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Subject[PATCH 2/4] aio: add a iocb refcount
Date
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@lst.de>
Tested-by: Avi Kivity <avi@scylladb.com>
---
fs/aio.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 27454594e37a..fe2018ada32c 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -18,6 +18,7 @@
#include <linux/export.h>
#include <linux/syscalls.h>
#include <linux/backing-dev.h>
+#include <linux/refcount.h>
#include <linux/uio.h>

#include <linux/sched/signal.h>
@@ -178,6 +179,7 @@ struct aio_kiocb {

struct list_head ki_list; /* the aio core uses this
* for cancellation */
+ refcount_t ki_refcnt;

/*
* If the aio_resfd field of the userspace iocb is not zero,
@@ -1015,6 +1017,7 @@ static inline struct aio_kiocb *aio_get_req(struct kioctx *ctx)

percpu_ref_get(&ctx->reqs);
INIT_LIST_HEAD(&req->ki_list);
+ refcount_set(&req->ki_refcnt, 0);
req->ki_ctx = ctx;
return req;
out_put:
@@ -1049,6 +1052,15 @@ static struct kioctx *lookup_ioctx(unsigned long ctx_id)
return ret;
}

+static inline void iocb_put(struct aio_kiocb *iocb)
+{
+ if (refcount_read(&iocb->ki_refcnt) == 0 ||
+ refcount_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 +1130,6 @@ 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 +1140,7 @@ 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
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