[PATCH AUTOSEL 6.6 29/58] io_uring: check if we need to reschedule during overflow flush

Sasha Levin
2024-10-04T18:25:55Z

From: Jens Axboe <axboe@kernel.dk>

[ Upstream commit eac2ca2d682f94f46b1973bdf5e77d85d77b8e53 ]

In terms of normal application usage, this list will always be empty.
And if an application does overflow a bit, it'll have a few entries.
However, nothing obviously prevents syzbot from running a test case
that generates a ton of overflow entries, and then flushing them can
take quite a while.

Check for needing to reschedule while flushing, and drop our locks and
do so if necessary. There's no state to maintain here as overflows
always prune from head-of-list, hence it's fine to drop and reacquire
the locks at the end of the loop.

Link: https://lore.kernel.org/io-uring/66ed061d.050a0220.29194.0053.GAE@google.com/
Reported-by: syzbot+5fca234bd7eb378ff78e@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
io_uring/io_uring.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 68504709f75cb..7ecfd314cf3cb 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -701,6 +701,21 @@ static void __io_cqring_overflow_flush(struct io_ring_ctx *ctx)
memcpy(cqe, &ocqe->cqe, cqe_size);
list_del(&ocqe->list);
kfree(ocqe);
+
+ /*
+ * For silly syzbot cases that deliberately overflow by huge
+ * amounts, check if we need to resched and drop and
+ * reacquire the locks if so. Nothing real would ever hit this.
+ * Ideally we'd have a non-posting unlock for this, but hard
+ * to care for a non-real case.
+ */
+ if (need_resched()) {
+ io_cq_unlock_post(ctx);
+ mutex_unlock(&ctx->uring_lock);
+ cond_resched();
+ mutex_lock(&ctx->uring_lock);
+ io_cq_lock(ctx);
+ }
}

if (list_empty(&ctx->cq_overflow_list)) {
--
2.43.0