On Mon, 18 Mar 2024 at 11:01, Marco Elver <el...@google.com> wrote:
>
> On Sun, 17 Mar 2024 at 22:55, Paul E. McKenney <paul...@kernel.org> wrote:
> >
> > On Fri, Mar 08, 2024 at 02:31:53PM -0800, Paul E. McKenney wrote:
> > > On Fri, Mar 08, 2024 at 11:02:28PM +0100, Marco Elver wrote:
> > > > On Fri, 8 Mar 2024 at 22:41, Paul E. McKenney <paul...@kernel.org>
> > > > wrote:
> > > > >
> > > > > Tasks Trace RCU needs a single-byte cmpxchg(), but no such thing
> > > > > exists.
> > > >
> > > > Because not all architectures support 1-byte cmpxchg?
> > > > What prevents us from implementing it?
> > >
> > > Nothing that I know of, but I didn't want to put up with the KCSAN report
> > > in the interim.
> >
> > And here is a lightly tested patch to emulate one-byte and two-byte
> > cmpxchg() for architectures that do not support it. This is just the
> > emulation, and would be followed up with patches to make the relevant
> > architectures make use of it.
> >
> > The one-byte emulation has been lightly tested on x86.
> >
> > Thoughts?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit d72e54166b56d8b373676e1e92a426a07d53899a
> > Author: Paul E. McKenney <paul...@kernel.org>
> > Date: Sun Mar 17 14:44:38 2024 -0700
> >
> > lib: Add one-byte and two-byte cmpxchg() emulation functions
> >
> > Architectures are required to provide four-byte cmpxchg() and 64-bit
> > architectures are additionally required to provide eight-byte cmpxchg().
> > However, there are cases where one-byte and two-byte cmpxchg()
> > would be extremely useful. Therefore, provide cmpxchg_emu_u8() and
> > cmpxchg_emu_u16() that emulated one-byte and two-byte cmpxchg() in terms
> > of four-byte cmpxchg().
> >
> > Signed-off-by: Paul E. McKenney <paul...@kernel.org>
> > Cc: Marco Elver <el...@google.com>
> > Cc: Andrew Morton <a...@linux-foundation.org>
> > Cc: Thomas Gleixner <t...@linutronix.de>
> > Cc: "Peter Zijlstra (Intel)" <pet...@infradead.org>
> > Cc: Douglas Anderson <diand...@chromium.org>
> > Cc: Petr Mladek <pmla...@suse.com>
> > Cc: <linux-a...@vger.kernel.org>
> >
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 154f994547632..eef11e9918ec7 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -1506,4 +1506,7 @@ config FUNCTION_ALIGNMENT
> > default 4 if FUNCTION_ALIGNMENT_4B
> > default 0
> >
> > +config ARCH_NEED_CMPXCHG_1_2_EMU
> > + bool
> > +
> > endmenu
> > diff --git a/include/linux/cmpxchg-emu.h b/include/linux/cmpxchg-emu.h
> > new file mode 100644
> > index 0000000000000..fee8171fa05eb
> > --- /dev/null
> > +++ b/include/linux/cmpxchg-emu.h
> > @@ -0,0 +1,16 @@
> > +/* SPDX-License-Identifier: GPL-2.0+ */
> > +/*
> > + * Emulated 1-byte and 2-byte cmpxchg operations for architectures
> > + * lacking direct support for these sizes. These are implemented in terms
> > + * of 4-byte cmpxchg operations.
> > + *
> > + * Copyright (C) 2024 Paul E. McKenney.
> > + */
> > +
> > +#ifndef __LINUX_CMPXCHG_EMU_H
> > +#define __LINUX_CMPXCHG_EMU_H
> > +
> > +uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new);
> > +uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t new);
> > +
> > +#endif /* __LINUX_CMPXCHG_EMU_H */
> > diff --git a/lib/Makefile b/lib/Makefile
> > index 6b09731d8e619..fecd7b8c09cbd 100644
> > --- a/lib/Makefile
> > +++ b/lib/Makefile
> > @@ -238,6 +238,7 @@ obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
> > lib-$(CONFIG_GENERIC_BUG) += bug.o
> >
> > obj-$(CONFIG_HAVE_ARCH_TRACEHOOK) += syscall.o
> > +obj-$(CONFIG_ARCH_NEED_CMPXCHG_1_2_EMU) += cmpxchg-emu.o
>
> Since you add instrumentation explicitly, we need to suppress
> instrumentation somehow. For the whole file this can be done with:
>
> KCSAN_SANITIZE_cmpxchg-emu.o := n
Hrm, I recall this doesn't actually work as-is because it also
disables instrument_read_write() instrumentation.
So I think the most reliable would be to use data_race() after all.
It'll be a bit slower because of double-instrumenting, but I think
that's not a major concern with an instrumented build anyway.
> Note, since you use cmpxchg, which pulls in its own
> instrument_read_write(), we can't use a function attribute (like
> __no_kcsan) if the whole-file no-instrumentation seems like overkill.
> Alternatively the cmpxchg could be wrapped into a data_race() (like
> your original RCU use case was doing).
>
> But I think "KCSAN_SANITIZE_cmpxchg-emu.o := n" would be my preferred way.
>
> With the explicit "instrument_read_write()" also note that this would
> do double-instrumentation with other sanitizers (KASAN, KMSAN). But I
> think we actually want to instrument the whole real access with those
> tools - would it be bad if we accessed some memory out-of-bounds, but
> that memory isn't actually used? I don't have a clear answer to that.
>
> Also, it might be useful to have an alignment check somewhere, because
> otherwise we end up with split atomic accesses (or whatever other bad
> thing the given arch does if that happens).
>
> Thanks,
> -- Marco
>
> > obj-$(CONFIG_DYNAMIC_DEBUG_CORE) += dynamic_debug.o
> > #ensure exported functions have prototypes
> > diff --git a/lib/cmpxchg-emu.c b/lib/cmpxchg-emu.c
> > new file mode 100644
> > index 0000000000000..508b55484c2b6
> > --- /dev/null
> > +++ b/lib/cmpxchg-emu.c
> > @@ -0,0 +1,68 @@
> > +/* SPDX-License-Identifier: GPL-2.0+ */
> > +/*
> > + * Emulated 1-byte and 2-byte cmpxchg operations for architectures
> > + * lacking direct support for these sizes. These are implemented in terms
> > + * of 4-byte cmpxchg operations.
> > + *
> > + * Copyright (C) 2024 Paul E. McKenney.
> > + */
> > +
> > +#include <linux/types.h>
> > +#include <linux/export.h>
> > +#include <linux/instrumented.h>
> > +#include <linux/atomic.h>
> > +#include <asm-generic/rwonce.h>
> > +
> > +union u8_32 {
> > + u8 b[4];
> > + u32 w;
> > +};
> > +
> > +/* Emulate one-byte cmpxchg() in terms of 4-byte cmpxchg. */
> > +uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
> > +{
> > + u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
> > + int i = ((uintptr_t)p) & 0x3;
> > + union u8_32 old32;
> > + union u8_32 new32;
> > + u32 ret;
> > +
> > + old32.w = READ_ONCE(*p32);
> > + do {
> > + if (old32.b[i] != old)
> > + return old32.b[i];
> > + new32.w = old32.w;
> > + new32.b[i] = new;
> > + instrument_atomic_read_write(p, 1);
> > + ret = cmpxchg(p32, old32.w, new32.w);
> > + } while (ret != old32.w);
> > + return old;
> > +}
> > +EXPORT_SYMBOL_GPL(cmpxchg_emu_u8);
> > +
> > +union u16_32 {
> > + u16 h[2];
> > + u32 w;
> > +};
> > +
> > +/* Emulate two-byte cmpxchg() in terms of 4-byte cmpxchg. */
> > +uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t new)
> > +{
> > + u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x1);
> > + int i = ((uintptr_t)p) & 0x1;
> > + union u16_32 old32;
> > + union u16_32 new32;
> > + u32 ret;
> > +
> > + old32.w = READ_ONCE(*p32);
> > + do {
> > + if (old32.h[i] != old)
> > + return old32.h[i];
> > + new32.w = old32.w;
> > + new32.h[i] = new;
> > + instrument_atomic_read_write(p, 2);
> > + ret = cmpxchg(p32, old32.w, new32.w);
> > + } while (ret != old32.w);
> > + return old;
> > +}
> > +EXPORT_SYMBOL_GPL(cmpxchg_emu_u16);
