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Paul Burton authored
Commit 8263db4d ("MIPS: cmpxchg: Implement __cmpxchg() as a function") refactored our implementation of __cmpxchg() to be a function rather than a macro, with the aim of making it easier to read & modify. Unfortunately the commit breaks use of cmpxchg() for signed 32 bit values when we have a 64 bit kernel with kernel_uses_llsc == false, because: - In cmpxchg_local() we cast the old value to the type the pointer points to, and then to an unsigned long. If the pointer points to a signed type smaller than 64 bits then the old value will be sign extended to 64 bits. That is, bits beyond the size of the pointed to type will be set to 1 if the old value is negative. In the case of a signed 32 bit integer with a negative value, bits 63:32 will all be set. - In __cmpxchg_asm() we load the value from memory, ie. dereference the pointer, and store the value as an unsigned integer (__ret) whose size matches the pointer. For a 32 bit cmpxchg() this means we store the value in a u32, because the pointer provided to __cmpxchg_asm() by __cmpxchg() is of type volatile u32 *. - __cmpxchg_asm() then checks whether the value in memory (__ret) matches the provided old value, by comparing the two values. This results in the u32 being promoted to a 64 bit unsigned long to match the old argument - however because both types are unsigned the value is zero extended, which does not match the sign extension performed on the old value in cmpxchg_local() earlier. This mismatch means that unfortunate cmpxchg() calls can incorrectly fail for 64 bit kernels with kernel_uses_llsc == false. This is the case on at least non-SMP Cavium Octeon kernels, which hardcode kernel_uses_llsc in their cpu-feature-overrides.h header. Using a v4.13-rc7 kernel configured using cavium_octeon_defconfig with SMP manually disabled, this presents itself as oddity when we reach userland - for example: can't run '/bin/mount': Text file busy can't run '/bin/mkdir': Text file busy can't run '/bin/mkdir': Text file busy can't run '/bin/mount': Text file busy can't run '/bin/hostname': Text file busy can't run '/etc/init.d/rcS': Text file busy can't run '/sbin/getty': Text file busy can't run '/sbin/getty': Text file busy It appears that some part of the init process, which is in this case buildroot's busybox init, is running successfully. It never manages to reach the login prompt though, and complains about /sbin/getty being busy repeatedly and indefinitely. Fix this by casting the old value provided to __cmpxchg_asm() to an appropriately sized unsigned integer, such that we consistently zero-extend avoiding the mismatch. The __cmpxchg_small() case for 8 & 16 bit values is unaffected because __cmpxchg_small() already masks provided values appropriately. Signed-off-by:
Paul Burton <paul.burton@imgtec.com> Fixes: 8263db4d ("MIPS: cmpxchg: Implement __cmpxchg() as a function") Cc: linux-mips@linux-mips.org Patchwork: https://patchwork.linux-mips.org/patch/17226/ Cc: linux-mips@linux-mips.org Signed-off-by:
Ralf Baechle <ralf@linux-mips.org>
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