linux-kernel - Re: ERROR: drivers: iscsi: iscsi

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Date:   Thu, 12 May 2022 12:10:55 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Test Bot <zgrieee@...il.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Mike Christie <michael.christie@...cle.com>,
        Mingzhe Zou <mingzhe.zou@...ystack.cn>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-scsi <linux-scsi@...r.kernel.org>,
        target-devel <target-devel@...r.kernel.org>
Subject: Re: ERROR: drivers: iscsi: iscsi_target.c

On Thu, May 12, 2022 at 11:42 AM Test Bot <zgrieee@...il.com> wrote:
>
> void iscsit_thread_get_cpumask(struct iscsi_conn *conn)
> {
>         int ord, cpu;
>         cpumask_t conn_allowed_cpumask;

Yeah, that's not how you are supposed to use 'cpumask_t'

This is why we have CONFIG_CPUMASK_OFFSTACK and 'cpumask_var_t', so
that the pattern is

        cpumask_var_t mask;

        if (!alloc_cpumask_var(&mask, GFP_KERNEL))
                return -ENOMEM;
        ... use 'mask' here as a  ...
        free_cpumask_var(mask);

and if the cpumask is small, it's allocated on the stack (and that
'alloc_cpumask_var()' becomes a no-op) and if it's huge, it has a real
allocation so that the stack frame doesn't grow too big.

The problem seems to have been introduced in this merge window by
commit d72d827f2f26 ("scsi: target: Add iscsi/cpus_allowed_list in
configfs"), but I didn't really look any closer than a plain "git
blame", so it might have happened before that too.

I also didn't check whether there was some explicit reason why the
code couldn't allocate the cpumask this way.

Btw, it's worth noting that 'cpumask_t' is always the full static
compile-time NR_CPUS bits in size, but a dynamically allocated
'cpumask_var_t' is only nr_cpumask_bits in size (ie the actual maximum
on that machine, as opposed to the theoretical maximum size). So they
are *not* exactly the same kind of 'cpumask_t' pointer in the end, but
no sane code should care (ie you have to do something else wrong for
the alloc_cpumask_var() pattern to not work).

           Linus