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SubjectACPI global lock macros
Hi Andy,

The ACPI_ACQUIRE_GLOBAL_LOCK() macro in include/asm-i386/acpi.h looks a
little odd:

#define ACPI_ACQUIRE_GLOBAL_LOCK(GLptr, Acq) \
do { \
int dummy; \
asm("1: movl (%1),%%eax;" \
"movl %%eax,%%edx;" \
"andl %2,%%edx;" \
"btsl $0x1,%%edx;" \
"adcl $0x0,%%edx;" \
"lock; cmpxchgl %%edx,(%1);" \
"jnz 1b;" \
"cmpb $0x3,%%dl;" \
"sbbl %%eax,%%eax" \
:"=a"(Acq),"=c"(dummy):"c"(GLptr),"i"(~1L):"dx"); \
} while(0)


When compiled, it results in:

266: mov 0x0,%ecx
268: R_386_32 acpi_gbl_common_fACS
26c: mov (%ecx),%eax
26e: mov %eax,%edx
270: and %ecx,%edx
272: bts $0x1,%edx
276: adc $0x0,%edx
279: lock cmpxchg %edx,(%ecx)
27d: jne 26c <acpi_ev_acquire_global_lock+0x2f>
27f: cmp $0x3,%dl
282: sbb %eax,%eax

So at location 270 we mask %edx with %ecx, which is the address of the
global lock. Unless the global lock is aligned on a 2-byte but not
4-byte boundary, which seems a little unlikely, then this is going to
clear both the owned and the pending bits in %edx, so we'll always think
that the lock is not owned. Shouldn't the andl be masking with %3 (which
is initialised as ~1) rather than %2 (the address of the lock)?

Given the comments above the definition, I'm guessing that the "dummy"
parameter was added later for some reason (to tell gcc that ecx would
get clobbered? - but it doesn't seem to be clobbered), and the parameter
substitutions in the asm weren't updated. Unless I'm missing something
fundamental, shouldn't the definition be something more like this:


#define ACPI_ACQUIRE_GLOBAL_LOCK(GLptr, Acq) \
do { \
asm volatile("1:movl (%1),%%eax;" \
"movl %%eax,%%edx;" \
"andl %2,%%edx;" \
"btsl $0x1,%%edx;" \
"adcl $0x0,%%edx;" \
"lock; cmpxchgl %%edx,(%1);" \
"jnz 1b;" \
"cmpb $0x3,%%dl;" \
"sbbl %0,%0" \
:"=r"(Acq):"r"(GLptr),"i"(~1L):"dx", "ax"); \
} while(0)

which compiles to:

2e5: mov 0x0,%ecx
2e7: R_386_32 acpi_gbl_common_fACS
2eb: mov (%ecx),%eax
2ed: mov %eax,%edx
2ef: and $0xfffffffe,%edx
2f2: bts $0x1,%edx
2f6: adc $0x0,%edx
2f9: lock cmpxchg %edx,(%ecx)
2fd: jne 2eb <acpi_ev_acquire_global_lock+0x37>
2ff: cmp $0x3,%dl
302: sbb %cl,%cl


which is identical to the ACPI spec reference implementation, apart from
returning the result in %cl rather than %al (since we're cleanly
separating clobbered registers from input/output params, and letting gcc
choose the param registers).

Alternatively it could be defined in C (as in ia64) which would reduce
the likelihood of asm bugs. (Although it wouldn't be safe to use
__cmpxchg(), as that uses LOCK_PREFIX which is empty on UP, rather than
an explicit "lock").

ACPI_RELEASE_GLOBAL_LOCK(), and the x86_64 variants of these, seem to
have similar issues.

Paul

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