Re: [PATCH v3 1/2] io_uring: change the poll events to be 32-bits

[Jens Axboe <axboe@kernel.dk>]

On 6/12/20 8:58 AM, Jens Axboe wrote:
> On 6/11/20 8:30 PM, Jiufei Xue wrote:
>> poll events should be 32-bits to cover EPOLLEXCLUSIVE.
>>
>> Signed-off-by: Jiufei Xue <jiufei.xue@linux.alibaba.com>
>> ---
>>  fs/io_uring.c                 | 4 ++--
>>  include/uapi/linux/io_uring.h | 2 +-
>>  tools/io_uring/liburing.h     | 2 +-
>>  3 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 47790a2..6250227 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -4602,7 +4602,7 @@ static void io_poll_queue_proc(struct file *file, struct wait_queue_head *head,
>>  static int io_poll_add_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>>  {
>>  	struct io_poll_iocb *poll = &req->poll;
>> -	u16 events;
>> +	u32 events;
>>  
>>  	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
>>  		return -EINVAL;
>> @@ -8196,7 +8196,7 @@ static int __init io_uring_init(void)
>>  	BUILD_BUG_SQE_ELEM(28, /* compat */   int, rw_flags);
>>  	BUILD_BUG_SQE_ELEM(28, /* compat */ __u32, rw_flags);
>>  	BUILD_BUG_SQE_ELEM(28, __u32,  fsync_flags);
>> -	BUILD_BUG_SQE_ELEM(28, __u16,  poll_events);
>> +	BUILD_BUG_SQE_ELEM(28, __u32,  poll_events);
>>  	BUILD_BUG_SQE_ELEM(28, __u32,  sync_range_flags);
>>  	BUILD_BUG_SQE_ELEM(28, __u32,  msg_flags);
>>  	BUILD_BUG_SQE_ELEM(28, __u32,  timeout_flags);
>> diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
>> index 92c2269..afc7edd 100644
>> --- a/include/uapi/linux/io_uring.h
>> +++ b/include/uapi/linux/io_uring.h
>> @@ -31,7 +31,7 @@ struct io_uring_sqe {
>>  	union {
>>  		__kernel_rwf_t	rw_flags;
>>  		__u32		fsync_flags;
>> -		__u16		poll_events;
>> +		__u32		poll_events;
>>  		__u32		sync_range_flags;
>>  		__u32		msg_flags;
>>  		__u32		timeout_flags;
> 
> We obviously have the space in there as most other flag members are 32-bits, but
> I'd want to double check if we're not changing the ABI here. Is this always
> going to be safe, on any platform, regardless of endianess etc?

Double checked, and as I feared, we can't safely do this. We'll have to
do something like the below, grabbing an unused bit of the poll mask
space and if that's set, then store the fact that EPOLLEXCLUSIVE is set.
So probably best to turn this just into one patch, since it doesn't make
a lot of sense to do it as a prep patch at that point.

This does have the benefit of not growing io_poll_iocb. With your patch,
it'd go beyond a cacheline, and hence bump the size of the entire
io_iocb as well, which would be very unfortunate.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 155f3d830ddb..64a98bf11943 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -350,6 +350,7 @@ struct io_poll_iocb {
 		u64			addr;
 	};
 	__poll_t			events;
+	bool				exclusive;
 	bool				done;
 	bool				canceled;
 	struct wait_queue_entry		wait;
@@ -4543,7 +4544,7 @@ static void io_poll_queue_proc(struct file *file, struct wait_queue_head *head,
 static int io_poll_add_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_poll_iocb *poll = &req->poll;
-	u16 events;
+	u32 events;
 
 	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
 		return -EINVAL;
@@ -4553,6 +4554,9 @@ static int io_poll_add_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe
 		return -EBADF;
 
 	events = READ_ONCE(sqe->poll_events);
+	if ((events & IORING_POLL_32BIT) &&
+	    (sqe->poll32_events & EPOLLEXCLUSIVE))
+		poll->exclusive = true;
 	poll->events = demangle_poll(events) | EPOLLERR | EPOLLHUP;
 
 	get_task_struct(current);
@@ -8155,6 +8159,7 @@ static int __init io_uring_init(void)
 	BUILD_BUG_SQE_ELEM(28, /* compat */ __u32, rw_flags);
 	BUILD_BUG_SQE_ELEM(28, __u32,  fsync_flags);
 	BUILD_BUG_SQE_ELEM(28, __u16,  poll_events);
+	BUILD_BUG_SQE_ELEM(28, __u32,  poll32_events);
 	BUILD_BUG_SQE_ELEM(28, __u32,  sync_range_flags);
 	BUILD_BUG_SQE_ELEM(28, __u32,  msg_flags);
 	BUILD_BUG_SQE_ELEM(28, __u32,  timeout_flags);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 92c22699a5a7..16d473d909eb 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -32,6 +32,7 @@ struct io_uring_sqe {
 		__kernel_rwf_t	rw_flags;
 		__u32		fsync_flags;
 		__u16		poll_events;
+		__u32		poll32_events;
 		__u32		sync_range_flags;
 		__u32		msg_flags;
 		__u32		timeout_flags;
@@ -60,6 +61,8 @@ struct io_uring_sqe {
 	};
 };
 
+#define IORING_POLL_32BIT	(1U << 15)
+
 enum {
 	IOSQE_FIXED_FILE_BIT,
 	IOSQE_IO_DRAIN_BIT,

-- 
Jens Axboe