Messages in this thread | | | From | Mike Frysinger <> | Date | Fri, 3 Jul 2009 14:12:58 -0400 | Subject | Re: making asm-generic/futex.h completely usable |
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On Fri, Jul 3, 2009 at 11:51, Arnd Bergmann wrote: > 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.
when i said "process context" i didnt mean it was running in usermode. i meant the context of the kernel execution is of a process, so it can sleep. since it needs to use get/put user, that means it can sleep fine.
> The irq-disable trick fundamentally does not work on SMP systems, > which need architecture specific atomic operations for this.
i'm guessing spin lock with irq semantics wouldnt work due to the possibility of get/put user sleeping ...
>> 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 ;-)
if the functions were able to be overridden on a per-function basis, that is easy to handle with CONFIG_SMP in the Blackfin header. -mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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