linux-kernel - [PATCH 5.15 174/917] io-wq: serialize hash clear with wakeup

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Message-Id: <20211115165434.675461830@linuxfoundation.org>
Date:   Mon, 15 Nov 2021 17:54:29 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Daniel Black <daniel@...iadb.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 5.15 174/917] io-wq: serialize hash clear with wakeup

From: Jens Axboe <axboe@...nel.dk>

commit d3e3c102d107bb84251455a298cf475f24bab995 upstream.

We need to ensure that we serialize the stalled and hash bits with the
wait_queue wait handler, or we could be racing with someone modifying
the hashed state after we find it busy, but before we then give up and
wait for it to be cleared. This can cause random delays or stalls when
handling buffered writes for many files, where some of these files cause
hash collisions between the worker threads.

Cc: stable@...r.kernel.org
Reported-by: Daniel Black <daniel@...iadb.org>
Fixes: e941894eae31 ("io-wq: make buffered file write hashed work map per-ctx")
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 fs/io-wq.c |   17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

--- a/fs/io-wq.c
+++ b/fs/io-wq.c
@@ -421,9 +421,10 @@ static inline unsigned int io_get_work_h
 	return work->flags >> IO_WQ_HASH_SHIFT;
 }
 
-static void io_wait_on_hash(struct io_wqe *wqe, unsigned int hash)
+static bool io_wait_on_hash(struct io_wqe *wqe, unsigned int hash)
 {
 	struct io_wq *wq = wqe->wq;
+	bool ret = false;
 
 	spin_lock_irq(&wq->hash->wait.lock);
 	if (list_empty(&wqe->wait.entry)) {
@@ -431,9 +432,11 @@ static void io_wait_on_hash(struct io_wq
 		if (!test_bit(hash, &wq->hash->map)) {
 			__set_current_state(TASK_RUNNING);
 			list_del_init(&wqe->wait.entry);
+			ret = true;
 		}
 	}
 	spin_unlock_irq(&wq->hash->wait.lock);
+	return ret;
 }
 
 static struct io_wq_work *io_get_next_work(struct io_wqe_acct *acct,
@@ -473,14 +476,21 @@ static struct io_wq_work *io_get_next_wo
 	}
 
 	if (stall_hash != -1U) {
+		bool unstalled;
+
 		/*
 		 * Set this before dropping the lock to avoid racing with new
 		 * work being added and clearing the stalled bit.
 		 */
 		set_bit(IO_ACCT_STALLED_BIT, &acct->flags);
 		raw_spin_unlock(&wqe->lock);
-		io_wait_on_hash(wqe, stall_hash);
+		unstalled = io_wait_on_hash(wqe, stall_hash);
 		raw_spin_lock(&wqe->lock);
+		if (unstalled) {
+			clear_bit(IO_ACCT_STALLED_BIT, &acct->flags);
+			if (wq_has_sleeper(&wqe->wq->hash->wait))
+				wake_up(&wqe->wq->hash->wait);
+		}
 	}
 
 	return NULL;
@@ -562,8 +572,11 @@ get_next:
 				io_wqe_enqueue(wqe, linked);
 
 			if (hash != -1U && !next_hashed) {
+				/* serialize hash clear with wake_up() */
+				spin_lock_irq(&wq->hash->wait.lock);
 				clear_bit(hash, &wq->hash->map);
 				clear_bit(IO_ACCT_STALLED_BIT, &acct->flags);
+				spin_unlock_irq(&wq->hash->wait.lock);
 				if (wq_has_sleeper(&wq->hash->wait))
 					wake_up(&wq->hash->wait);
 				raw_spin_lock(&wqe->lock);