linux-kernel - Re: [PATCH v2 1/4] seqlock: introduce scoped_seqlock_read() and scoped_seqlock_read

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Message-ID: <aOznWZGV9qyGWoE5@redhat.com>
Date: Mon, 13 Oct 2025 13:50:11 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Boqun Feng <boqun.feng@...il.com>,
	David Howells <dhowells@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Li RongQing <lirongqing@...du.com>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] seqlock: introduce scoped_seqlock_read() and
 scoped_seqlock_read_irqsave()

On 10/13, Peter Zijlstra wrote:
>
> On Fri, Oct 10, 2025 at 03:14:39PM +0200, Oleg Nesterov wrote:
> > 	static inline void __scoped_seqlock_cleanup(struct ss_tmp *sst)
> > 	{
> > 		if (sst->state & ss_lock)
> > 			spin_unlock(&sst->lock.lock);
> > 		if (sst->state & ss_lock_irqsave)
> > 			spin_unlock_irqrestore(&sst->lock.lock, sst->data);
> > 	}
> > 
> > 	static inline void
> > 	__scoped_seqlock_next(struct ss_tmp *sst, enum ss_state target)
> > 	{
> > 		switch (sst->state) {
> > 		case ss_lock:
> > 		case ss_lock_irqsave:
> > 			sst->state |= ss_done;
> 
> So GCC is clever enough to see through this scheme, but Clang gets
> confused and generates worse code. Specifically it emits the whole
> __scoped_seqlock_cleanup() sequence, testing both bits and both unlock
> options.

OK, thanks and sorry for the noise then.

> So while that additional pointer might seem wasteful, it actually makes
> the state tracking easier and allows the compiler to more easily throw
> away stuff.

Great ;)

Oleg.