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

[Jeff Moyer <jmoyer@redhat.com>]

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.

-Jeff