is queued. If the work's
* callback appears to be running, cancel_work_sync() will block until it
* has completed.
*
* It is possible to use this function if the work re-queues itself. It can
* cancel the work even if it migrates to another workqueue, however in that
* case it only guarantees that work->func() has completed on the last queued
* workqueue.
*
* cancel_work_sync(&delayed_work->work) should be used only if ->timer is not
* pending, otherwise it goes into a busy-wait loop until the timer expires.
*
* The caller must ensure that workqueue_struct on which this work was last
* queued can't be destroyed before this function returns.
*/
void cancel_work_sync(struct work_struct *work)
{
while (!try_to_grab_pending(work))
cpu_relax();
wait_on_work(work);
work_clear_pending(work);
}
EXPORT_SYMBOL_GPL(cancel_work_sync);
/**
* cancel_rearming_delayed_work - reliably kill off a delayed work.
* @dwork: the delayed work struct
*
* It is possible to use this function if @dwork rearms itself via queue_work()
* or queue_delayed_work(). See also the comment for cancel_work_sync().
*/
void cancel_rearming_delayed_work(struct delayed_work *dwork)
{
while (!del_timer(&dwork->timer) &&
!try_to_grab_pending(&dwork->work))
cpu_relax();
wait_on_work(&dwork->work);
work_clear_pending(&dwork->work);
}
EXPORT_SYMBOL(cancel_rearming_delayed_work);
static struct workqueue_struct *keventd_wq;
static struct workqueue_struct *keventd_wq __read_mostly;
/**
* schedule_work - put work task in global workqueue
@ -531,28 +595,6 @@ void flush_scheduled_work(void)
}
EXPORT_SYMBOL(flush_scheduled_work);
/**
* cancel_rearming_delayed_work - kill off a delayed work whose handler rearms the delayed work.
* @dwork: the delayed work struct
*
* Note that the work callback function may still be running on return from
* cancel_delayed_work(). Run flush_workqueue() or cancel_work_sync() to wait
* on it.
*/
void cancel_rearming_delayed_work(struct delayed_work *dwork)
{
struct cpu_workqueue_struct *cwq = get_wq_data(&dwork->work);
/* Was it ever queued ? */
if (cwq != NULL) {
struct workqueue_struct *wq = cwq->wq;
while (!cancel_delayed_work(dwork))
flush_workqueue(wq);
}
}
EXPORT_SYMBOL(cancel_rearming_delayed_work);
/**
* execute_in_process_context - reliably execute the routine with user context
* @fn: the function to execute