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2c0b8a7578
Actually, on 386, cmpxchg and cmpxchg_local fall back on cmpxchg_386_u8/16/32: it disables interruptions around non atomic updates to mimic the cmpxchg behavior. The comment: /* Poor man's cmpxchg for 386. Unsuitable for SMP */ already present in cmpxchg_386_u32 tells much about how this cmpxchg implementation should not be used in a SMP context. However, the cmpxchg_local can perfectly use this fallback, since it only needs to be atomic wrt the local cpu. This patch adds a cmpxchg_486_u64 and uses it as a fallback for cmpxchg64 and cmpxchg64_local on 80386 and 80486. Q: but why is it called cmpxchg_486 when the other functions are called A: Because the standard cmpxchg is missing only on 386, but cmpxchg8b is missing both on 386 and 486. Citing Intel's Instruction set reference: cmpxchg: This instruction is not supported on Intel processors earlier than the Intel486 processors. cmpxchg8b: This instruction encoding is not supported on Intel processors earlier than the Pentium processors. Q: What's the reason to have cmpxchg64_local on 32 bit architectures? Without that need all this would just be a few simple defines. A: cmpxchg64_local on 32 bits architectures takes unsigned long long parameters, but cmpxchg_local only takes longs. Since we have cmpxchg8b to execute a 8 byte cmpxchg atomically on pentium and +, it makes sense to provide a flavor of cmpxchg and cmpxchg_local using this instruction. Also, for 32 bits architectures lacking the 64 bits atomic cmpxchg, it makes sense _not_ to define cmpxchg64 while cmpxchg could still be available. Moreover, the fallback for cmpxchg8b on i386 for 386 and 486 is a However, cmpxchg64_local will be emulated by disabling interrupts on all architectures where it is not supported atomically. Therefore, we *could* turn cmpxchg64_local into a cmpxchg_local, but it would make the 386/486 fallbacks ugly, make its design different from cmpxchg/cmpxchg64 (which really depends on atomic operations and cannot be emulated) and require the __cmpxchg_local to be expressed as a macro rather than an inline function so the parameters would not be fixed to unsigned long long in every case. So I think cmpxchg64_local makes sense there, but I am open to suggestions. Q: Are there any callers? A: I am actually using it in LTTng in my timestamping code. I use it to work around CPUs with asynchronous TSCs. I need to update 64 bits values atomically on this 32 bits architecture. Changelog: - Ran though checkpatch. Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> Cc: Andi Kleen <ak@suse.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> |
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| crypto | ||
| ia32 | ||
| kernel | ||
| lguest | ||
| lib | ||
| mach-default | ||
| mach-es7000 | ||
| mach-generic | ||
| mach-visws | ||
| mach-voyager | ||
| math-emu | ||
| mm | ||
| oprofile | ||
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| power | ||
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| video | ||
| xen | ||
| Kconfig | ||
| Kconfig.cpu | ||
| Kconfig.debug | ||
| Makefile | ||
| Makefile_32 | ||
| Makefile_32.cpu | ||
| Makefile_64 | ||