Re: git: 234683726708 - main - devclass: make devclass_alloc_unit use M_NOWAIT

> On Mar 6, 2025, at 7:03 PM, Mateusz Guzik <mjg@FreeBSD.org> wrote:
> 
> The branch main has been updated by mjg:
> 
> URL: https://cgit.FreeBSD.org/src/commit/?id=234683726708cf5212d672d676d30056d4133859
> 
> commit 234683726708cf5212d672d676d30056d4133859
> Author:     Mateusz Guzik <mjg@FreeBSD.org>
> AuthorDate: 2025-03-06 11:01:49 +0000
> Commit:     Mateusz Guzik <mjg@FreeBSD.org>
> CommitDate: 2025-03-06 11:01:49 +0000
> 
>    devclass: make devclass_alloc_unit use M_NOWAIT
> 
>    The only caller already does this.
> 
>    The routine can be called with a mutex held making M_WAITOK illegal.
> 
>    Sponsored by:   Rubicon Communications, LLC ("Netgate")
> ---
> sys/kern/subr_bus.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/sys/kern/subr_bus.c b/sys/kern/subr_bus.c
> index 9506e471705c..0422352bba51 100644
> --- a/sys/kern/subr_bus.c
> +++ b/sys/kern/subr_bus.c
> @@ -1208,6 +1208,7 @@ devclass_get_sysctl_tree(devclass_t dc)
> static int
> devclass_alloc_unit(devclass_t dc, device_t dev, int *unitp)
> {
> +	device_t *devices;
> 	const char *s;
> 	int unit = *unitp;
> 
> @@ -1264,8 +1265,11 @@ devclass_alloc_unit(devclass_t dc, device_t dev, int *unitp)
> 		int newsize;
> 
> 		newsize = unit + 1;
> -		dc->devices = reallocf(dc->devices,
> -		    newsize * sizeof(*dc->devices), M_BUS, M_WAITOK);
> +		devices = reallocf(dc->devices,
> +		    newsize * sizeof(*dc->devices), M_BUS, M_NOWAIT);

I'd recommend against this. From the commit message of f3d3c63442ff, Warner said,
> In addition, transition to M_WAITOK since this is a sleepable context
So, the M_WAITOK is intentional.

Rather than reverting this, the caller devclass_add_device() should use M_WAITOK.

```
-       dev->nameunit = malloc(buflen, M_BUS, M_NOWAIT|M_ZERO);
-       if (!dev->nameunit)
-               return (ENOMEM);
+       dev->nameunit = malloc(buflen, M_BUS, M_WAITOK | M_ZERO);
```

Best regards,
Zhenlei

> +		if (devices == NULL)
> +			return (ENOMEM);
> +		dc->devices = devices;
> 		memset(dc->devices + dc->maxunit, 0,
> 		    sizeof(device_t) * (newsize - dc->maxunit));
> 		dc->maxunit = newsize;