linux-kernel - Re: [GIT PULL] alpha: cleanups and build fixes for 6.10

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Date: Sat, 11 May 2024 18:26:10 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Linux-Arch <linux-arch@...r.kernel.org>,
	linux-alpha@...r.kernel.org,
	Richard Henderson <richard.henderson@...aro.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	Matt Turner <mattst88@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [GIT PULL] alpha: cleanups and build fixes for 6.10

On Sat, May 11, 2024 at 10:08:50PM +0200, Arnd Bergmann wrote:
> On Sat, May 11, 2024, at 21:37, Paul E. McKenney wrote:
> > On Sat, May 11, 2024 at 08:49:08PM +0200, John Paul Adrian Glaubitz wrote:
> >
> > 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?
> 
> I think it's slightly worse: gcc will actually do a 64-bit
> read-modify-write rather than a 32-bit one, and it doesn't
> use atomic ll/sc when storing into an _Atomic struct member:
> 
> echo '#include <stdatomic.h>^M struct s { _Atomic char c; _Atomic char s[7]; long l; }; void f(struct s *s) { atomic_store(&s->c, 3); }' | alpha-linux-gcc-14  -xc - -S -o- -O2 -mcpu=ev5
> 
> f:
> 	.frame $30,0,$26,0
> $LFB0:
> 	.cfi_startproc
> 	.prologue 0
> 	mb
> 	lda $1,3($31)
> 	insbl $1,$16,$1
> 	ldq_u $2,0($16)
> 	mskbl $2,$16,$2
> 	bis $1,$2,$1
> 	stq_u $1,0($16)
> 	bis $31,$31,$31
> 	mb
> 	ret $31,($26),1
> 	.cfi_endproc
> $LFE0:
> 	.end f
> 
> compared to -mcpu=ev56:
> 
> f:
> 	.frame $30,0,$26,0
> $LFB0:
> 	.cfi_startproc
> 	.prologue 0
> 	mb
> 	lda $1,3($31)
> 	stb $1,0($16)
> 	bis $31,$31,$31
> 	mb
> 	ret $31,($26),1
> 	.cfi_endproc
> $LFE0:
> 	.end f

Thank you, Arnd!

And that breaks things because it can clobber concurrent stores to
other bytes in that enclosing machine word.

							Thanx, Paul