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asm-generic: avoid __generic_cmpxchg_local warnings 

Code that passes a 32-bit constant into cmpxchg() produces a harmless
sparse warning because of the truncation in the branch that is not taken:

fs/erofs/zdata.c: note: in included file (through /home/arnd/arm-soc/arch/arm/include/asm/cmpxchg.h, /home/arnd/arm-soc/arch/arm/include/asm/atomic.h, /home/arnd/arm-soc/include/linux/atomic.h, ...):
include/asm-generic/cmpxchg-local.h:29:33: warning: cast truncates bits from constant value (5f0ecafe becomes fe)
include/asm-generic/cmpxchg-local.h:33:34: warning: cast truncates bits from constant value (5f0ecafe becomes cafe)
include/asm-generic/cmpxchg-local.h:29:33: warning: cast truncates bits from constant value (5f0ecafe becomes fe)
include/asm-generic/cmpxchg-local.h:30:42: warning: cast truncates bits from constant value (5f0edead becomes ad)
include/asm-generic/cmpxchg-local.h:33:34: warning: cast truncates bits from constant value (5f0ecafe becomes cafe)
include/asm-generic/cmpxchg-local.h:34:44: warning: cast truncates bits from constant value (5f0edead becomes dead)

This was reported as a regression to Matt's recent __generic_cmpxchg_local
patch, though this patch only added more warnings on top of the ones
that were already there.

Rewording the truncation to use an explicit bitmask instead of a cast
to a smaller type avoids the warning but otherwise leaves the code
unchanged.

I had another look at why the cast is even needed for atomic_cmpxchg(),
and as Matt describes the problem here is that atomic_t contains a
signed 'int', but cmpxchg() takes an 'unsigned long' argument, and
converting between the two leads to a 64-bit sign-extension of
negative 32-bit atomics.

I checked the other implementations of arch_cmpxchg() and did not find
any others that run into the same problem as __generic_cmpxchg_local(),
but it's easy to be on the safe side here and always convert the
signed int into an unsigned int when calling arch_cmpxchg(), as this
will work even when any of the arch_cmpxchg() implementations run
into the same problem.

Fixes: 6246541522 ("locking/atomic: cmpxchg: Make __generic_cmpxchg_local compare against zero-extended 'old' value")
Reviewed-by: Matt Evans <mev@rivosinc.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
This commit is contained in:
Arnd Bergmann 2023-03-02 09:53:31 +01:00
parent 05d3855b4d
commit 656e9007ef
3 changed files with 11 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
include/asm-generic/atomic.h
View File

@ -130,7 +130,7 @@ ATOMIC_OP(xor, ^)
#define arch_atomic_read(v) READ_ONCE((v)->counter)
#define arch_atomic_set(v, i) WRITE_ONCE(((v)->counter), (i))
#define arch_atomic_xchg(ptr, v) (arch_xchg(&(ptr)->counter, (v)))
#define arch_atomic_cmpxchg(v, old, new) (arch_cmpxchg(&((v)->counter), (old), (new)))
#define arch_atomic_xchg(ptr, v) (arch_xchg(&(ptr)->counter, (u32)(v)))
#define arch_atomic_cmpxchg(v, old, new) (arch_cmpxchg(&((v)->counter), (u32)(old), (u32)(new)))
#endif /* __ASM_GENERIC_ATOMIC_H */

12
include/asm-generic/cmpxchg-local.h
View File

@ -26,16 +26,16 @@ static inline unsigned long __generic_cmpxchg_local(volatile void *ptr,
raw_local_irq_save(flags);
switch (size) {
case 1: prev = *(u8 *)ptr;
if (prev == (u8)old)
*(u8 *)ptr = (u8)new;
if (prev == (old & 0xffu))
*(u8 *)ptr = (new & 0xffu);
break;
case 2: prev = *(u16 *)ptr;
if (prev == (u16)old)
*(u16 *)ptr = (u16)new;
if (prev == (old & 0xffffu))
*(u16 *)ptr = (new & 0xffffu);
break;
case 4: prev = *(u32 *)ptr;
if (prev == (u32)old)
*(u32 *)ptr = (u32)new;
if (prev == (old & 0xffffffffffu))
*(u32 *)ptr = (new & 0xffffffffu);
break;
case 8: prev = *(u64 *)ptr;
if (prev == old)

6
include/asm-generic/cmpxchg.h
View File

@ -32,7 +32,7 @@ unsigned long __generic_xchg(unsigned long x, volatile void *ptr, int size)
#else
local_irq_save(flags);
ret = *(volatile u8 *)ptr;
*(volatile u8 *)ptr = x;
*(volatile u8 *)ptr = (x & 0xffu);
local_irq_restore(flags);
return ret;
#endif /* __xchg_u8 */
@ -43,7 +43,7 @@ unsigned long __generic_xchg(unsigned long x, volatile void *ptr, int size)
#else
local_irq_save(flags);
ret = *(volatile u16 *)ptr;
*(volatile u16 *)ptr = x;
*(volatile u16 *)ptr = (x & 0xffffu);
local_irq_restore(flags);
return ret;
#endif /* __xchg_u16 */
@ -54,7 +54,7 @@ unsigned long __generic_xchg(unsigned long x, volatile void *ptr, int size)
#else
local_irq_save(flags);
ret = *(volatile u32 *)ptr;
*(volatile u32 *)ptr = x;
*(volatile u32 *)ptr = (x & 0xffffffffu);
local_irq_restore(flags);
return ret;
#endif /* __xchg_u32 */