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author | Arnd Bergmann <arnd@arndb.de> | 2023-03-02 11:53:31 +0300 |
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committer | Arnd Bergmann <arnd@arndb.de> | 2023-04-04 18:58:11 +0300 |
commit | 656e9007ef5862746cdf7ac16267c8e06e7b0989 (patch) | |
tree | ef0e41da56c4b51f697f168e83a2c4c062020fe3 /include/asm-generic/cmpxchg-local.h | |
parent | 05d3855b4d21ef3c2df26be1cbba9d2c68915fcb (diff) | |
download | linux-656e9007ef5862746cdf7ac16267c8e06e7b0989.tar.xz |
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: 624654152284 ("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>
Diffstat (limited to 'include/asm-generic/cmpxchg-local.h')
-rw-r--r-- | include/asm-generic/cmpxchg-local.h | 12 |
1 files changed, 6 insertions, 6 deletions
diff --git a/include/asm-generic/cmpxchg-local.h b/include/asm-generic/cmpxchg-local.h index c3e7315b7c1d..3df9f59a544e 100644 --- a/include/asm-generic/cmpxchg-local.h +++ b/include/asm-generic/cmpxchg-local.h @@ -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) |