linux-kernel - Re: [RFC PATCH v2] fuse: fix race in fuse_notify

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Message-ID: <CAJfpegsrGO25sJe1GQBVe=Ea5jhkpr7WjpQOHKxkL=gJTk+y8g@mail.gmail.com>
Date: Mon, 24 Feb 2025 14:36:17 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: Luis Henriques <luis@...lia.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Bernd Schubert <bernd@...ernd.com>, Teng Qin <tqin@...ptrading.com>
Subject: Re: [RFC PATCH v2] fuse: fix race in fuse_notify_store()

On Thu, 30 Jan 2025 at 11:16, Luis Henriques <luis@...lia.com> wrote:
>
> Userspace filesystems can push data for a specific inode without it being
> explicitly requested.  This can be accomplished by using NOTIFY_STORE.
> However, this may race against another process performing different
> operations on the same inode.
>
> If, for example, there is a process reading from it, it may happen that it
> will block waiting for data to be available (locking the folio), while the
> FUSE server will also block trying to lock the same folio to update it with
> the inode data.
>
> The easiest solution, as suggested by Miklos, is to allow the userspace
> filesystem to skip locked folios.

Not sure.

The easiest solution is to make the server perform the two operations
independently.  I.e. never trigger a notification from a request.

This is true of other notifications, e.g. doing FUSE_NOTIFY_DELETE
during e.g. FUSE_RMDIR will deadlock on i_mutex.

Or am I misunderstanding the problem?

Thanks,
Miklos