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

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Message-ID: <CAJfpeguQTZ8KcdffKvY8kknZVnBH6h3Tz1GSESwBjXSz_25TLw@mail.gmail.com>
Date: Mon, 24 Feb 2025 15:39:38 +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>, 
	Matt Harvey <mharvey@...ptrading.com>
Subject: Re: [RFC PATCH v2] fuse: fix race in fuse_notify_store()

On Mon, 24 Feb 2025 at 15:30, Luis Henriques <luis@...lia.com> wrote:
>
> On Mon, Feb 24 2025, Miklos Szeredi wrote:
>
> > 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.
>
> Hmmm... OK, the NOTIFY_DELETE and NOTIFY_INVAL_ENTRY deadlocks are
> documented (in libfuse, at least).  So, maybe this one could be added to
> the list of notifications that could deadlock.  However, IMHO, it would be
> great if this could be fixed instead.
>
> > Or am I misunderstanding the problem?
>
> I believe the initial report[1] actually adds a specific use-case where
> the deadlock can happen when the server performs the two operations
> independently.  For example:
>
>   - An application reads 4K of data at offset 0
>   - The server gets a read request.  It performs the read, and gets more
>     data than the data requested (say 4M)
>   - It caches this data in userspace and replies to VFS with 4K of data
>   - The server does a notify_store with the reminder data
>   - In the meantime the userspace application reads more 4K at offset 4K
>
> The last 2 operations can race and the server may deadlock if the
> application already has locked the page where data will be read into.

I don't see the deadlock.  If the race was won by the read, then it
will proceed with FUSE_READ and fetch the data from the server.  When
this is finished,  NOTIFY_STORE will overwrite the page with the same
data.

Thanks,
Miklos