arm64: futex: Consolidate 'old == new' check in __lsui_cmpxchg32()

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The LSUI futex implementation relies on a cmpxchg() loop to implement
FUTEX_OP_XOR, as the architecture doesn't provide unprivileged *EOR
atomics. Since the unprivileged 'CAST' instructions used to implement
the cmpxchg() can only operate on 64-bit memory locations, the
__lsui_cmpxchg32() helper function performs a song and dance to marshall
the 32-bit futex value into the correct part of a 64-bit register and
fill the remaining bytes with the neighbouring data.

A consequence of this structure is that the 'CAST' failure/success
condition ends up being split into two separate 32-bit checks across
__lsui_cmpxchg32() and its caller. This is a little fiddly to read and
introduces some additional local variables which can be avoided if the
check is done in one place.

Tweak __lsui_cmpxchg32() so that it performs the full 64-bit check on
the value returned from the 'CAST' instruction and returns success to
its caller only in the case that the cmpxchg() operation has succeeded.
With that in place, simplify the outer loop in __lsui_futex_atomic_eor()
to pass 'oldval' by reference and return unless the cmpxchg() operation
returns -EAGAIN. __lsui_futex_cmpxchg() then swallows the -EAGAIN if the
futex word has changed.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Yeoreum Yun <yeoreum.yun@arm.com>
Reviewed-by: Yeoreum Yun <yeoreum.yun@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

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This commit is contained in:

Will Deacon 2026-07-23 14:03:04 +01:00

parent 9315e22b0c

commit 25ef34b541

1 changed files with 22 additions and 39 deletions

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arch/arm64/include/asm/futex.h

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