linux-kernel - Re: [RFC PATCH 0/3] gpu: nova-core: add basic timer subdevice implementation

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Message-ID: <CAPM=9tyu84z4Xk5X0fykO3Dazby2UqRgwtN4woNKe4Z2yMyDZg@mail.gmail.com>
Date: Tue, 18 Feb 2025 11:46:26 +1000
From: Dave Airlie <airlied@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Alexandre Courbot <acourbot@...dia.com>, John Hubbard <jhubbard@...dia.com>, 
	Ben Skeggs <bskeggs@...dia.com>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, nouveau@...ts.freedesktop.org, 
	dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH 0/3] gpu: nova-core: add basic timer subdevice implementation

> 1. How to avoid unnecessary calls to try_access().
>
> This is why I made Boot0.read() take a &RevocableGuard<'_, Bar0> as argument. I
> think we can just call try_access() once and then propage the guard through the
> callchain, where necessary.

Nope, you can't do that, RevocableGuard holds a lock and things
explode badly in lockdep if you do.

[ 39.960247] =============================
[ 39.960265] [ BUG: Invalid wait context ]
[ 39.960282] 6.12.0-rc2+ #151 Not tainted
[ 39.960298] -----------------------------
[ 39.960316] modprobe/2006 is trying to lock:
[ 39.960335] ffffa08dd7783a68
(drivers/gpu/nova-core/gsp/sharedq.rs:259){....}-{3:3}, at:
_RNvMs0_NtNtCs6v51TV2h8sK_6nova_c3gsp7sharedqNtB5_26GSPSharedQueuesr535_113_018rpc_push+0x34/0x4c0
[nova_core]
[ 39.960413] other info that might help us debug this:
[ 39.960434] context-{4:4}
[ 39.960447] 2 locks held by modprobe/2006:
[ 39.960465] #0: ffffa08dc27581b0 (&dev->mutex){....}-{3:3}, at:
__driver_attach+0x111/0x260
[ 39.960505] #1: ffffffffad55ac10 (rcu_read_lock){....}-{1:2}, at:
rust_helper_rcu_read_lock+0x11/0x80
[ 39.960545] stack backtrace:
[ 39.960559] CPU: 8 UID: 0 PID: 2006 Comm: modprobe Not tainted 6.12.0-rc2+ #151
[ 39.960586] Hardware name: System manufacturer System Product
Name/PRIME X370-PRO, BIOS 6231 08/31/2024
[ 39.960618] Call Trace:
[ 39.960632] <TASK>

was one time I didn't drop a revocable before proceeding to do other things,

Dave.