io_uring: return back safer resurrect

mainline inclusion
from mainline-v5.13-rc1
commit f70865db
category: bugfix
bugzilla: 185739
CVE: NA

-----------------------------------------------

Revert of revert of "io_uring: wait potential ->release() on resurrect",
which adds a helper for resurrect not racing completion reinit, as was
removed because of a strange bug with no clear root or link to the
patch.

Was improved, instead of rcu_synchronize(), just wait_for_completion()
because we're at 0 refs and it will happen very shortly. Specifically
use non-interruptible version to ignore all pending signals that may
have ended prior interruptible wait.

This reverts commit cb5e1b81.
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/7a080c20f686d026efade810b116b72f88abaff9.1618101759.git.asml.silence@gmail.comSigned-off-by: NJens Axboe <axboe@kernel.dk>

conflicts:
fs/io_uring.c
Signed-off-by: NYe Bin <yebin10@huawei.com>
Reviewed-by: NZhang Yi <yi.zhang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/io_uring.c

@@ -8669,6 +8669,18 @@ static bool io_register_op_must_quiesce(int op)
 	}
 }
 
+static void io_refs_resurrect(struct percpu_ref *ref, struct completion *compl)
+{
+	bool got = percpu_ref_tryget(ref);
+
+	/* already at zero, wait for ->release() */
+	if (!got)
+		wait_for_completion(compl);
+	percpu_ref_resurrect(ref);
+	if (got)
+		percpu_ref_put(ref);
+}
+
 static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 			       void __user *arg, unsigned nr_args)
 	__releases(ctx->uring_lock)
@@ -8699,9 +8711,8 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 		ret = wait_for_completion_interruptible(&ctx->ref_comp);
 		mutex_lock(&ctx->uring_lock);
 		if (ret) {
-			percpu_ref_resurrect(&ctx->refs);
-			ret = -EINTR;
-			goto out;
+			io_refs_resurrect(&ctx->refs, &ctx->ref_comp);
+			return ret;
 		}
 	}
 
@@ -8772,7 +8783,6 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
 	if (io_register_op_must_quiesce(opcode)) {
 		/* bring the ctx back to life */
 		percpu_ref_reinit(&ctx->refs);
-out:
 		reinit_completion(&ctx->ref_comp);
 	}
 	return ret;