linux-kernel - Re: [PATCH] slab: fix kmalloc_nolock() context check for PREEMPT

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Message-ID: <ce970598-fbef-4b58-bde6-fb099f1cb6be@suse.cz>
Date: Tue, 13 Jan 2026 14:59:38 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Swaraj Gaikwad <swarajgaikwad1925@...il.com>, lgoncalv@...hat.com
Cc: akpm@...ux-foundation.org, ast@...nel.org, bigeasy@...utronix.de,
 cl@...two.org, clrkwllms@...nel.org, david.hunter.linux@...il.com,
 harry.yoo@...cle.com, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 linux-rt-devel@...ts.linux.dev, rientjes@...gle.com,
 roman.gushchin@...ux.dev, rostedt@...dmis.org, skhan@...uxfoundation.org,
 syzbot+b1546ad4a95331b2101e@...kaller.appspotmail.com,
 Hao Li <hao.li@...ux.dev>
Subject: Re: [PATCH] slab: fix kmalloc_nolock() context check for PREEMPT_RT

On 1/5/26 8:19 PM, Vlastimil Babka wrote:
> On 12/19/25 14:51, Swaraj Gaikwad wrote:
>> I agree that we can simplify this. I think !preemptible() would be
>> the most descriptive choice here, as it directly expresses the
>> constraint, we cannot take a sleeping lock when preemption is
>> disabled.
>> The updated check would be:
>>     if (IS_ENABLED(CONFIG_PREEMPT_RT) && !preemptible())
>>
>> I'll send a v2 with this change unless there are objections.

Ping? Are you planning to? Otherwise we have to do it ourselves as we
can't leave things broken?

Also Sebastian do you agree? Thanks.

> I think it's the best solution for now, so please do. We can change it back
> later with sheaves, as Hao Li pointed out. It will limit PREEMPT_RT
> kmalloc_nolock() for now. Steven explained to me it's the tracepoints
> disabling preemption, but that will be also improved soon:
> 
> https://lore.kernel.org/all/20251219120607.7371034e@gandalf.local.home/
> 
> 
>> Swaraj
>