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McKenney" To: John Paul Adrian Glaubitz Cc: Arnd Bergmann , Linus Torvalds , linux-kernel@vger.kernel.org, Linux-Arch , linux-alpha@vger.kernel.org, Richard Henderson , Ivan Kokshaysky , Matt Turner , Alexander Viro Subject: Re: [GIT PULL] alpha: cleanups and build fixes for 6.10 Message-ID: Reply-To: paulmck@kernel.org References: <71feb004-82ef-4c7b-9e21-0264607e4b20@app.fastmail.com> <6e6dae45ffbf7a6ab54175695a3e21207c6f5126.camel@physik.fu-berlin.de> <46543a98-4767-471a-91be-20fb60ab138b@paulmck-laptop> <7432d241b538819b603194bfb3a306faf360d4b1.camel@physik.fu-berlin.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7432d241b538819b603194bfb3a306faf360d4b1.camel@physik.fu-berlin.de> On Sat, May 11, 2024 at 08:49:08PM +0200, John Paul Adrian Glaubitz wrote: > Hi Paul, > > On Fri, 2024-05-10 at 15:28 -0700, Paul E. McKenney wrote: > > > I'm still against dropping pre-EV56 so quickly without a proper phaseout period. > > > Why not wait for the next LTS release? AFAIK pre-EV56 support is not broken, is > > > it? > > > > Sadly, yes, it is, and it has been broken in mainline for almost two > > years. > > Could you elaborate what exactly is broken? I'm just trying to understand the reasoning. First, let's make sure that I completely and correctly understand the situation. The pre-EV56 Alphas have no byte store instruction, correct? If that is in fact correct, what code is generated for a volatile store to a single byte for those CPUs? For example, for this example? char c; ... WRITE_ONCE(c, 3); The rumor I heard is that the compilers will generate a non-atomic read-modify-write instruction sequence in this case, first reading the 32-bit word containing that byte into a register, then substituting the value to be stored into corresponding byte of that register, and finally doing a 32-bit store from that register. Is that the case, or am I confused? Thanx, Paul PS: Or, if you prefer, this example is equivalent: volatile char c; ... c = 3;