linux-kernel - Re: [RFC PATCH v v2 0/4] avoid the extra atomic on a ref when closing a fd

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Message-ID: <20250305021936.71e837ea@pumpkin>
Date: Wed, 5 Mar 2025 02:19:36 +0000
From: David Laight <david.laight.linux@...il.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz,
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH v v2 0/4] avoid the extra atomic on a ref when
 closing a fd

On Tue,  4 Mar 2025 19:35:02 +0100
Mateusz Guzik <mjguzik@...il.com> wrote:

> The stock kernel transitioning the file to no refs held penalizes the
> caller with an extra atomic to block any increments.
> 
> For cases where the file is highly likely to be going away this is
> easily avoidable.

Have you looked at the problem caused by epoll() ?
The epoll code has a 'hidden' extra reference to the fd.
This doesn't usualy matter, but some of the driver callbacks add and
remove an extra reference - which doesn't work well if fput() has
just decremented it to zero.

The fput code might need to do a 'decrement not one' so that the
epoll tidyup can be done while the refcount is still one.

That would save the extra atomic pair that (IIRC) got added into
the epoll callback code.

Thoughts?

	David