linux-kernel - Re: [PATCH v5 5/5] rqspinlock: Use smp_cond_load_acquire

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Message-ID: <CAP01T778qzHqNYanGtQ_UB9av9BtxLYdf2xWnJWtsf=-w=gANA@mail.gmail.com>
Date: Thu, 11 Sep 2025 20:56:31 +0200
From: Kumar Kartikeya Dwivedi <memxor@...il.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Ankur Arora <ankur.a.arora@...cle.com>, linux-kernel@...r.kernel.org, 
	linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	bpf@...r.kernel.org, arnd@...db.de, will@...nel.org, peterz@...radead.org, 
	akpm@...ux-foundation.org, mark.rutland@....com, harisokn@...zon.com, 
	cl@...two.org, ast@...nel.org, zhenglifeng1@...wei.com, 
	xueshuai@...ux.alibaba.com, joao.m.martins@...cle.com, 
	boris.ostrovsky@...cle.com, konrad.wilk@...cle.com
Subject: Re: [PATCH v5 5/5] rqspinlock: Use smp_cond_load_acquire_timeout()

On Thu, 11 Sept 2025 at 16:32, Catalin Marinas <catalin.marinas@....com> wrote:
>
> On Wed, Sep 10, 2025 at 08:46:55PM -0700, Ankur Arora wrote:
> > Switch out the conditional load inerfaces used by rqspinlock
> > to smp_cond_read_acquire_timeout().
> > This interface handles the timeout check explicitly and does any
> > necessary amortization, so use check_timeout() directly.
>
> It's worth mentioning that the default smp_cond_load_acquire_timeout()
> implementation (without hardware support) only spins 200 times instead
> of 16K times in the rqspinlock code. That's probably fine but it would
> be good to have confirmation from Kumar or Alexei.
>
> > diff --git a/kernel/bpf/rqspinlock.c b/kernel/bpf/rqspinlock.c
> > index 5ab354d55d82..4d2c12d131ae 100644
> > --- a/kernel/bpf/rqspinlock.c
> > +++ b/kernel/bpf/rqspinlock.c
> [...]
> > @@ -313,11 +307,8 @@ EXPORT_SYMBOL_GPL(resilient_tas_spin_lock);
> >   */
> >  static DEFINE_PER_CPU_ALIGNED(struct qnode, rqnodes[_Q_MAX_NODES]);
> >
> > -#ifndef res_smp_cond_load_acquire
> > -#define res_smp_cond_load_acquire(v, c) smp_cond_load_acquire(v, c)
> > -#endif
> > -
> > -#define res_atomic_cond_read_acquire(v, c) res_smp_cond_load_acquire(&(v)->counter, (c))
> > +#define res_atomic_cond_read_acquire_timeout(v, c, t)                \
> > +     smp_cond_load_acquire_timeout(&(v)->counter, (c), (t))
>
> BTW, we have atomic_cond_read_acquire() which accesses the 'counter' of
> an atomic_t. You might as well add an atomic_cond_read_acquire_timeout()
> in atomic.h than open-code the atomic_t internals here.
>

+1, and then drop res_atomic_cond_read_acquire_timeout from this file.

> Otherwise the patch looks fine to me, much simpler than the previous
> attempt.
>
> Reviewed-by: Catalin Marinas <catalin.marinas@....com>