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From: Nadav Amit <nadav.amit@gmail.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: io-uring@vger.kernel.org, linux-kernel@vger.kernel.org,
	Nadav Amit <namit@vmware.com>,
	Pavel Begunkov <asml.silence@gmail.com>
Subject: [PATCH 2/2] io_uring: Use WRITE_ONCE() when writing to sq_flags
Date: Sat,  7 Aug 2021 17:13:42 -0700	[thread overview]
Message-ID: <20210808001342.964634-3-namit@vmware.com> (raw)
In-Reply-To: <20210808001342.964634-1-namit@vmware.com>

From: Nadav Amit <namit@vmware.com>

The compiler should be forbidden from any strange optimization for async
writes to user visible data-structures. Without proper protection, the
compiler can cause write-tearing or invent writes that would confuse the
userspace.

However, there are writes to sq_flags which are not protected by
WRITE_ONCE(). Use WRITE_ONCE() for these writes.

This is purely a theoretical issue. Presumably, any compiler is very
unlikely to do such optimizations.

Fixes: 75b28affdd6a ("io_uring: allocate the two rings together")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
---
 fs/io_uring.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index f39244d35f90..c78b487d5a80 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1491,7 +1491,8 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
 	all_flushed = list_empty(&ctx->cq_overflow_list);
 	if (all_flushed) {
 		clear_bit(0, &ctx->check_cq_overflow);
-		ctx->rings->sq_flags &= ~IORING_SQ_CQ_OVERFLOW;
+		WRITE_ONCE(ctx->rings->sq_flags,
+			   ctx->rings->sq_flags & ~IORING_SQ_CQ_OVERFLOW);
 	}
 
 	if (posted)
@@ -1570,7 +1571,9 @@ static bool io_cqring_event_overflow(struct io_ring_ctx *ctx, u64 user_data,
 	}
 	if (list_empty(&ctx->cq_overflow_list)) {
 		set_bit(0, &ctx->check_cq_overflow);
-		ctx->rings->sq_flags |= IORING_SQ_CQ_OVERFLOW;
+		WRITE_ONCE(ctx->rings->sq_flags,
+			   ctx->rings->sq_flags | IORING_SQ_CQ_OVERFLOW);
+
 	}
 	ocqe->cqe.user_data = user_data;
 	ocqe->cqe.res = res;
@@ -6780,14 +6783,16 @@ static inline void io_ring_set_wakeup_flag(struct io_ring_ctx *ctx)
 {
 	/* Tell userspace we may need a wakeup call */
 	spin_lock_irq(&ctx->completion_lock);
-	ctx->rings->sq_flags |= IORING_SQ_NEED_WAKEUP;
+	WRITE_ONCE(ctx->rings->sq_flags,
+		   ctx->rings->sq_flags | IORING_SQ_NEED_WAKEUP);
 	spin_unlock_irq(&ctx->completion_lock);
 }
 
 static inline void io_ring_clear_wakeup_flag(struct io_ring_ctx *ctx)
 {
 	spin_lock_irq(&ctx->completion_lock);
-	ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
+	WRITE_ONCE(ctx->rings->sq_flags,
+		   ctx->rings->sq_flags & ~IORING_SQ_NEED_WAKEUP);
 	spin_unlock_irq(&ctx->completion_lock);
 }
 
-- 
2.25.1


  parent reply	other threads:[~2021-08-08  0:14 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-08  0:13 [PATCH 0/2] io_uring: bug fixes Nadav Amit
2021-08-08  0:13 ` [PATCH 1/2] io_uring: clear TIF_NOTIFY_SIGNAL when running task work Nadav Amit
2021-08-08 12:55   ` Pavel Begunkov
2021-08-08 17:31     ` Nadav Amit
2021-08-09  4:07       ` Hao Xu
2021-08-09  4:50         ` Nadav Amit
2021-08-09 10:35           ` Pavel Begunkov
2021-08-09 10:18       ` Pavel Begunkov
2021-08-09 21:48   ` Olivier Langlois
2021-08-10  8:28     ` Nadav Amit
2021-08-10 13:33       ` Olivier Langlois
2021-08-10 21:32       ` Pavel Begunkov
2021-08-11  2:33         ` Nadav Amit
2021-08-11  2:51           ` Jens Axboe
2021-08-11  5:40             ` I/O cancellation in io-uring (was: io_uring: clear TIF_NOTIFY_SIGNAL ...) Nadav Amit
2021-08-08  0:13 ` Nadav Amit [this message]
2021-08-09 13:53 ` [PATCH 0/2] io_uring: bug fixes Jens Axboe

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