linux-kernel - Re: [PATCH RFC cmpxchg 2/8] sparc: Emulate one-byte and two-byte cmpxchg

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Date: Tue, 2 Apr 2024 04:37:53 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com,
	"David S. Miller" <davem@...emloft.net>,
	Andreas Larsson <andreas@...sler.com>,
	Palmer Dabbelt <palmer@...osinc.com>, Arnd Bergmann <arnd@...db.de>,
	Marco Elver <elver@...gle.com>
Subject: Re: [PATCH RFC cmpxchg 2/8] sparc: Emulate one-byte and two-byte
 cmpxchg

On Tue, Apr 02, 2024 at 01:07:58AM +0100, Al Viro wrote:

> It does, IIRC.
> 
> > Would you like to do that patch?  If so, I would be happy to drop mine
> > in favor of yours.  If not, could I please have your Signed-off-by so
> > I can do the Co-developed-by dance?
> 
> Will do once I dig my way from under the pile of mail (sick for a week
> and subscribed to l-k, among other lists)...

FWIW, parisc is in the same situation - atomics-by-cached-spinlocks.
've a candidate branch, will post if it survives build...

Re parisc: why does it bother with arch_cmpxchg_local()?  Default is
	* save and disable local interrupts
	* read the current value, compare to old
	* if equal, store new there
	* restore local interrupts
For 32bit case parisc goes for __cmpxchg_u32(), which is
	* if (SMP) choose the spinlock (indexed by hash of address)
	* save and disable local interrupes
	* if (SMP) arch_spin_lock(spinlock)
	* read the current value, compare to old
	* if equal, store new there
	* if (SMP) arch_spin_unlock(spinlock)
	* restore local interrupts
In UP case it's identical to generic; on SMP it's strictly more work.
Unless I'm very confused about cmpxchg_local() semantics, the
callers do not expect atomicity wrt other CPUs, so why do we bother?