Re: [PATCH v3 RESEND] perf: Fix refcount warning on event->mmap_count increment

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jan 19, 2026 at 11:49:56AM -0700, Will Rosenberg wrote:

> 
> Notes:
>     v2 -> v3: Update patch to error out instead of incrementing.
>     
>     Thank you, this is a much better solution. I was not thinking
>     that the mmap itself was unintended.
>     
>     I believe you are missing a "!" in your patch. After adding
>     that, I tested the patch, and it fixed the bug.

D'0h indeed. Sometimes typing is so very hard ;-)

>     I also wanted to check my understanding of the race with
>     perf_mmap_close() to double check this patch will not cause
>     an issue. perf_mmap_rb() should always hold the
>     event->mmap_mutex, so there should be no race on
>     event->mmap_count with perf_mmap_close()'s
>     refcount_dec_and_mutex_lock(). If there was a race, we would
>     risk returning -EBUSY when we should "continue as if !event->rb."

Since we're failing perf_mmap_rb(), it won't call ->close(), right?

Also, we already have an error path on data_page_nr() mismatch.

The caller of perf_mmap_rb() has if (ret) return ret; nothing is
modified before calling perf_mmap_rb() and perf_mmap_rb() itself hasn't
modified anytyhing yet at the point of failure.

So afaict we're good.


Anyway, thanks for the patch, I'll get it applied!