On Mon, Mar 18, 2024 at 04:43:38PM +0100, Marco Elver wrote:
> On Mon, 18 Mar 2024 at 11:01, Marco Elver <elver@xxxxxxxxxx> wrote:
> >
> > On Sun, 17 Mar 2024 at 22:55, Paul E. McKenney <paulmck@xxxxxxxxxx> 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 <paulmck@xxxxxxxxxx> 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 <paulmck@xxxxxxxxxx>
> > > 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 <paulmck@xxxxxxxxxx>
> > > Cc: Marco Elver <elver@xxxxxxxxxx>
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> > > Cc: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > > Cc: Petr Mladek <pmladek@xxxxxxxx>
> > > Cc: <linux-arch@xxxxxxxxxxxxxxx>
> > >
> > > 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.
And I have added data_race(), thank you!
> > 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).
Excellent point, added.
I also fixed an embarrassing pointer-arithmetic bug which the act of
coding the alignment check uncovered, so two for one! ;-)
Please see below for a patch to the patched code.
Thanx, Paul
> > 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);
diff --git a/lib/cmpxchg-emu.c b/lib/cmpxchg-emu.c
index 508b55484c2b6..b904f954dd4fc 100644
--- a/lib/cmpxchg-emu.c
+++ b/lib/cmpxchg-emu.c
@@ -11,6 +11,8 @@
#include <linux/export.h>
#include <linux/instrumented.h>
#include <linux/atomic.h>
+#include <linux/panic.h>
+#include <linux/bug.h>
#include <asm-generic/rwonce.h>
union u8_32 {
@@ -34,7 +36,7 @@ uintptr_t cmpxchg_emu_u8(volatile u8 *p, uintptr_t old, uintptr_t new)
new32.w = old32.w;
new32.b[i] = new;
instrument_atomic_read_write(p, 1);
- ret = cmpxchg(p32, old32.w, new32.w);
+ ret = data_race(cmpxchg(p32, old32.w, new32.w));
} while (ret != old32.w);
return old;
}
@@ -48,12 +50,13 @@ union u16_32 {
/* 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;
+ u32 *p32 = (u32 *)(((uintptr_t)p) & ~0x3);
+ int i = (((uintptr_t)p) & 0x2) / 2;
union u16_32 old32;
union u16_32 new32;
u32 ret;
+ WARN_ON_ONCE(((uintptr_t)p) & 0x1);
old32.w = READ_ONCE(*p32);
do {
if (old32.h[i] != old)
@@ -61,7 +64,7 @@ uintptr_t cmpxchg_emu_u16(volatile u16 *p, uintptr_t old, uintptr_t new)
new32.w = old32.w;
new32.h[i] = new;
instrument_atomic_read_write(p, 2);
- ret = cmpxchg(p32, old32.w, new32.w);
+ ret = data_race(cmpxchg(p32, old32.w, new32.w));
} while (ret != old32.w);
return old;
}
