linux-kernel - [PATCH 2/4] aio: add a iocb refcount

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Message-Id: <20180726082903.30273-3-hch@lst.de>
Date:   Thu, 26 Jul 2018 10:29:01 +0200
From:   Christoph Hellwig <hch@....de>
To:     viro@...iv.linux.org.uk
Cc:     Avi Kivity <avi@...lladb.com>, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 2/4] aio: add a iocb refcount

This is needed to prevent races caused by the way the ->poll API works.
To avoid introducing overhead for other users of the iocbs we initialize
it to zero and only do refcount operations if it is non-zero in the
completion path.

Signed-off-by: Christoph Hellwig <hch@....de>
---
 fs/aio.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 27454594e37a..7f3c159b3e2e 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -178,6 +178,7 @@ struct aio_kiocb {
 
 	struct list_head	ki_list;	/* the aio core uses this
 						 * for cancellation */
+	atomic_t		ki_refcnt;
 
 	/*
 	 * If the aio_resfd field of the userspace iocb is not zero,
@@ -1015,6 +1016,7 @@ static inline struct aio_kiocb *aio_get_req(struct kioctx *ctx)
 
 	percpu_ref_get(&ctx->reqs);
 	INIT_LIST_HEAD(&req->ki_list);
+	atomic_set(&req->ki_refcnt, 0);
 	req->ki_ctx = ctx;
 	return req;
 out_put:
@@ -1049,6 +1051,15 @@ static struct kioctx *lookup_ioctx(unsigned long ctx_id)
 	return ret;
 }
 
+static inline void iocb_put(struct aio_kiocb *iocb)
+{
+	if (atomic_read(&iocb->ki_refcnt) == 0 ||
+	    atomic_dec_and_test(&iocb->ki_refcnt)) {
+		percpu_ref_put(&iocb->ki_ctx->reqs);
+		kmem_cache_free(kiocb_cachep, iocb);
+	}
+}
+
 /* aio_complete
  *	Called when the io request on the given iocb is complete.
  */
@@ -1118,8 +1129,6 @@ static void aio_complete(struct aio_kiocb *iocb, long res, long res2)
 		eventfd_ctx_put(iocb->ki_eventfd);
 	}
 
-	kmem_cache_free(kiocb_cachep, iocb);
-
 	/*
 	 * We have to order our ring_info tail store above and test
 	 * of the wait list below outside the wait lock.  This is
@@ -1130,8 +1139,7 @@ static void aio_complete(struct aio_kiocb *iocb, long res, long res2)
 
 	if (waitqueue_active(&ctx->wait))
 		wake_up(&ctx->wait);
-
-	percpu_ref_put(&ctx->reqs);
+	iocb_put(iocb);
 }
 
 /* aio_read_events_ring
-- 
2.18.0