Re: uprobe splat in PREEMP_RT

[Sebastian Sewior <bigeasy@linutronix.de>]

On 2025-04-02 11:10:45 [+0200], Oleg Nesterov wrote:
> Add Peter.
> 
> I never understood why __seqprop_preemptible() returns false.
> Stupid question, perhaps
> 
> 	--- x/include/linux/seqlock.h
> 	+++ x/include/linux/seqlock.h
> 	@@ -213,12 +213,11 @@ static inline unsigned __seqprop_sequenc
> 	 
> 	 static inline bool __seqprop_preemptible(const seqcount_t *s)
> 	 {
> 	-	return false;
> 	+	return true;
> 	 }
> 	 
> 	 static inline void __seqprop_assert(const seqcount_t *s)
> 	 {
> 	-	lockdep_assert_preemption_disabled();
> 	 }
> 	 
> 	 #define __SEQ_RT	IS_ENABLED(CONFIG_PREEMPT_RT)
> 
> makes more sense?
> 
> Then we can remove the no longer necessary preempt_disable()'s
> before write_seqcount_begin() in other users of seqcount_t.

This depends on locktype that is coupled with the seqcount.
If the lock disables preemption and relies on it then it must be somehow
enforced on PREEMPT_RT or rely on the lock+unlock mechnanism to avoid
deadlocks. Also it needs to be ensured that you don't have two writer
since preemption is allowed.

> Oleg.

Sebastian