rw-semaphore asm constraints patch fix
PATCH 1/1 rw-semaphore asm constraints patch fix
Hi Linus,
The attached patch should fix the compile error that asm-i386/rwsem.h causes with old gcc's whilst maintaining the asm constraints correctly.
David
include/asm-i386/rwsem.h +2 -2
@@ -164,7 +164,7 @@ " jmp 1b\n" LOCK_SECTION_END "# ending __up_read\n" - : /*"+m"(sem->count),*/ "+d"(tmp) - : "a"(sem) + : "=m"(sem->count), "=d"(tmp) + : "0"(sem->count), "1"(tmp), "a"(sem) : "memory", "cc"); }
Quoted 3 lines · David Howells
The attached patch should fix the compile error that
asm-i386/rwsem.hcauses with old gcc's whilst maintaining the asm constraints correctly.
Doesn't work.
include/asm-i386/rwsem.h line 168
Quoted 2 lines
include/asm-i386/rwsem.h +2
-0+ : "=m"(sem->count), "=d"(tmp) + : "0"(sem->count), "1"(tmp), "a"(sem)
A "m" constraint cannot be used together with a subsequent number (ie the "=m" -> "0" thing is illegal - the numbering traditionally only works on hard registers). It may work with some compilers and with the right register pressure, but it absolutely _will_ cause problems on other setups.
I should know, I tried to work around gcc stupidities in this area enough times, and cursed how various things didn't work with various compilers.
The best thing to do is probably to mark "sem->count" a memory input only, which together with the memory clobber should be ok.
Linus