ERR_PTR abuses the high bits of a pointer to transport error information.
This is only safe for kernel pointers and not user pointers. Fix
io_buffer_select and its helpers to just return NULL for failure and get
rid of this abuse.
io_uring: add support for ring mapped supplied buffers
Provided buffers allow an application to supply io_uring with buffers
that can then be grabbed for a read/receive request, when the data
source is ready to deliver data. The existing scheme relies on using
IORING_OP_PROVIDE_BUFFERS to do that, but it can be difficult to use
in real world applications. It's pretty efficient if the application
is able to supply back batches of provided buffers when they have been
consumed and the application is ready to recycle them, but if
fragmentation occurs in the buffer space, it can become difficult to
supply enough buffers at the time. This hurts efficiency.
Add a register op, IORING_REGISTER_PBUF_RING, which allows an application
to setup a shared queue for each buffer group of provided buffers. The
application can then supply buffers simply by adding them to this ring,
and the kernel can consume then just as easily. The ring shares the head
with the application, the tail remains private in the kernel.
Provided buffers setup with IORING_REGISTER_PBUF_RING cannot use
IORING_OP_{PROVIDE,REMOVE}_BUFFERS for adding or removing entries to the
ring, they must use the mapped ring. Mapped provided buffer rings can
co-exist with normal provided buffers, just not within the same group ID.
To gauge overhead of the existing scheme and evaluate the mapped ring
approach, a simple NOP benchmark was written. It uses a ring of 128
entries, and submits/completes 32 at the time. 'Replenish' is how
many buffers are provided back at the time after they have been
consumed:
Test Replenish NOPs/sec
================================================================
No provided buffers NA ~30M
Provided buffers 32 ~16M
Provided buffers 1 ~10M
Ring buffers 32 ~27M
Ring buffers 1 ~27M
The ring mapped buffers perform almost as well as not using provided
buffers at all, and they don't care if you provided 1 or more back at
the same time. This means application can just replenish as they go,
rather than need to batch and compact, further reducing overhead in the
application. The NOP benchmark above doesn't need to do any compaction,
so that overhead isn't even reflected in the above test.
Jens Axboe [Sun, 15 May 2022 17:19:43 +0000 (11:19 -0600)]
io_uring: fix locking state for empty buffer group
io_provided_buffer_select() must drop the submit lock, if needed, even
in the error handling case. Failure to do so will leave us with the
ctx->uring_lock held, causing spew like:
====================================
WARNING: iou-wrk-366/368 still has locks held! 5.18.0-rc6-00294-gdf8dc7004331 #994 Not tainted
------------------------------------
1 lock held by iou-wrk-366/368:
#0: ffff0000c72598a8 (&ctx->uring_lock){+.+.}-{3:3}, at: io_ring_submit_lock+0x20/0x48
Hao Xu [Sat, 14 May 2022 14:20:45 +0000 (22:20 +0800)]
io_uring: let fast poll support multishot
For operations like accept, multishot is a useful feature, since we can
reduce a number of accept sqe. Let's integrate it to fast poll, it may
be good for other operations in the future.
Hao Xu [Sat, 14 May 2022 14:20:44 +0000 (22:20 +0800)]
io_uring: add REQ_F_APOLL_MULTISHOT for requests
Add a flag to indicate multishot mode for fast poll. currently only
accept use it, but there may be more operations leveraging it in the
future. Also add a mask IO_APOLL_MULTI_POLLED which stands for
REQ_F_APOLL_MULTI | REQ_F_POLLED, to make the code short and cleaner.
Dylan Yudaken [Thu, 12 May 2022 09:18:34 +0000 (02:18 -0700)]
io_uring: only wake when the correct events are set
The check for waking up a request compares the poll_t bits, however this
will always contain some common flags so this always wakes up.
For files with single wait queues such as sockets this can cause the
request to be sent to the async worker unnecesarily. Further if it is
non-blocking will complete the request with EAGAIN which is not desired.
Here exclude these common events, making sure to not exclude POLLERR which
might be important.
Pavel Begunkov [Fri, 13 May 2022 10:24:56 +0000 (11:24 +0100)]
io_uring: avoid io-wq -EAGAIN looping for !IOPOLL
If an opcode handler semi-reliably returns -EAGAIN, io_wq_submit_work()
might continue busily hammer the same handler over and over again, which
is not ideal. The -EAGAIN handling in question was put there only for
IOPOLL, so restrict it to IOPOLL mode only where there is no other
recourse than to retry as we cannot wait.
Jens Axboe [Mon, 9 May 2022 15:29:14 +0000 (09:29 -0600)]
io_uring: add flag for allocating a fully sparse direct descriptor space
Currently to setup a fully sparse descriptor space upfront, the app needs
to alloate an array of the full size and memset it to -1 and then pass
that in. Make this a bit easier by allowing a flag that simply does
this internally rather than needing to copy each slot separately.
This works with IORING_REGISTER_FILES2 as the flag is set in struct
io_uring_rsrc_register, and is only allow when the type is
IORING_RSRC_FILE as this doesn't make sense for registered buffers.
Jens Axboe [Mon, 9 May 2022 15:11:01 +0000 (09:11 -0600)]
io_uring: bump max direct descriptor count to 1M
We currently limit these to 32K, but since we're now backing the table
space with vmalloc when needed, there's no reason why we can't make it
bigger. The total space is limited by RLIMIT_NOFILE as well.
Jens Axboe [Sat, 7 May 2022 16:08:31 +0000 (10:08 -0600)]
io_uring: allow allocated fixed files for accept
If the application passes in IORING_FILE_INDEX_ALLOC as the file_slot,
then that's a hint to allocate a fixed file descriptor rather than have
one be passed in directly.
This can be useful for having io_uring manage the direct descriptor space,
and also allows multi-shot support to work with fixed files.
Normal accept direct requests will complete with 0 for success, and < 0
in case of error. If io_uring is asked to allocated the direct descriptor,
then the direct descriptor is returned in case of success.
Jens Axboe [Sat, 7 May 2022 20:18:44 +0000 (14:18 -0600)]
io_uring: allow allocated fixed files for openat/openat2
If the application passes in IORING_FILE_INDEX_ALLOC as the file_slot,
then that's a hint to allocate a fixed file descriptor rather than have
one be passed in directly.
This can be useful for having io_uring manage the direct descriptor space.
Normal open direct requests will complete with 0 for success, and < 0
in case of error. If io_uring is asked to allocated the direct descriptor,
then the direct descriptor is returned in case of success.
Jens Axboe [Sat, 7 May 2022 20:20:40 +0000 (14:20 -0600)]
io_uring: add basic fixed file allocator
Applications currently always pick where they want fixed files to go.
In preparation for allowing these types of commands with multishot
support, add a basic allocator in the fixed file table.
Jens Axboe [Sun, 1 May 2022 18:08:35 +0000 (12:08 -0600)]
io_uring: eliminate the need to track provided buffer ID separately
We have io_kiocb->buf_index which is used for either fixed buffers, or
for provided buffers. For the latter, it's used to hold the buffer group
ID for buffer selection. Post selection, req->kbuf->bid is used to get
the buffer ID.
Store the buffer ID, when selected, in req->buf_index. If we do end up
recycling the buffer, reset it back to the buffer group ID.
io_uring: move provided and fixed buffers into the same io_kiocb area
These are mutually exclusive - if you use provided buffers, then you
cannot use fixed buffers and vice versa. Move them into the same spot
in the io_kiocb, which is also advantageous for provided buffers as
they get near the submit side hot cacheline.
Jens Axboe [Sun, 1 May 2022 16:52:44 +0000 (10:52 -0600)]
io_uring: get rid of hashed provided buffer groups
Use a plain array for any group ID that's less than 64, and punt
anything beyond that to an xarray. 64 fits in a page even for 4KB
page sizes and with the planned additions.
This makes the expected group usage faster by avoiding a hash and lookup
to find our list, and it uses less memory upfront by not allocating any
memory for provided buffers unless it's actually being used.
Suggested-by: Pavel Begunkov <asml.silence@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
io_uring: always use req->buf_index for the provided buffer group
The read/write opcodes use it already, but the recv/recvmsg do not. If
we switch them over and read and validate this at init time while we're
checking if the opcode supports it anyway, then we can do it in one spot
and we don't have to pass in a separate group ID for io_buffer_select().
After the recent changes, this is direct call to io_buffer_select()
anyway. With this change, there are no wrappers left for provided
buffer selection.
For all of send/sendmsg and recv/recvmsg we have the local 'sr' variable,
yet some cases still use req->sr_msg which sr points to. Use 'sr'
consistently.
io_uring: add POLL_FIRST support for send/sendmsg and recv/recvmsg
If IORING_RECVSEND_POLL_FIRST is set for recv/recvmsg or send/sendmsg,
then we arm poll first rather than attempt a receive or send upfront.
This can be useful if we expect there to be no data (or space) available
for the request, as we can then avoid wasting time on the initial
issue attempt.
Don't punt this check to the op prep handlers, add the support to
io_op_defs and we can check them while setting up the request.
This reduces the text size by 500 bytes on aarch64, and makes this less
fragile by having the check in one spot and needing opcodes to opt in
to IOPOLL or ioprio support.
io_uring: replace smp_mb() with smp_mb__after_atomic() in io_sq_thread()
The IORING_SQ_NEED_WAKEUP flag is now set using atomic_or() which
implies a full barrier on some architectures but it is not required to
do so. Use the more appropriate smp_mb__after_atomic() which avoids the
extra barrier on those architectures.
If IORING_SETUP_COOP_TASKRUN is set to use cooperative scheduling for
running task_work, then IORING_SETUP_TASKRUN_FLAG can be set so the
application can tell if task_work is pending in the kernel for this
ring. This allows use cases like io_uring_peek_cqe() to still function
appropriately, or for the task to know when it would be useful to
call io_uring_wait_cqe() to run pending events.
io_uring: use TWA_SIGNAL_NO_IPI if IORING_SETUP_COOP_TASKRUN is used
If this is set, io_uring will never use an IPI to deliver a task_work
notification. This can be used in the common case where a single task or
thread communicates with the ring, and doesn't rely on
io_uring_cqe_peek().
This provides a noticeable win in performance, both from eliminating
the IPI itself, but also from avoiding interrupting the submitting
task unnecessarily.
io_uring: set task_work notify method at init time
While doing so, switch SQPOLL to TWA_SIGNAL_NO_IPI as well, as that
just does a task wakeup and then we can remove the special wakeup we
have in task_work_add.
The only difference between set_notify_signal() and __set_notify_signal()
is that the former checks if it needs to deliver an IPI to force a
reschedule. As the io-wq workers never leave the kernel, and IPI is never
needed, they simply need a wakeup.
io_uring: serialize ctx->rings->sq_flags with atomic_or/and
Rather than require ctx->completion_lock for ensuring that we don't
clobber the flags, use the atomic bitop helpers instead. This removes
the need to grab the completion_lock, in preparation for needing to set
or clear sq_flags when we don't know the status of this lock.
task_work: allow TWA_SIGNAL without a rescheduling IPI
Some use cases don't always need an IPI when sending a TWA_SIGNAL
notification. Add TWA_SIGNAL_NO_IPI, which is just like TWA_SIGNAL, except
it doesn't send an IPI to the target task. It merely sets
TIF_NOTIFY_SIGNAL and wakes up the task.
This can be useful in avoiding a forceful transition to the kernel if the
task is running in userspace. Depending on the task_work in question, it
may be quite fine waiting for the next reschedule or kernel enter anyway,
or the use case may even have other mechanisms for hinting to the task
that a transition may be useful. This can drive more cooperative
scheduling of task_work.
If IO_URING_SCM_ALL isn't set, as it would not be on 32-bit builds,
then we trigger a warning:
fs/io_uring.c: In function '__io_sqe_files_unregister':
fs/io_uring.c:8992:13: warning: unused variable 'i' [-Wunused-variable]
8992 | int i;
| ^
Move the ifdef up to include the 'i' variable declaration.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au> Fixes: 8bd895003118 ("io_uring: store SCM state in io_fixed_file->file_ptr") Signed-off-by: Jens Axboe <axboe@kernel.dk>
Right now io_uring will not actively inform userspace if a CQE is
dropped. This is extremely rare, requiring a CQ ring overflow, as well as
a GFP_ATOMIC kmalloc failure. However the consequences could cause for
example applications to go into an undefined state, possibly waiting for a
CQE that never arrives.
Return an error code (EBADR) in these cases. Since this is expected to be
incredibly rare, try and avoid as much as possible affecting the hot code
paths, and so it only is returned lazily and when there is no other
available CQEs.
Once the error is returned, reset the error condition assuming the user is
either ok with it or will clean up appropriately.
io_uring: rework io_uring_enter to simplify return value
io_uring_enter returns the count submitted preferrably over an error
code. In some code paths this check is not required, so reorganise the
code so that the check is only done as needed.
This is also a prep for returning error codes only in waiting scenarios.
We currently check REQ_F_POLLED before arming async poll for a
notification to retry. If it's set, then we don't allow poll and will
punt to io-wq instead. This is done to prevent a situation where a buggy
driver will repeatedly return that there's space/data available yet we
get -EAGAIN.
However, if we already transferred data, then it should be safe to rely
on poll again. Gate the check on whether or not REQ_F_PARTIAL_IO is
also set.
io_uring: support MSG_WAITALL for IORING_OP_SEND(MSG)
Like commit cb9342f20212 for recv/recvmsg, support MSG_WAITALL for the
send side. If this flag is set and we do a short send, retry for a
stream of seqpacket socket.
Rather than match on a specific key, be it user_data or file, allow
canceling any request that we can lookup. Works like
IORING_ASYNC_CANCEL_ALL in that it cancels multiple requests, but it
doesn't key off user_data or the file.
Can't be set with IORING_ASYNC_CANCEL_FD, as that's a key selector.
Only one may be used at the time.
io_uring: allow IORING_OP_ASYNC_CANCEL with 'fd' key
Currently sqe->addr must contain the user_data of the request being
canceled. Introduce the IORING_ASYNC_CANCEL_FD flag, which tells the
kernel that we're keying off the file fd instead for cancelation. This
allows canceling any request that a) uses a file, and b) was assigned the
file based on the value being passed in.
The current cancelation will lookup and cancel the first request it
finds based on the key passed in. Add a flag that allows to cancel any
request that matches they key. It completes with the number of requests
found and canceled, or res < 0 if an error occured.
io_uring: pass in struct io_cancel_data consistently
In preparation for being able to not only key cancel off the user_data,
pass in the io_cancel_data struct for the various functions that deal
with request cancelation.
Pavel Begunkov [Wed, 20 Apr 2022 12:40:54 +0000 (13:40 +0100)]
io_uring: move timeout locking in io_timeout_cancel()
Move ->timeout_lock grabbing inside of io_timeout_cancel(), so
we can do io_req_task_queue_fail() outside of the lock. It's much nicer
than relying on triple nested locking.
io_uring: store SCM state in io_fixed_file->file_ptr
A previous commit removed SCM accounting for non-unix sockets, as those
are the only ones that can cause a fixed file reference. While that is
true, it also means we're now dereferencing the file as part of the
workqueue driven __io_sqe_files_unregister() after the process has
exited. This isn't safe for SCM files, as unix gc may have already
reaped them when the process exited. KASAN complains about this:
We have an extra bit we can use in file_ptr on 64-bit, use that to store
whether this file is SCM'ed or not, avoiding the need to look at the
file contents itself. This does mean that 32-bit will be stuck with SCM
for all registered files, just like 64-bit did before the referenced
commit.
Fixes: fbd4ef2c417a ("io_uring: don't scm-account for non af_unix sockets") Signed-off-by: Jens Axboe <axboe@kernel.dk>
Pavel Begunkov [Mon, 18 Apr 2022 19:51:14 +0000 (20:51 +0100)]
io_uring: add a helper for putting rsrc nodes
Add a simple helper to encapsulating dropping rsrc nodes references,
it's cleaner and will help if we'd change rsrc refcounting or play with
percpu_ref_put() [no]inlining.
Pavel Begunkov [Mon, 18 Apr 2022 19:51:13 +0000 (20:51 +0100)]
io_uring: store rsrc node in req instead of refs
req->fixed_rsrc_refs keeps a pointer to rsrc node pcpu references, but
it's more natural just to store rsrc node directly. There were some
reasons for that in the past but not anymore.
Pavel Begunkov [Mon, 18 Apr 2022 19:51:12 +0000 (20:51 +0100)]
io_uring: refactor io_assign_file error path
All io_assign_file() callers do error handling themselves,
req_set_fail() in the io_assign_file()'s fail path needlessly bloats the
kernel and is not the best abstraction to have. Simplify the error path.
Pavel Begunkov [Fri, 15 Apr 2022 21:08:28 +0000 (22:08 +0100)]
io_uring: refactor io_queue_sqe()
io_queue_sqe() is a part of the submission path and we try hard to keep
it inlined, so shed some extra bytes from it by moving the error
checking part into io_queue_sqe_arm_apoll() and renaming it accordingly.
note: io_queue_sqe_arm_apoll() is not inlined, thus the patch doesn't
change the number of function calls for the apoll path.
Pavel Begunkov [Fri, 15 Apr 2022 21:08:25 +0000 (22:08 +0100)]
io_uring: helper for prep+queuing linked timeouts
We try to aggresively inline the submission path, so it's a good idea to
not pollute it with colder code. One of them is linked timeout
preparation + queue, which can be extracted into a function.
Pavel Begunkov [Fri, 15 Apr 2022 21:08:23 +0000 (22:08 +0100)]
io_uring: kill io_put_req_deferred()
We have several spots where a call to io_fill_cqe_req() is immediately
followed by io_put_req_deferred(). Replace them with
__io_req_complete_post() and get rid of io_put_req_deferred() and
io_fill_cqe_req().
> size ./fs/io_uring.o
text data bss dec hex filename
86942 13734 8 100684 1894c ./fs/io_uring.o
> size ./fs/io_uring.o
text data bss dec hex filename
86438 13654 8 100100 18704 ./fs/io_uring.o
Pavel Begunkov [Fri, 15 Apr 2022 21:08:20 +0000 (22:08 +0100)]
io_uring: clean poll tw PF_EXITING handling
When we meet PF_EXITING in io_poll_check_events(), don't overcomplicate
the code with io_poll_mark_cancelled() but just return -ECANCELED and
the callers will deal with the rest.
Pavel Begunkov [Tue, 12 Apr 2022 14:09:51 +0000 (15:09 +0100)]
io_uring: optimise io_get_cqe()
io_get_cqe() is expensive because of a bunch of loads, masking, etc.
However, most of the time we should have enough of entries in the CQ,
so we can cache two pointers representing a range of contiguous CQE
memory we can use. When the range is exhausted we'll go through a slower
path to set up a new range. When there are no CQEs avaliable, pointers
will naturally point to the same address.
Pavel Begunkov [Tue, 12 Apr 2022 14:09:50 +0000 (15:09 +0100)]
io_uring: optimise submission left counting
Considering all inlining io_submit_sqe() is huge and usually ends up
calling some other functions.
We decrement @left in io_submit_sqes() just before calling
io_submit_sqe() and use it later after the call. Considering how huge
io_submit_sqe() is, there is not much hope @left will be treated
gracefully by compilers.
Decrement it after the call, not only it's easier on register spilling
and probably saves stack write/read, but also at least for x64 uses
CPU flags set by the dec instead of doing (read/write and tests).
Pavel Begunkov [Tue, 12 Apr 2022 14:09:49 +0000 (15:09 +0100)]
io_uring: optimise submission loop invariant
Instead of keeping @submitted in io_submit_sqes(), which for each
iteration requires comparison with the initial number of SQEs, store the
number of SQEs left to submit. We'll need nr only for when we're done
with SQE handling.
note: if we can't allocate a req for the first SQE we always has been
returning -EAGAIN to the userspace, save this behaviour by looking into
the cache in a slow path.
Pavel Begunkov [Tue, 12 Apr 2022 14:09:47 +0000 (15:09 +0100)]
io_uring: helper for empty req cache checks
Add io_req_cache_empty(), which checks if there are requests in the
inline req cache or not. It'll be needed in the future, but also nicely
cleans up a few spots poking into ->free_list directly.
Pavel Begunkov [Tue, 12 Apr 2022 14:09:45 +0000 (15:09 +0100)]
io_uring: shrink final link flush
All good users should not set IOSQE_IO_*LINK flags for the last request
of a link. io_uring flushes collected links at the end of submission,
but it's not the optimal way and so we don't care too much about it.
Replace io_queue_sqe() call with io_queue_sqe_fallback() as the former
one is inlined and will generate a bunch of extra code. This will also
help compilers with the submission path inlining.
> size ./fs/io_uring.o
text data bss dec hex filename
87265 13734 8 101007 18a8f ./fs/io_uring.o
> size ./fs/io_uring.o
text data bss dec hex filename
87073 13734 8 100815 189cf ./fs/io_uring.o
Pavel Begunkov [Tue, 12 Apr 2022 14:09:44 +0000 (15:09 +0100)]
io_uring: memcpy CQE from req
We can do CQE filling a bit more efficiently when req->cqe is fully
filled by memcpy()'ing it to the userspace instead of doing it field by
field. It's easier on register spilling, removes a couple of extra
loads/stores and write combines two u32 memory writes.
Pavel Begunkov [Tue, 12 Apr 2022 14:09:43 +0000 (15:09 +0100)]
io_uring: explicitly keep a CQE in io_kiocb
We already have req->{result,user_data,cflags}, which mimic struct
io_uring_cqe and are intended to store CQE data. Combine them into a
struct io_uring_cqe field.
Pavel Begunkov [Thu, 7 Apr 2022 12:40:03 +0000 (13:40 +0100)]
io_uring: don't pass around fixed index for scm
There is an old API nuisance where io_uring's SCM accounting functions
traverse fixed file tables and so requires them to be set in advance,
which leads to some implicit rules of how io_sqe_file_register() should
be used.
__io_sqe_files_scm() now works with only one file at a time, pass a file
directly and get rid of all fixed table dereferencing inside. Clean
io_sqe_file_register() callers.
Pavel Begunkov [Thu, 7 Apr 2022 12:40:01 +0000 (13:40 +0100)]
io_uring: uniform SCM accounting
Channel all SCM accounting through io_sqe_file_register(), so we do it
uniformely for updates and initial registration and can kill duplicated
code. Registration might be slightly slower in some case, but first we
skip most of SCM accounting now so it's not a problem. Moreover, it's
nicer for an empty set registration as we don't even try to allocate
skb for them anymore.
Pavel Begunkov [Wed, 6 Apr 2022 20:33:56 +0000 (21:33 +0100)]
io_uring: don't scm-account for non af_unix sockets
io_uring deals with file reference loops by registering all fixed files
in the SCM/GC infrastrucure. However, only a small subset of all file
types can keep long-term references to other files and those that don't
are not interesting for the garbage collector as they can't be in a
reference loop. They neither can be directly recycled by GC nor affect
loop searching.
Let's skip io_uring SCM accounting for loop-less files, i.e. all but
af_unix sockets, quite imroving fixed file updates performance and
greatly helpnig with memory footprint.
Jens Axboe [Fri, 25 Mar 2022 22:39:57 +0000 (16:39 -0600)]
io_uring: move finish_wait() outside of loop in cqring_wait()
We don't need to call this for every loop. This is particularly
troublesome if we are task_work intensive, and get woken more often than
we desire due to that.
Just do it at the end, that's always safe as we initialize the waitqueue
list head anyway. This can save a considerable amount of hammering on
the waitqueue lock, which is also hot from the request completion side.
Pavel Begunkov [Fri, 25 Mar 2022 11:52:15 +0000 (11:52 +0000)]
io_uring: partially uninline io_put_task()
In most cases io_put_task() is called from the submitter task and go
through a higly optimised fast path, which has to be inlined. The other
branch though is bulkier and we don't care about it as much because it
implies atomics and other heavy calls. Extract it into a helper, which
is expected not to be inlined.
[before] size ./fs/io_uring.o
text data bss dec hex filename
89328 13646 8 102982 19246 ./fs/io_uring.o
[after] size ./fs/io_uring.o
text data bss dec hex filename
89096 13646 8 102750 1915e ./fs/io_uring.o
Pavel Begunkov [Fri, 25 Mar 2022 11:52:14 +0000 (11:52 +0000)]
io_uring: cleanup conditional submit locking
Refactor io_ring_submit_[un]lock(), make it accept issue_flags and
remove manual IO_URING_F_UNLOCKED checks. It also allows us to place
lockdep annotations inside instead of sprinkling them in a bunch of
places. There is only one user that doesn't fit now, so hand code
locking in __io_rsrc_put_work().
Pavel Begunkov [Tue, 22 Mar 2022 14:07:58 +0000 (14:07 +0000)]
io_uring: optimise mutex locking for submit+iopoll
Both submittion and iopolling requires holding uring_lock. IOPOLL can
users do them together in a single syscall, however it would still do 2
pairs of lock/unlock. Optimise this case combining locking into one
lock/unlock pair, which especially nice for low QD.
Syscall should only iopoll for events when it's a IOPOLL ring and is not
SQPOLL. Instead of check both flags every time we can save it in ring
flags so it's easier to use. We don't care much about an extra if there,
however it will be inconvenient to copy-paste this chunk with checks in
future patches.
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