linux-kernel - [RFC PATCH 04/13] fs/userfaultfd: simplify locks in userfaultfd_ctx

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Message-Id: <20201129004548.1619714-5-namit@vmware.com>
Date:   Sat, 28 Nov 2020 16:45:39 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     linux-fsdevel@...r.kernel.org
Cc:     Nadav Amit <namit@...are.com>, Jens Axboe <axboe@...nel.dk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: [RFC PATCH 04/13] fs/userfaultfd: simplify locks in userfaultfd_ctx_read

From: Nadav Amit <namit@...are.com>

Small refactoring to reduce the number of locations in which locks are
released in userfaultfd_ctx_read(), as this makes the understanding of
the code and its changes harder.

No functional change intended.

Cc: Jens Axboe <axboe@...nel.dk>
Cc: Andrea Arcangeli <aarcange@...hat.com>
Cc: Peter Xu <peterx@...hat.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: io-uring@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Cc: linux-mm@...ck.org
Signed-off-by: Nadav Amit <namit@...are.com>
---
 fs/userfaultfd.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 4fe07c1a44c6..fedf7c1615d5 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -1039,6 +1039,7 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
 		set_current_state(TASK_INTERRUPTIBLE);
 		spin_lock(&ctx->fault_pending_wqh.lock);
 		uwq = find_userfault(ctx);
+		ret = -EAGAIN;
 		if (uwq) {
 			/*
 			 * Use a seqcount to repeat the lockless check
@@ -1077,11 +1078,11 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
 
 			/* careful to always initialize msg if ret == 0 */
 			*msg = uwq->msg;
-			spin_unlock(&ctx->fault_pending_wqh.lock);
 			ret = 0;
-			break;
 		}
 		spin_unlock(&ctx->fault_pending_wqh.lock);
+		if (!ret)
+			break;
 
 		spin_lock(&ctx->event_wqh.lock);
 		uwq = find_userfault_evt(ctx);
@@ -1099,17 +1100,14 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
 				 * reference on it.
 				 */
 				userfaultfd_ctx_get(fork_nctx);
-				spin_unlock(&ctx->event_wqh.lock);
-				ret = 0;
-				break;
+			} else {
+				userfaultfd_event_complete(ctx, uwq);
 			}
-
-			userfaultfd_event_complete(ctx, uwq);
-			spin_unlock(&ctx->event_wqh.lock);
 			ret = 0;
-			break;
 		}
 		spin_unlock(&ctx->event_wqh.lock);
+		if (!ret)
+			break;
 
 		if (signal_pending(current)) {
 			ret = -ERESTARTSYS;
-- 
2.25.1