io_uring: drop ctx->uring_lock before acquiring sqd->lock

The SQPOLL thread dictates the lock order, and we hold the ctx->uring_lock
for all the registration opcodes. We also hold a ref to the ctx, and we
do drop the lock for other reasons to quiesce, so it's fine to drop the
ctx lock temporarily to grab the sqd->lock. This fixes the following
lockdep splat:

======================================================
WARNING: possible circular locking dependency detected
5.14.0-syzkaller #0 Not tainted
------------------------------------------------------
syz-executor.5/25433 is trying to acquire lock:
ffff888023426870 (&sqd->lock){+.+.}-{3:3}, at: io_register_iowq_max_workers fs/io_uring.c:10551 [inline]
ffff888023426870 (&sqd->lock){+.+.}-{3:3}, at: __io_uring_register fs/io_uring.c:10757 [inline]
ffff888023426870 (&sqd->lock){+.+.}-{3:3}, at: __do_sys_io_uring_register+0x10aa/0x2e70 fs/io_uring.c:10792

but task is already holding lock:
ffff8880885b40a8 (&ctx->uring_lock){+.+.}-{3:3}, at: __do_sys_io_uring_register+0x2e1/0x2e70 fs/io_uring.c:10791

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&ctx->uring_lock){+.+.}-{3:3}:
       __mutex_lock_common kernel/locking/mutex.c:596 [inline]
       __mutex_lock+0x131/0x12f0 kernel/locking/mutex.c:729
       __io_sq_thread fs/io_uring.c:7291 [inline]
       io_sq_thread+0x65a/0x1370 fs/io_uring.c:7368
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

-> #0 (&sqd->lock){+.+.}-{3:3}:
       check_prev_add kernel/locking/lockdep.c:3051 [inline]
       check_prevs_add kernel/locking/lockdep.c:3174 [inline]
       validate_chain kernel/locking/lockdep.c:3789 [inline]
       __lock_acquire+0x2a07/0x54a0 kernel/locking/lockdep.c:5015
       lock_acquire kernel/locking/lockdep.c:5625 [inline]
       lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5590
       __mutex_lock_common kernel/locking/mutex.c:596 [inline]
       __mutex_lock+0x131/0x12f0 kernel/locking/mutex.c:729
       io_register_iowq_max_workers fs/io_uring.c:10551 [inline]
       __io_uring_register fs/io_uring.c:10757 [inline]
       __do_sys_io_uring_register+0x10aa/0x2e70 fs/io_uring.c:10792
       do_syscall_x64 arch/x86/entry/common.c:50 [inline]
       do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
       entry_SYSCALL_64_after_hwframe+0x44/0xae

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&ctx->uring_lock);
                               lock(&sqd->lock);
                               lock(&ctx->uring_lock);
  lock(&sqd->lock);

 *** DEADLOCK ***

Fixes: 2e480058ddc2 ("io-wq: provide a way to limit max number of workers")
Reported-by: syzbot+97fa56483f69d677969f@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -10551,7 +10551,14 @@ static int io_register_iowq_max_workers(struct io_ring_ctx *ctx,
 	if (ctx->flags & IORING_SETUP_SQPOLL) {
 		sqd = ctx->sq_data;
 		if (sqd) {
+			/*
+			 * Observe the correct sqd->lock -> ctx->uring_lock
+			 * ordering. Fine to drop uring_lock here, we hold
+			 * a ref to the ctx.
+			 */
+			mutex_unlock(&ctx->uring_lock);
 			mutex_lock(&sqd->lock);
+			mutex_lock(&ctx->uring_lock);
 			tctx = sqd->thread->io_uring;
 		}
 	} else {