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Message-ID: <ZZeo3-XzV9GBZuMe@LeoBras>
Date: Fri, 5 Jan 2024 03:59:44 -0300
From: Leonardo Bras <leobras@...hat.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Leonardo Bras <leobras@...hat.com>,
Will Deacon <will@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Mark Rutland <mark.rutland@....com>,
Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>,
Guo Ren <guoren@...nel.org>,
Andrea Parri <parri.andrea@...il.com>,
Geert Uytterhoeven <geert@...ux-m68k.org>,
Ingo Molnar <mingo@...nel.org>,
Andrzej Hajda <andrzej.hajda@...el.com>,
linux-kernel@...r.kernel.org,
linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v1 1/5] riscv/cmpxchg: Deduplicate xchg() asm functions
On Thu, Jan 04, 2024 at 09:18:15PM -0800, Boqun Feng wrote:
> On Fri, Jan 05, 2024 at 01:45:42AM -0300, Leonardo Bras wrote:
> [...]
> > > > According to gcc.gnu.org:
> > > >
> > > > ---
> > > > "memory" [clobber]:
> > > >
> > > > The "memory" clobber tells the compiler that the assembly code
> > > > performs memory reads or writes to items other than those listed in
> > > > the input and output operands (for example, accessing the memory
> > > > pointed to by one of the input parameters). To ensure memory contains
> > >
> > > Note here it says "other than those listed in the input and output
> > > operands", and in the above asm block, the memory pointed by "__ptr" is
> > > already marked as read-and-write by the asm block via "+A" (*__ptr), so
> > > the compiler knows the asm block may modify the memory pointed by
> > > "__ptr", therefore in _relaxed() case, "memory" clobber can be avoided.
> >
> > Thanks for pointing that out!
> > That helped me improve my understanding on constraints for asm operands :)
> > (I ended up getting even more info from the gcc manual)
> >
> > So "+A" constraints means the operand will get read/write and it's an
> > address stored into a register.
> >
> > >
> > > Here is an example showing the difference, considering the follow case:
> > >
> > > this_val = *this;
> > > that_val = *that;
> > > xchg_relaxed(this, 1);
> > > reread_this = *this;
> > >
> > > by the semantics of _relaxed, compilers can optimize the above into
> > >
> > > this_val = *this;
> > > xchg_relaxed(this, 1);
> > > that_val = *that;
> > > reread_this = *this;
> > >
> >
> > Seems correct, since there is no barrier().
> >
> > > but the "memory" clobber in the xchg_relexed() will provide this.
> >
> > By 'this' here you mean the barrier? I mean, IIUC "memory" clobber will
> > avoid the above optimization, right?
> >
>
> Right, seems I mis-typed "provide" (I meant "prevent")
>
> > > Needless to say the '"+A" (*__ptr)' prevents compiler from the following
> > > optimization:
> > >
> > > this_val = *this;
> > > that_val = *that;
> > > xchg_relaxed(this, 1);
> > > reread_this = this_val;
> > >
> > > since the compiler knows the asm block will read and write *this.
> >
> > Right, the compiler knows that address will be wrote by the asm block, and
> > so it reloads the value instead of re-using the old one.
> >
>
> Correct.
>
> >
> > A question, though:
>
> Good question ;-)
>
> > Do we need the "memory" clobber in any other xchg / cmpxchg asm?
>
> The "memory" clobber is needed for others, see below:
>
> > I mean, usually the only write to memory will happen in the *__ptr, which
> > should be safe by "+A".
> >
> > I understand that since the others are not "relaxed" they will need to
> > have a barrier, but is not the compiler supposed to understand the barrier
> > instruction and avoid compiler reordering / optimizations across given
> > instruction ?
> >
>
> The barrier semantics (ACQUIRE/RELEASE/FULL) is provided by the combined
> effort of both 1) preventing compiler optimization by "memory" clobber
> and 2) preventing CPU/memory reordering by arch-specific instructions.
>
> In other words, an asm block contains a hardware barrier instruction
> should always have the "memory" clobber, otherwise, there are
> possiblities that compilers reorder the asm block therefore break the
> ordering provided by the hardware instructions.
Oh, I see.
So this means the compiler does not check for memory barrier instructions
before reordering loads/stores. Right?
Meaning it needs a way to signal a compiler barrier, on top of the barrier
instructions.
Thanks for helping me improve my understanding of this!
Leo
>
> Regards,
> Boqun
>
> >
> > Thanks!
> > Leo
> >
> > > Regards,
> > > Boqun
> > >
> > > > correct values, GCC may need to flush specific register values to
> > > > memory before executing the asm. Further, the compiler does not assume
> > > > that any values read from memory before an asm remain unchanged after
> > > > that asm ; it reloads them as needed. Using the "memory" clobber
> > > > effectively forms a read/write memory barrier for the compiler.
> > > >
> > > > Note that this clobber does not prevent the processor from doing
> > > > speculative reads past the asm statement. To prevent that, you need
> > > > processor-specific fence instructions.
> > > > ---
> > > >
> > > > IIUC above text says that having memory accesses to *__ptr would require
> > > > above asm to have the "memory" clobber, so memory accesses don't get
> > > > reordered by the compiler.
> > > >
> > > > By above affirmation, all asm in this file should have the "memory"
> > > > clobber, since all atomic operations will change memory pointed by an input
> > > > ptr. Is that correct?
> > > >
> > > > Thanks!
> > > > Leo
> > > >
> > > >
> > > > >
> > > > > Regards,
> > > > > Boqun
> > > > >
> > > > > > - break; \
> > > > > > - case 8: \
> > > > > > - __asm__ __volatile__ ( \
> > > > > > - " amoswap.d %0, %2, %1\n" \
> > > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > > - : "r" (__new) \
> > > > > > - : "memory"); \
> > > > > > - break; \
> > > > > > - default: \
> > > > > > - BUILD_BUG(); \
> > > > > > - } \
> > > > > > - __ret; \
> > > > > > -})
> > > > > > -
> > > > > > -#define arch_xchg_relaxed(ptr, x) \
> > > > > > -({ \
> > > > > > - __typeof__(*(ptr)) _x_ = (x); \
> > > > > > - (__typeof__(*(ptr))) __xchg_relaxed((ptr), \
> > > > > > - _x_, sizeof(*(ptr))); \
> > > > > > + __asm__ __volatile__ ( \
> > > > > > + prepend \
> > > > > > + " amoswap" sfx " %0, %2, %1\n" \
> > > > > > + append \
> > > > > > + : "=r" (r), "+A" (*(p)) \
> > > > > > + : "r" (n) \
> > > > > > + : "memory"); \
> > > > > > })
> > > > > >
> > > > > > -#define __xchg_acquire(ptr, new, size) \
> > > > > > +#define _arch_xchg(ptr, new, sfx, prepend, append) \
> > > > > > ({ \
> > > > > > __typeof__(ptr) __ptr = (ptr); \
> > > > > > - __typeof__(new) __new = (new); \
> > > > > > - __typeof__(*(ptr)) __ret; \
> > > > > > - switch (size) { \
> > > > > > + __typeof__(*(__ptr)) __new = (new); \
> > > > > > + __typeof__(*(__ptr)) __ret; \
> > > > > > + switch (sizeof(*__ptr)) { \
> > > > > > case 4: \
> > > > > > - __asm__ __volatile__ ( \
> > > > > > - " amoswap.w %0, %2, %1\n" \
> > > > > > - RISCV_ACQUIRE_BARRIER \
> > > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > > - : "r" (__new) \
> > > > > > - : "memory"); \
> > > > > > + __arch_xchg(".w" sfx, prepend, append, \
> > > > > > + __ret, __ptr, __new); \
> > > > > > break; \
> > > > > > case 8: \
> > > > > > - __asm__ __volatile__ ( \
> > > > > > - " amoswap.d %0, %2, %1\n" \
> > > > > > - RISCV_ACQUIRE_BARRIER \
> > > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > > - : "r" (__new) \
> > > > > > - : "memory"); \
> > > > > > + __arch_xchg(".d" sfx, prepend, append, \
> > > > > > + __ret, __ptr, __new); \
> > > > > > break; \
> > > > > > default: \
> > > > > > BUILD_BUG(); \
> > > > > > } \
> > > > > > - __ret; \
> > > > > > + (__typeof__(*(__ptr)))__ret; \
> > > > > > })
> > > > > >
> > > > > > -#define arch_xchg_acquire(ptr, x) \
> > > > > > -({ \
> > > > > > - __typeof__(*(ptr)) _x_ = (x); \
> > > > > > - (__typeof__(*(ptr))) __xchg_acquire((ptr), \
> > > > > > - _x_, sizeof(*(ptr))); \
> > > > > > -})
> > > > > > +#define arch_xchg_relaxed(ptr, x) \
> > > > > > + _arch_xchg(ptr, x, "", "", "")
> > > > > >
> > > > > > -#define __xchg_release(ptr, new, size) \
> > > > > > -({ \
> > > > > > - __typeof__(ptr) __ptr = (ptr); \
> > > > > > - __typeof__(new) __new = (new); \
> > > > > > - __typeof__(*(ptr)) __ret; \
> > > > > > - switch (size) { \
> > > > > > - case 4: \
> > > > > > - __asm__ __volatile__ ( \
> > > > > > - RISCV_RELEASE_BARRIER \
> > > > > > - " amoswap.w %0, %2, %1\n" \
> > > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > > - : "r" (__new) \
> > > > > > - : "memory"); \
> > > > > > - break; \
> > > > > > - case 8: \
> > > > > > - __asm__ __volatile__ ( \
> > > > > > - RISCV_RELEASE_BARRIER \
> > > > > > - " amoswap.d %0, %2, %1\n" \
> > > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > > - : "r" (__new) \
> > > > > > - : "memory"); \
> > > > > > - break; \
> > > > > > - default: \
> > > > > > - BUILD_BUG(); \
> > > > > > - } \
> > > > > > - __ret; \
> > > > > > -})
> > > > > > +#define arch_xchg_acquire(ptr, x) \
> > > > > > + _arch_xchg(ptr, x, "", "", RISCV_ACQUIRE_BARRIER)
> > > > > >
> > > > > > #define arch_xchg_release(ptr, x) \
> > > > > > -({ \
> > > > > > - __typeof__(*(ptr)) _x_ = (x); \
> > > > > > - (__typeof__(*(ptr))) __xchg_release((ptr), \
> > > > > > - _x_, sizeof(*(ptr))); \
> > > > > > -})
> > > > > > -
> > > > > > -#define __arch_xchg(ptr, new, size) \
> > > > > > -({ \
> > > > > > - __typeof__(ptr) __ptr = (ptr); \
> > > > > > - __typeof__(new) __new = (new); \
> > > > > > - __typeof__(*(ptr)) __ret; \
> > > > > > - switch (size) { \
> > > > > > - case 4: \
> > > > > > - __asm__ __volatile__ ( \
> > > > > > - " amoswap.w.aqrl %0, %2, %1\n" \
> > > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > > - : "r" (__new) \
> > > > > > - : "memory"); \
> > > > > > - break; \
> > > > > > - case 8: \
> > > > > > - __asm__ __volatile__ ( \
> > > > > > - " amoswap.d.aqrl %0, %2, %1\n" \
> > > > > > - : "=r" (__ret), "+A" (*__ptr) \
> > > > > > - : "r" (__new) \
> > > > > > - : "memory"); \
> > > > > > - break; \
> > > > > > - default: \
> > > > > > - BUILD_BUG(); \
> > > > > > - } \
> > > > > > - __ret; \
> > > > > > -})
> > > > > > + _arch_xchg(ptr, x, "", RISCV_RELEASE_BARRIER, "")
> > > > > >
> > > > > > #define arch_xchg(ptr, x) \
> > > > > > -({ \
> > > > > > - __typeof__(*(ptr)) _x_ = (x); \
> > > > > > - (__typeof__(*(ptr))) __arch_xchg((ptr), _x_, sizeof(*(ptr))); \
> > > > > > -})
> > > > > > + _arch_xchg(ptr, x, ".aqrl", "", "")
> > > > > >
> > > > > > #define xchg32(ptr, x) \
> > > > > > ({ \
> > > > > > --
> > > > > > 2.43.0
> > > > > >
> > > > >
> > > >
> > >
> >
>
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