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io_uring: fix racy REQ_F_LINK_TIMEOUT clearing 

io_link_timeout_fn() removes REQ_F_LINK_TIMEOUT from the link head's
flags, it's not atomic and may race with what the head is doing.

If io_link_timeout_fn() doesn't clear the flag, as forced by this patch,
then it may happen that for "req -> link_timeout1 -> link_timeout2",
__io_kill_linked_timeout() would find link_timeout2 and try to cancel
it, so miscounting references. Teach it to ignore such double timeouts
by marking the active one with a new flag in io_prep_linked_timeout().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Pavel Begunkov 2020-10-19 16:39:16 +01:00 committed by Jens Axboe
parent 4d52f33899
commit 900fad45dc
1 changed files with 13 additions and 4 deletions
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17
fs/io_uring.c
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@ -585,6 +585,7 @@ enum {
REQ_F_BUFFER_SELECTED_BIT,
REQ_F_NO_FILE_TABLE_BIT,
REQ_F_WORK_INITIALIZED_BIT,
REQ_F_LTIMEOUT_ACTIVE_BIT,
/* not a real bit, just to check we're not overflowing the space */
__REQ_F_LAST_BIT,
@ -614,7 +615,7 @@ enum {
REQ_F_CUR_POS = BIT(REQ_F_CUR_POS_BIT),
/* must not punt to workers */
REQ_F_NOWAIT = BIT(REQ_F_NOWAIT_BIT),
/* has linked timeout */
/* has or had linked timeout */
REQ_F_LINK_TIMEOUT = BIT(REQ_F_LINK_TIMEOUT_BIT),
/* regular file */
REQ_F_ISREG = BIT(REQ_F_ISREG_BIT),
@ -628,6 +629,8 @@ enum {
REQ_F_NO_FILE_TABLE = BIT(REQ_F_NO_FILE_TABLE_BIT),
/* io_wq_work is initialized */
REQ_F_WORK_INITIALIZED = BIT(REQ_F_WORK_INITIALIZED_BIT),
/* linked timeout is active, i.e. prepared by link's head */
REQ_F_LTIMEOUT_ACTIVE = BIT(REQ_F_LTIMEOUT_ACTIVE_BIT),
};
struct async_poll {
@ -1871,6 +1874,12 @@ static bool __io_kill_linked_timeout(struct io_kiocb *req)
link = list_first_entry(&req->link_list, struct io_kiocb, link_list);
if (link->opcode != IORING_OP_LINK_TIMEOUT)
return false;
/*
* Can happen if a linked timeout fired and link had been like
* req -> link t-out -> link t-out [-> ...]
*/
if (!(link->flags & REQ_F_LTIMEOUT_ACTIVE))
return false;
list_del_init(&link->link_list);
wake_ev = io_link_cancel_timeout(link);
@ -6106,10 +6115,9 @@ static enum hrtimer_restart io_link_timeout_fn(struct hrtimer *timer)
if (!list_empty(&req->link_list)) {
prev = list_entry(req->link_list.prev, struct io_kiocb,
link_list);
if (refcount_inc_not_zero(&prev->refs)) {
if (refcount_inc_not_zero(&prev->refs))
list_del_init(&req->link_list);
prev->flags &= ~REQ_F_LINK_TIMEOUT;
} else
else
prev = NULL;
}
@ -6166,6 +6174,7 @@ static struct io_kiocb *io_prep_linked_timeout(struct io_kiocb *req)
if (!nxt || nxt->opcode != IORING_OP_LINK_TIMEOUT)
return NULL;
nxt->flags |= REQ_F_LTIMEOUT_ACTIVE;
req->flags |= REQ_F_LINK_TIMEOUT;
return nxt;
}