io_uring: fix io_kiocb.flags modification race in IOPOLL mode

While testing io_uring in arm, we found sometimes io_sq_thread() keeps
polling io requests even though there are not inflight io requests in
block layer. After some investigations, found a possible race about
io_kiocb.flags, see below race codes:
  1) in the end of io_write() or io_read()
    req->flags &= ~REQ_F_NEED_CLEANUP;
    kfree(iovec);
    return ret;

  2) in io_complete_rw_iopoll()
    if (res != -EAGAIN)
        req->flags |= REQ_F_IOPOLL_COMPLETED;

In IOPOLL mode, io requests still maybe completed by interrupt, then
above codes are not safe, concurrent modifications to req->flags, which
is not protected by lock or is not atomic modifications. I also had
disassemble io_complete_rw_iopoll() in arm:
   req->flags |= REQ_F_IOPOLL_COMPLETED;
   0xffff000008387b18 <+76>:    ldr     w0, [x19,#104]
   0xffff000008387b1c <+80>:    orr     w0, w0, #0x1000
   0xffff000008387b20 <+84>:    str     w0, [x19,#104]

Seems that the "req->flags |= REQ_F_IOPOLL_COMPLETED;" is  load and
modification, two instructions, which obviously is not atomic.

To fix this issue, add a new iopoll_completed in io_kiocb to indicate
whether io request is completed.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5b02491..61fca5a 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -529,7 +529,6 @@ enum {
        REQ_F_INFLIGHT_BIT,
        REQ_F_CUR_POS_BIT,
        REQ_F_NOWAIT_BIT,
-       REQ_F_IOPOLL_COMPLETED_BIT,
        REQ_F_LINK_TIMEOUT_BIT,
        REQ_F_TIMEOUT_BIT,
        REQ_F_ISREG_BIT,
@@ -574,8 +573,6 @@ enum {
        REQ_F_CUR_POS           = BIT(REQ_F_CUR_POS_BIT),
        /* must not punt to workers */
        REQ_F_NOWAIT            = BIT(REQ_F_NOWAIT_BIT),
-       /* polled IO has completed */
-       REQ_F_IOPOLL_COMPLETED  = BIT(REQ_F_IOPOLL_COMPLETED_BIT),
        /* has linked timeout */
        REQ_F_LINK_TIMEOUT      = BIT(REQ_F_LINK_TIMEOUT_BIT),
        /* timeout request */
@@ -640,6 +637,8 @@ struct io_kiocb {
        struct io_async_ctx             *io;
        int                             cflags;
        u8                              opcode;
+       /* polled IO has completed */
+       u8                              iopoll_completed;
 
        u16                             buf_index;
 
@@ -1798,7 +1797,7 @@ static int io_do_iopoll(struct io_ring_ctx *ctx, unsigned int *nr_events,
                 * If we find a request that requires polling, break out
                 * and complete those lists first, if we have entries there.
                 */
-               if (req->flags & REQ_F_IOPOLL_COMPLETED) {
+               if (READ_ONCE(req->iopoll_completed)) {
                        list_move_tail(&req->list, &done);
                        continue;
                }
@@ -1979,7 +1978,7 @@ static void io_complete_rw_iopoll(struct kiocb *kiocb, long res, long res2)
                req_set_fail_links(req);
        req->result = res;
        if (res != -EAGAIN)
-               req->flags |= REQ_F_IOPOLL_COMPLETED;
+               WRITE_ONCE(req->iopoll_completed, 1);
 }
 
 /*
@@ -2012,7 +2011,7 @@ static void io_iopoll_req_issued(struct io_kiocb *req)
         * For fast devices, IO may have already completed. If it has, add
         * it to the front so we find it first.
         */
-       if (req->flags & REQ_F_IOPOLL_COMPLETED)
+       if (READ_ONCE(req->iopoll_completed))
                list_add(&req->list, &ctx->poll_list);
        else
                list_add_tail(&req->list, &ctx->poll_list);
@@ -2140,6 +2139,7 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe,
                kiocb->ki_flags |= IOCB_HIPRI;
                kiocb->ki_complete = io_complete_rw_iopoll;
                req->result = 0;
+               req->iopoll_completed = 0;
        } else {
                if (kiocb->ki_flags & IOCB_HIPRI)
                        return -EINVAL;