linux-kernel - Re: [PATCH RESEND 1/3] locking: add rwsem_is_write

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Message-ID: <aWr0LtTf1rU7iae8@tardis-2.local>
Date: Sat, 17 Jan 2026 10:30:06 +0800
From: Boqun Feng <boqun.feng@...il.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Hildenbrand <david@...nel.org>,
	"Liam R . Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>,
	Shakeel Butt <shakeel.butt@...ux.dev>, Jann Horn <jannh@...gle.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev, Ingo Molnar <mingo@...hat.com>,
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RESEND 1/3] locking: add rwsem_is_write_locked(), update
 non-lockdep asserts

On Fri, Jan 16, 2026 at 04:21:29PM +0000, Lorenzo Stoakes wrote:
> On Fri, Jan 16, 2026 at 04:57:43PM +0100, Sebastian Andrzej Siewior wrote:
> > On 2026-01-16 15:50:24 [+0000], Lorenzo Stoakes wrote:
> > > No, but we need to be able to assert that one of two locks are held and we
> > > don't want the failure of one being held to cause an assert when the other
> > > isn't.
> >
> > But why don't you use the lockdep based check? That assert only ensures
> 
> Not sure what you mean, the checks I'm adding don't exist yet.
> 
> > that it is locked at the time you did the check. This does not mean you
> > are owner - it could be owned by another task which is unrelated to your
> > cause.
> 
> Yup I'm aware that lockdep tests more than a simple assert.
> 
> I wasn't aware this was possible with the lockdep primitives, mea culpa.
> 
> Also this came out of a previous discussion where I added a similar
> predicate vma_is_detached() and Suren suggested similar for the locks.
> 
> Anyway, I went and looked and yes I see there's lockdep_is_held() for
> instance.
> 
> However, I'd STILL need to do what I'm doing here to account for
> CONFIG_DEBUG_VM && !CONFIG_LOCKDEP configurations right?
> 

There is an idea about a light weight lockdep where we only remain
tracking the held locks in a per-task stack and skip the whole
dependency checking, that would provide lock holding
information without the full cost of LOCKDEP, but that requires some
work and I'm not sure whether it fulfills what you need for DEBUG_VM
tests (each task_struct would have some extra space and lock/unlock
would do extra book-keeping).

> So I'll respin later with if (IS_ENABLED(CONFIG_LOCKDEP)) ...
> And sprinkle with some lockdep_is_held() and see how that works.
> 

Yeah, for LOCKDEP=y cases, please do use lockdep_is_held() or
lockdep_is_held_type(), those would provide the accurate information.

> I mean rwsem_is_locked() is already specified, so naming is going to be a
> thing now but I guess:
> 
> static inline bool rwsem_is_locked_nolockdep(const struct rw_semaphore *sem)
> {
> 	return rw_base_is_locked(&sem->rwbase);
> }
> 
> static inline bool rwsem_is_locked(const struct rw_semaphore *sem)
> {
> 	if (IS_ENABLED(CONFIG_LOCKDEP))
> 		return lockdep_is_held(sem);
> 
> 	return rwsem_is_locked_nolockdep(sem);
> }
> 
> And obviously equivalent for the write case is what's necessary now right?
> 

Assuming we want CONFIG_LOCKDEP=n cases to work without extra
book-keeping, I think we could use rwsem_owner() for write cases, and
name the function as rwsem_is_write_held(), which tells you whether
the current thread is the owner of the write lock (we are lucky here
because rwsem is one of those locks remember their owners ;-)). This
would cover the use case of MM without introducing another is_locked()
function. Peter & Sebastian, how do you like (or not hate ;-)) that
idea?

Regards,
Boqun

> Or am I misunderstanding you?
> 
> >
> > Sebastian
> 
> Thanks, Lorenzo