Re: [Adeos-main] non-atomic bitop in __adeos_stall_root()

[Philippe Gerum <rpm@xenomai.org>]

On Thu, 2005-01-27 at 21:26, Michael Neuhauser wrote:
> Hello!
> 
> During my work on Adeos for ARM I've noticed the following: In
> __adeos_stall_root() (both 2.4 & 2.6) the stall flag is set like this:
> 
> 	adeos_get_cpu(flags);
> 	__set_bit(IPIPE_STALL_FLAG,&adp_root->cpudata[cpuid].status);
> 	adeos_put_cpu(flags);
> 
> I don't understand why the non-atomic __set_bit() is used instead of the
> atomic set_bit(). Maybe someone can shed some light on this.
> 
> Suppose we have a non-SMP machine. In this case, adeos_get_cpu() is a
> nop. Hence, hw-irqs may be enabled when __set_bit() is executed. On ARM,
> __set_bit() is implemented as
> 
> 	read from memory; modify register; write to memory
> 
> Assume an interrupt happens after the read and before the write and that
> some other bit of "status" is modified during the interrupt
> => this modification is lost as the interrupted __set_bit() uses the
> value of "status" before the interrupt!
> 
> This may not happen on i386 where __set_bit() is implemented as a single
> asm instruction (which I assume is interrupt safe). But as
> __adeos_stall_root() is contained in include/linux/adeos.h (i.e. generic
> code) I think the atomic bitop has to be used.
> 
> __adeos_test_and_stall_root() and adeos_unstall_pipeline_from() also use
> some non-atomic bitop without ensuring that hw-interrupts are disabled.
> 

Ack, it's a bug non-x86 wise; likely a collision between two past change
sets, namely using adeos_get_cpu() when adeos_lock_cpu() would have been
overkill on UP, and set/clear_bits() changed to __set/__clear_bits() for
the same reason given that the interrupts were guaranteed to be off at
that time. The problem is that #1 came after #2, and introduced this bug
recently.

> Mike
-- 

Philippe.