linux-kernel - Re: [PATCH v5 05/14] tools: Add testing support for changes to rcu and slab for sheaves

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Message-ID: <711d539d-da4b-4f77-9be6-ca11b6fa5162@suse.cz>
Date: Tue, 26 Aug 2025 11:32:34 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>,
 Uladzislau Rezki <urezki@...il.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
 maple-tree@...ts.infradead.org
Subject: Re: [PATCH v5 05/14] tools: Add testing support for changes to rcu
 and slab for sheaves

On 8/22/25 18:28, Suren Baghdasaryan wrote:
>> diff --git a/tools/testing/shared/linux/rcupdate.h b/tools/testing/shared/linux/rcupdate.h
>> index fed468fb0c78db6f33fb1900c7110ab5f3c19c65..c95e2f0bbd93798e544d7d34e0823ed68414f924 100644
>> --- a/tools/testing/shared/linux/rcupdate.h
>> +++ b/tools/testing/shared/linux/rcupdate.h
>> @@ -9,4 +9,26 @@
>>  #define rcu_dereference_check(p, cond) rcu_dereference(p)
>>  #define RCU_INIT_POINTER(p, v) do { (p) = (v); } while (0)
>>
>> +void kmem_cache_free_active(void *objp);
>> +static unsigned long kfree_cb_offset = 0;
>> +
>> +static inline void kfree_rcu_cb(struct rcu_head *head)
>> +{
>> +       void *objp = (void *) ((unsigned long)head - kfree_cb_offset);
>> +
>> +       kmem_cache_free_active(objp);
>> +}
>> +
>> +#ifndef offsetof
>> +#define offsetof(TYPE, MEMBER) __builtin_offsetof(TYPE, MEMBER)
>> +#endif
>> +
> 
> We need a comment here that concurrent kfree_rcu() calls are not
> supported because they would override each other's kfree_cb_offset.

I think it's a bit more complex and related to the commit log sentence "This
only works with one kmem_cache, and only the first one used.". The first
call to kfree_rcu sets kfree_cb_offset (but what if the rhv offset is
actually 0?) so the others won't update it. So concurrent calls will work as
far as from the same cache thus same offset. But I'd like Liam's
confirmation and the comment text, if possible :)

> Kinda obvious but I think unusual limitations should be explicitly
> called out.
> 
>> +#define kfree_rcu(ptr, rhv)                                            \
>> +do {                                                                   \
>> +       if (!kfree_cb_offset)                                           \
>> +               kfree_cb_offset = offsetof(typeof(*(ptr)), rhv);        \
>> +                                                                       \
>> +       call_rcu(&ptr->rhv, kfree_rcu_cb);                              \
>> +} while (0)
> 
> Any specific reason kfree_rcu() is a macro and not a static inline function?

Think it's needed for the typeof() to work. The kernel's kfree_rcu() is
similar in this aspect.

>> +
>>  #endif
>>
>> --
>> 2.50.1
>>