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

[Michael Neuhauser <mike@domain.hid>]

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.

Mike
-- 
Dr. Michael Neuhauser                phone: +43 1 789 08 49 - 30
Firmix Software GmbH                   fax: +43 1 789 08 49 - 55
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