linux-kernel - Re: [PATCH v2 01/28] coresight: Change device mode to atomic type

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Message-ID: <20250702103845.GV794930@e132581.arm.com>
Date: Wed, 2 Jul 2025 11:38:45 +0100
From: Leo Yan <leo.yan@....com>
To: Yeoreum Yun <yeoreum.yun@....com>
Cc: Suzuki K Poulose <suzuki.poulose@....com>,
	Mike Leach <mike.leach@...aro.org>,
	James Clark <james.clark@...aro.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Yabin Cui <yabinc@...gle.com>, Keita Morisaki <keyz@...gle.com>,
	Yuanfang Zhang <quic_yuanfang@...cinc.com>,
	coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 01/28] coresight: Change device mode to atomic type

Hi Levi,

On Wed, Jul 02, 2025 at 10:49:01AM +0100, Yeoreum Yun wrote:
> Hi Leo,
> 
> >  {
> > -	return local_cmpxchg(&csdev->mode, CS_MODE_DISABLED, new_mode) ==
> > -	       CS_MODE_DISABLED;
> > +	int curr = CS_MODE_DISABLED;
> > +
> > +	return atomic_try_cmpxchg_acquire(&csdev->mode, &curr, new_mode);
> >  }
> 
> Just question. why is acquire symentic enough in here?

My understanding is that acquire semantics ensure ordering between
cmpxchg_acquire() and all memory accesses that follow it. However, it
does not guarantee that memory accesses appearing before the acquire
are ordered as well.

This is exactly what we want in the driver. We must ensure to first grab
an active device mode, then it is safe to proceed later operations (e.g.
set configurations in driver data and access registers).

> before this change, local_cmpxchg seems to use full_fenced.

Not really. Arm64 has atomic instruction for cmpxchg, it does not use
full_fenced. It should run into the path of arch_cmpxchg().

Thanks,
Leo