Re: [PATCH] io_uring: use cpu_online() to check p->sq_thread_cpu instead of cpu_possible()

[Jeff Moyer <jmoyer@redhat.com>]

Jens Axboe <axboe@kernel.dk> writes:

> On 5/1/19 5:56 AM, Jeff Moyer wrote:
>> Shenghui Wang <shhuiw@foxmail.com> writes:
>> 
>>> This issue is found by running liburing/test/io_uring_setup test.
>>>
>>> When test run, the testcase "attempt to bind to invalid cpu" would not
>>> pass with messages like:
>>>    io_uring_setup(1, 0xbfc2f7c8), \
>>> flags: IORING_SETUP_SQPOLL|IORING_SETUP_SQ_AFF, \
>>> resv: 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000, \
>>> sq_thread_cpu: 2
>>>    expected -1, got 3
>>>    FAIL
>>>
>>> On my system, there is:
>>>    CPU(s) possible : 0-3
>>>    CPU(s) online   : 0-1
>>>    CPU(s) offline  : 2-3
>>>    CPU(s) present  : 0-1
>>>
>>> The sq_thread_cpu 2 is offline on my system, so the bind should fail.
>>> But cpu_possible() will pass the check. We shouldn't be able to bind
>>> to an offline cpu. Use cpu_online() to do the check.
>>>
>>> After the change, the testcase run as expected: EINVAL will be returned
>>> for cpu offlined.
>>>
>>> Signed-off-by: Shenghui Wang <shhuiw@foxmail.com>
>>> ---
>>>  fs/io_uring.c | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 0e9fb2cb1984..aa3d39860a1c 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -2241,7 +2241,7 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
>>>  	ctx->sqo_mm = current->mm;
>>>  
>>>  	ret = -EINVAL;
>>> -	if (!cpu_possible(p->sq_thread_cpu))
>>> +	if (!cpu_online(p->sq_thread_cpu))
>>>  		goto err;
>>>  
>>>  	if (ctx->flags & IORING_SETUP_SQPOLL) {
>>> @@ -2258,7 +2258,7 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
>>>  
>>>  			cpu = array_index_nospec(p->sq_thread_cpu, NR_CPUS);
>>>  			ret = -EINVAL;
>>> -			if (!cpu_possible(p->sq_thread_cpu))
>>> +			if (!cpu_online(p->sq_thread_cpu))
>>>  				goto err;
>>>  
>>>  			ctx->sqo_thread = kthread_create_on_cpu(io_sq_thread,
>> 
>> Hmm.  Why are we doing this check twice?  Oh... Jens, I think you
>> braino'd commit 917257daa0fea.  Have a look.  You probably wanted to get
>> rid of the first check for cpu_possible.
>
> Added a fixup patch the other day:
>
> http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=362bf8670efccebca22efda1ee5a5ee831ec5efb

@@ -2333,13 +2329,14 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
 			ctx->sq_thread_idle = HZ;
 
 		if (p->flags & IORING_SETUP_SQ_AFF) {
-			int cpu;
+			int cpu = p->sq_thread_cpu;
 
-			cpu = array_index_nospec(p->sq_thread_cpu, NR_CPUS);
 			ret = -EINVAL;
-			if (!cpu_possible(p->sq_thread_cpu))
+			if (cpu >= nr_cpu_ids || !cpu_possible(cpu))
 				goto err;
 
+			cpu = array_index_nospec(cpu, nr_cpu_ids);
+

Why do you do the array_index_nospec last?  Why wouldn't that be written
as:

	if (p->flags & IORING_SETUP_SQ_AFF) {
		int cpu = array_index_nospec(p->sq_thread_cpu, nr_cpu_ids);

		ret = -EINVAL;
		if (!cpu_possible(cpu))
			goto err;

		ctx->sqo_thread = kthread_create_on_cpu(io_sq_thread,
						ctx, cpu,
						"io_uring-sq");
	} else {
...

That would take away some head-scratching for me.

Cheers,
Jeff