24.08.2015 20:43, David Miller пишет:
From: Eugene Shatokhin <eugene.shatokhin@xxxxxxxxxx> Date: Wed, 19 Aug 2015 14:59:01 +0300So the following might be possible, although unlikely: CPU0 CPU1 clear_bit: read dev->flags clear_bit: clear EVENT_RX_KILL in the read value dev->flags=0; clear_bit: write updated dev->flags As a result, dev->flags may become non-zero again.Is this really possible?
On x86, it is not possible, so this is not a problem. Perhaps, for ARM too. As for the other architectures supported by the kernel - not sure, no common guarantees, it seems. Anyway, this is not a critical issue, I agree.
OK, let us leave things as they are for this one and fix the rest.
Stores really are "atomic" in the sense that the do their update in one indivisible operation. Atomic operations like clear_bit also will behave that way. If a clear_bit is in progress, the "dev->flags=0" store will not be able to grab the cache line exclusively until the clear_bit is done. So I think the above sequent of events is completely impossible. Once a clear_bit starts, a write by another foreign agent on the bus is absolutely impossible to legally occur until the clear_bit completes. I think this is a non-issue.
Regards, Eugene -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
