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workqueue: Allow cancel_work_sync() and disable_work() from atomic contexts on BH work items

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Now that work_grab_pending() can always grab the PENDING bit without
sleeping, the only thing that prevents allowing cancel_work_sync() of a BH
work item from an atomic context is the flushing of the in-flight instance.

When we're flushing a BH work item for cancel_work_sync(), we know that the
work item is not queued and must be executing in a BH context, which means
that it's safe to busy-wait for its completion from a non-hardirq atomic
context.

This patch updates __flush_work() so that it busy-waits when flushing a BH
work item for cancel_work_sync(). might_sleep() is pushed from
start_flush_work() to its callers - when operating on a BH work item,
__cancel_work_sync() now enforces !in_hardirq() instead of might_sleep().

This allows cancel_work_sync() and disable_work() to be called from
non-hardirq atomic contexts on BH work items.

v3: In __flush_work(), test WORK_OFFQ_BH to tell whether a work item being
    canceled can be busy waited instead of making start_flush_work() return
    the pool. (Lai)

v2: Lai pointed out that __flush_work() was accessing pool->flags outside
    the RCU critical section protecting the pool pointer. Fix it by testing
    and remembering the result inside the RCU critical section.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
This commit is contained in:
Tejun Heo 2024-03-25 07:21:03 -10:00
parent 456a78eef2
commit 134874e2ee
1 changed files with 55 additions and 19 deletions
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74
kernel/workqueue.c
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@ -4105,8 +4105,6 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
struct pool_workqueue *pwq;
struct workqueue_struct *wq;
might_sleep();
rcu_read_lock();
pool = get_work_pool(work);
if (!pool) {
@ -4158,6 +4156,7 @@ already_gone:
static bool __flush_work(struct work_struct *work, bool from_cancel)
{
struct wq_barrier barr;
unsigned long data;
if (WARN_ON(!wq_online))
return false;
@ -4165,13 +4164,41 @@ static bool __flush_work(struct work_struct *work, bool from_cancel)
if (WARN_ON(!work->func))
return false;
if (start_flush_work(work, &barr, from_cancel)) {
wait_for_completion(&barr.done);
destroy_work_on_stack(&barr.work);
return true;
} else {
if (!start_flush_work(work, &barr, from_cancel))
return false;
/*
* start_flush_work() returned %true. If @from_cancel is set, we know
* that @work must have been executing during start_flush_work() and
* can't currently be queued. Its data must contain OFFQ bits. If @work
* was queued on a BH workqueue, we also know that it was running in the
* BH context and thus can be busy-waited.
*/
data = *work_data_bits(work);
if (from_cancel &&
!WARN_ON_ONCE(data & WORK_STRUCT_PWQ) && (data & WORK_OFFQ_BH)) {
/*
* On RT, prevent a live lock when %current preempted soft
* interrupt processing or prevents ksoftirqd from running by
* keeping flipping BH. If the BH work item runs on a different
* CPU then this has no effect other than doing the BH
* disable/enable dance for nothing. This is copied from
* kernel/softirq.c::tasklet_unlock_spin_wait().
*/
while (!try_wait_for_completion(&barr.done)) {
if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
local_bh_disable();
local_bh_enable();
} else {
cpu_relax();
}
}
} else {
wait_for_completion(&barr.done);
}
destroy_work_on_stack(&barr.work);
return true;
}
/**
@ -4187,6 +4214,7 @@ static bool __flush_work(struct work_struct *work, bool from_cancel)
*/
bool flush_work(struct work_struct *work)
{
might_sleep();
return __flush_work(work, false);
}
EXPORT_SYMBOL_GPL(flush_work);
@ -4276,6 +4304,11 @@ static bool __cancel_work_sync(struct work_struct *work, u32 cflags)
ret = __cancel_work(work, cflags | WORK_CANCEL_DISABLE);
if (*work_data_bits(work) & WORK_OFFQ_BH)
WARN_ON_ONCE(in_hardirq());
else
might_sleep();
/*
* Skip __flush_work() during early boot when we know that @work isn't
* executing. This allows canceling during early boot.
@ -4302,19 +4335,19 @@ EXPORT_SYMBOL(cancel_work);
* cancel_work_sync - cancel a work and wait for it to finish
* @work: the work to cancel
*
* Cancel @work and wait for its execution to finish. This function
* can be used even if the work re-queues itself or migrates to
* another workqueue. On return from this function, @work is
* guaranteed to be not pending or executing on any CPU.
* Cancel @work and wait for its execution to finish. This function can be used
* even if the work re-queues itself or migrates to another workqueue. On return
* from this function, @work is guaranteed to be not pending or executing on any
* CPU as long as there aren't racing enqueues.
*
* cancel_work_sync(&delayed_work->work) must not be used for
* delayed_work's. Use cancel_delayed_work_sync() instead.
* cancel_work_sync(&delayed_work->work) must not be used for delayed_work's.
* Use cancel_delayed_work_sync() instead.
*
* The caller must ensure that the workqueue on which @work was last
* queued can't be destroyed before this function returns.
* Must be called from a sleepable context if @work was last queued on a non-BH
* workqueue. Can also be called from non-hardirq atomic contexts including BH
* if @work was last queued on a BH workqueue.
*
* Return:
* %true if @work was pending, %false otherwise.
* Returns %true if @work was pending, %false otherwise.
*/
bool cancel_work_sync(struct work_struct *work)
{
@ -4384,8 +4417,11 @@ EXPORT_SYMBOL_GPL(disable_work);
* Similar to disable_work() but also wait for @work to finish if currently
* executing.
*
* Must be called from a sleepable context. Returns %true if @work was pending,
* %false otherwise.
* Must be called from a sleepable context if @work was last queued on a non-BH
* workqueue. Can also be called from non-hardirq atomic contexts including BH
* if @work was last queued on a BH workqueue.
*
* Returns %true if @work was pending, %false otherwise.
*/
bool disable_work_sync(struct work_struct *work)
{