[PATCH 1/2] drm/panthor: Replace sleep locks with spinlocks in fdinfo path

[Boris Brezillon]

On Tue, 11 Feb 2025 11:39:49 +0000
Tvrtko Ursulin <tursulin at ursulin.net> wrote:

> On 10/02/2025 16:08, Adrián Larumbe wrote:
> > Hi Tvrtko,  
> 
> Thanks!
> 
> > [18153.770244] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:562
> > [18153.771059] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 203412, name: cat
> > [18153.771757] preempt_count: 1, expected: 0
> > [18153.772164] RCU nest depth: 0, expected: 0
> > [18153.772538] INFO: lockdep is turned off.
> > [18153.772898] CPU: 4 UID: 0 PID: 203412 Comm: cat Tainted: G        W          6.14.0-rc1-panthor-next-rk3588-fdinfo+ #1
> > [18153.772906] Tainted: [W]=WARN
> > [18153.772908] Hardware name: Radxa ROCK 5B (DT)
> > [18153.772911] Call trace:
> > [18153.772913]  show_stack+0x24/0x38 (C)
> > [18153.772927]  dump_stack_lvl+0x3c/0x98
> > [18153.772935]  dump_stack+0x18/0x24
> > [18153.772941]  __might_resched+0x298/0x2b0
> > [18153.772948]  __might_sleep+0x6c/0xb0
> > [18153.772953]  __mutex_lock_common+0x7c/0x1950
> > [18153.772962]  mutex_lock_nested+0x38/0x50
> > [18153.772969]  panthor_fdinfo_gather_group_samples+0x80/0x138 [panthor]
> > [18153.773042]  panthor_show_fdinfo+0x80/0x228 [panthor]
> > [18153.773109]  drm_show_fdinfo+0x1a4/0x1e0 [drm]
> > [18153.773397]  seq_show+0x274/0x358
> > [18153.773404]  seq_read_iter+0x1d4/0x630  
> 
> There is a mutex_lock literally in seq_read_iter.
> 
> So colour me confused. What created the atomic context between then and 
> Panthor code?! I just don't see it.

It's actually the xa_lock() we take when gathering fdinfo data that
puts us in this atomic context. In other words, the fix is correct, but
the explanation is wrong :-).