On 7/21/26 7:46 PM, Vlastimil Babka (SUSE) wrote: > On 7/20/26 14:44, Harry Yoo (Oracle) wrote: >> As suggested by Vlastimil Babka [1], kfree_rcu_sheaf() can be used >> on PREEMPT_RT if we always assume spinning is not allowed on PREEMPT_RT. >> This is because local_trylock and spinlock_t are safe to use with >> trylock and unlock as long as the kernel does not spin and the context >> is not NMI and not hardirq. >> >> Now that __kfree_rcu_sheaf() knows how to handle SLAB_FREE_NOLOCK, >> relax the limitation and try the sheaves path on PREEMPT_RT as well. >> >> Keep the lockdep map on non RT kernels. However, do not use the lockdep >> map on PREEMPT_RT to avoid suppressing valid lockdep warnings. >> >> As pointed by Vlastimil Babka [2], on PREEMPT_RT it is unnecessary to >> defer call_rcu() under IRQ-disabled section or raw spinlock. However, >> let us avoid adding more complexity as the scenario is not supposed >> to be common on PREEMPT_RT, with a hope that call_rcu_nolock() will be >> soon supported in RCU. >> >> Link: >> https://lore.kernel.org/linux-mm/[email protected] >> [1] >> Link: >> https://lore.kernel.org/linux-mm/[email protected] >> [2] >> Suggested-by: Vlastimil Babka (SUSE) <[email protected]> >> Signed-off-by: Harry Yoo (Oracle) <[email protected]> > > Reviewed-by: Vlastimil Babka (SUSE) <[email protected]>
Thanks!
> Nit:
>
>> ---
>> mm/slab_common.c | 12 ++++++++++--
>> mm/slub.c | 17 ++++++++++-------
>> 2 files changed, 20 insertions(+), 9 deletions(-)
>>
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 9c9ae1384f47..8c631bf97cd5 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -1595,6 +1595,14 @@ static bool kfree_rcu_sheaf(void *obj)
>> {
>> struct kmem_cache *s;
>> struct slab *slab;
>> + unsigned int free_flags = SLAB_FREE_DEFAULT;
>> +
>> + /*
>> + * It is not safe to spin on PREEMPT_RT because the kernel might be
>> + * holding a raw spinlock and slab acquires sleeping locks.
>> + */
>> + if (IS_ENABLED(CONFIG_PREEMPT_RT))
>> + free_flags = SLAB_FREE_NOLOCK;
>>
>> if (is_vmalloc_addr(obj))
>> return false;
>> @@ -1605,7 +1613,7 @@ static bool kfree_rcu_sheaf(void *obj)
>>
>> s = slab->slab_cache;
>> if (likely(!IS_ENABLED(CONFIG_NUMA) || slab_nid(slab) == numa_mem_id()))
>> - return __kfree_rcu_sheaf(s, obj, SLAB_FREE_DEFAULT);
>> + return __kfree_rcu_sheaf(s, obj, free_flags);
>>
>> return false;
>> }
>> @@ -1954,7 +1962,7 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>> if (!head)
>> might_sleep();
>>
>> - if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kfree_rcu_sheaf(ptr))
>> + if (kfree_rcu_sheaf(ptr))
>> return;
>>
>> // Queue the object but don't yet schedule the batch.
>> diff --git a/mm/slub.c b/mm/slub.c
>> index 8afa6b47b1f2..deac315d0f23 100644
>> --- a/mm/slub.c
>> +++ b/mm/slub.c
>> @@ -6065,12 +6065,13 @@ static void rcu_free_sheaf(struct rcu_head *head)
>> * kvfree_call_rcu() can be called while holding a raw_spinlock_t. Since
>> * __kfree_rcu_sheaf() may acquire a spinlock_t (sleeping lock on
>> PREEMPT_RT),
>> * this would violate lock nesting rules. Therefore, kvfree_call_rcu()
>> avoids
>> - * this problem by bypassing the sheaves layer entirely on PREEMPT_RT.
>> + * this problem by passing allow_spin = false on PREEMPT_RT.
>
> by passing SLAB_FREE_NOLOCK ?
Ouch, I noticed this but forgot to update.
Will adjust, thanks.
--
Cheers,
Harry / Hyeonggon
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