Hello,
I write in relation to a email I sent two weeks ago about atomicity.
Henrik, I have bought the book "Symmetric Multiprocessing and Caching
for Kernel Programmers" and it's great.
Anyway, what I am confused about is if this instruction is atomic:
if(flags & FLAG)
{
}
where flag is a constant such as 0x02. Is this atomic? The flags
variable is word-sized and hopefully aligned in its size boundary.
Now, my question is: If this is atomic, why are these kernel functions needed?
test_bit() ---> It ensures the operation is atomic
__test_bit() ----> It doesnt ensure atomicity
The book "Linux Kernel Development" by Robert Love says that i should
use this functions whenever i need to work with bits in a variable.
However, it seems more readable for me to use the classical way :
flags |= FLAG --> to set a bit
flags &= ~FLAG --> to clear a bit
(old_flags ^ new_flags) & FLAG --> test if the flag has changed
...an so on. Of course, these last examples wouldnt be atomic, right?
Because it's not just reading the flags variable. Instead, it envolves
read-modify-write the variable. So I should use a locking scheme for
this.
But, if I assure there is one and only one writer (using a spinlock,
semaphore, ...) and use the above instructions to set/clear/change
bits is the (flags & FLAG) condition atomic? And why the need for
test_bit and __test_bit()?
Well, I hope my questions are clear, and someone can help me. I know,
I could just use test_bit() and forget about this. But, however, the
networking code uses flags the other way, for example in
dev_change_flags() [net/core/dev.c].
It acquires the rtnl semaphore (in function dev_ioctl) before changing
anything in dev->flags, so there is no more than one writer at a
certain time. But in dev_get_flags() [net/core/dev.c] it reads the
flags, without acquiring any lock for that purpose.
Thank you for your time.
Regards
Aritz
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