io-wq: ensure we have a stable view of ->cur_work for cancellations

worker->cur_work is currently protected by the lock of the wqe that the
worker belongs to. When we send a signal to a worker, we need a stable
view of ->cur_work, so we need to hold that lock. But this doesn't work
so well, since we have the opposite order potentially on queueing work.
If POLL_ADD is used with a signalfd, then io_poll_wake() is called with
the signal lock, and that sometimes needs to insert work items.

Add a specific worker lock that protects the current work item. Then we
can guarantee that the task we're sending a signal is currently
processing the exact work we think it is.

Reported-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io-wq.c b/fs/io-wq.c
index 26d81540c1fc54abe076793afbb6c84a2b1f5aee..4031b75541beb1d7b6ba2c5fcf74b61598b68c81 100644 (file)
--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -49,7 +49,9 @@ struct io_worker {
        struct task_struct *task;
        wait_queue_head_t wait;
        struct io_wqe *wqe;
+
        struct io_wq_work *cur_work;
+       spinlock_t lock;
 
        struct rcu_head rcu;
        struct mm_struct *mm;
@@ -323,7 +325,6 @@ static void __io_worker_busy(struct io_wqe *wqe, struct io_worker *worker,
                hlist_nulls_add_head_rcu(&worker->nulls_node,
                                                &wqe->busy_list.head);
        }
-       worker->cur_work = work;
 
        /*
         * If worker is moving from bound to unbound (or vice versa), then
@@ -402,17 +403,6 @@ static void io_worker_handle_work(struct io_worker *worker)
        do {
                unsigned hash = -1U;
 
-               /*
-                * Signals are either sent to cancel specific work, or to just
-                * cancel all work items. For the former, ->cur_work must
-                * match. ->cur_work is NULL at this point, since we haven't
-                * assigned any work, so it's safe to flush signals for that
-                * case. For the latter case of cancelling all work, the caller
-                * wil have set IO_WQ_BIT_CANCEL.
-                */
-               if (signal_pending(current))
-                       flush_signals(current);
-
                /*
                 * If we got some work, mark us as busy. If we didn't, but
                 * the list isn't empty, it means we stalled on hashed work.
@@ -432,6 +422,14 @@ static void io_worker_handle_work(struct io_worker *worker)
                if (!work)
                        break;
 next:
+               /* flush any pending signals before assigning new work */
+               if (signal_pending(current))
+                       flush_signals(current);
+
+               spin_lock_irq(&worker->lock);
+               worker->cur_work = work;
+               spin_unlock_irq(&worker->lock);
+
                if ((work->flags & IO_WQ_WORK_NEEDS_FILES) &&
                    current->files != work->files) {
                        task_lock(current);
@@ -457,8 +455,12 @@ next:
                old_work = work;
                work->func(&work);
 
-               spin_lock_irq(&wqe->lock);
+               spin_lock_irq(&worker->lock);
                worker->cur_work = NULL;
+               spin_unlock_irq(&worker->lock);
+
+               spin_lock_irq(&wqe->lock);
+
                if (hash != -1U) {
                        wqe->hash_map &= ~BIT_ULL(hash);
                        wqe->flags &= ~IO_WQE_FLAG_STALLED;
@@ -577,6 +579,7 @@ static void create_io_worker(struct io_wq *wq, struct io_wqe *wqe, int index)
        worker->nulls_node.pprev = NULL;
        init_waitqueue_head(&worker->wait);
        worker->wqe = wqe;
+       spin_lock_init(&worker->lock);
 
        worker->task = kthread_create_on_node(io_wqe_worker, worker, wqe->node,
                                "io_wqe_worker-%d/%d", index, wqe->node);
@@ -783,21 +786,20 @@ struct io_cb_cancel_data {
 static bool io_work_cancel(struct io_worker *worker, void *cancel_data)
 {
        struct io_cb_cancel_data *data = cancel_data;
-       struct io_wqe *wqe = data->wqe;
        unsigned long flags;
        bool ret = false;
 
        /*
         * Hold the lock to avoid ->cur_work going out of scope, caller
-        * may deference the passed in work.
+        * may dereference the passed in work.
         */
-       spin_lock_irqsave(&wqe->lock, flags);
+       spin_lock_irqsave(&worker->lock, flags);
        if (worker->cur_work &&
            data->cancel(worker->cur_work, data->caller_data)) {
                send_sig(SIGINT, worker->task, 1);
                ret = true;
        }
-       spin_unlock_irqrestore(&wqe->lock, flags);
+       spin_unlock_irqrestore(&worker->lock, flags);
 
        return ret;
 }
@@ -864,13 +866,20 @@ enum io_wq_cancel io_wq_cancel_cb(struct io_wq *wq, work_cancel_fn *cancel,
 static bool io_wq_worker_cancel(struct io_worker *worker, void *data)
 {
        struct io_wq_work *work = data;
+       unsigned long flags;
+       bool ret = false;
 
+       if (worker->cur_work != work)
+               return false;
+
+       spin_lock_irqsave(&worker->lock, flags);
        if (worker->cur_work == work) {
                send_sig(SIGINT, worker->task, 1);
-               return true;
+               ret = true;
        }
+       spin_unlock_irqrestore(&worker->lock, flags);
 
-       return false;
+       return ret;
 }
 
 static enum io_wq_cancel io_wqe_cancel_work(struct io_wqe *wqe,