Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: making asm-generic/futex.h completely usable | Date | Fri, 3 Jul 2009 17:51:46 +0200 |
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On Friday 03 July 2009, Mike Frysinger wrote: > ive been reading up on futexes of late and have been poking around the > futex kernel pieces to see what is needed to get Blackfin working with > it. > > Blackfin falls into the "no hardware atomic instructions" category as > can easily be seen in our atomic.h: disable interrupts, do > load/stores, restore interrupts. my understanding of the > futex_atomic_op_inuser() function is that this runs in process > context, so this same interrupt trick should work fine. which leads > me to wonder why doesnt the asm-generic/futex.h header already take > this approach ?
The code does not run in user space, the name indicates that it operates on variables in user space while running in the kernel.
The irq-disable trick fundamentally does not work on SMP systems, which need architecture specific atomic operations for this.
> seems to me that the SuperH implementation fits the bill nicely. > their arch/sh/include/asm/futex-irq.h looks like it could be literally > straight copied into asm-generic/futex.h thus making it fully > functional for everyone by default.
That sounds reasonable for the arch/sh/include/asm/futex.h file, but the futex-irq.h file would still be reserved for non-SMP architectures. I believe that out of the architectures currently using asm-generic/futex.h, blackfin is the only one that supports SMP, so you still lose ;-)
On blackfin, being NOMMU, you could problably do a respective implementation for SMP, along the lines of
/* include/asm-generic/futex-nommu-smp.h */ static inline int atomic_futex_op_xchg_set(int oparg, int __user *uaddr, int *oldval) { int *addr = (__force void *)uaddr; /* only valid on NOMMU */ if (!access_ok(VERIFY_WRITE, uaddr, 4)) return -EFAULT; *oldval = xchg(addr, oparg); return 0; }
static inline int atomic_futex_op_xchg_add(int oparg, int __user *uaddr, int *oldval) { int *addr = (__force void *)uaddr; /* only valid on NOMMU */ int tmp; if (!access_ok(VERIFY_WRITE, uaddr, 4)) return -EFAULT; tmp = *addr; while ((*oldval = cmpxchg(addr, tmp, tmp + oparg)) != tmp) tmp = *oldval; return 0; }
I.e. if your user space access is trivial, you can implement this using the standard cmpxchg and xchg functions even on SMP.
Arnd <><
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