On Mon, Sep 08, 2014 at 03:55:11PM +0300, Alexander Shishkin wrote:

> Fair enough. Then I'd like to disable the ACTIVE ones before freeing AUX
> stuff and then re-enabling them since perf_event_{en,dis}able() already
> provide the convenient cross-cpu calls, which would also avoid
> concurrency between pmu::{add,del} callbacks and this unmap path. Makes
> sense?

But why? The buffer stuff is RCU freed, so if the hardware observes
pages and does get_page_unless_zero() on them its good. The memory will
not be freed from underneath the hardware writer because of the
get_page().

Then when the buffer is full and we 'swap', we'll find there is no next
buffer. At that point we can not provide a new buffer, effectively
stopping the hardware writes and release the old buffer, freeing the
memory.