[Intel-xe] [PATCH 3/3] drm/drm_exec: Work around a WW mutex lockdep oddity
Christian König
ckoenig.leichtzumerken at gmail.com
Wed Sep 6 08:34:00 UTC 2023
Am 05.09.23 um 16:29 schrieb Thomas Hellström:
> Hi, Christian
>
> On 9/5/23 15:14, Christian König wrote:
>> Am 05.09.23 um 10:58 schrieb Thomas Hellström:
>>> If *any* object of a certain WW mutex class is locked, lockdep will
>>> consider *all* mutexes of that class as locked. Also the lock
>>> allocation
>>> tracking code will apparently register only the address of the first
>>> mutex locked in a sequence.
>>> This has the odd consequence that if that first mutex is unlocked and
>>> its memory then freed, the lock alloc tracking code will assume that
>>> memory
>>> is freed with a held lock in there.
>>>
>>> For now, work around that for drm_exec by releasing the first grabbed
>>> object lock last.
>>>
>>> Related lock alloc tracking warning:
>>> [ 322.660067] =========================
>>> [ 322.660070] WARNING: held lock freed!
>>> [ 322.660074] 6.5.0-rc7+ #155 Tainted: G U N
>>> [ 322.660078] -------------------------
>>> [ 322.660081] kunit_try_catch/4981 is freeing memory
>>> ffff888112adc000-ffff888112adc3ff, with a lock still held there!
>>> [ 322.660089] ffff888112adc1a0
>>> (reservation_ww_class_mutex){+.+.}-{3:3}, at:
>>> drm_exec_lock_obj+0x11a/0x600 [drm_exec]
>>> [ 322.660104] 2 locks held by kunit_try_catch/4981:
>>> [ 322.660108] #0: ffffc9000343fe18
>>> (reservation_ww_class_acquire){+.+.}-{0:0}, at:
>>> test_early_put+0x22f/0x490 [drm_exec_test]
>>> [ 322.660123] #1: ffff888112adc1a0
>>> (reservation_ww_class_mutex){+.+.}-{3:3}, at:
>>> drm_exec_lock_obj+0x11a/0x600 [drm_exec]
>>> [ 322.660135]
>>> stack backtrace:
>>> [ 322.660139] CPU: 7 PID: 4981 Comm: kunit_try_catch Tainted: G
>>> U N 6.5.0-rc7+ #155
>>> [ 322.660146] Hardware name: ASUS System Product Name/PRIME B560M-A
>>> AC, BIOS 0403 01/26/2021
>>> [ 322.660152] Call Trace:
>>> [ 322.660155] <TASK>
>>> [ 322.660158] dump_stack_lvl+0x57/0x90
>>> [ 322.660164] debug_check_no_locks_freed+0x20b/0x2b0
>>> [ 322.660172] slab_free_freelist_hook+0xa1/0x160
>>> [ 322.660179] ? drm_exec_unlock_all+0x168/0x2a0 [drm_exec]
>>> [ 322.660186] __kmem_cache_free+0xb2/0x290
>>> [ 322.660192] drm_exec_unlock_all+0x168/0x2a0 [drm_exec]
>>> [ 322.660200] drm_exec_fini+0xf/0x1c0 [drm_exec]
>>> [ 322.660206] test_early_put+0x289/0x490 [drm_exec_test]
>>> [ 322.660215] ? __pfx_test_early_put+0x10/0x10 [drm_exec_test]
>>> [ 322.660222] ? __kasan_check_byte+0xf/0x40
>>> [ 322.660227] ? __ksize+0x63/0x140
>>> [ 322.660233] ? drmm_add_final_kfree+0x3e/0xa0 [drm]
>>> [ 322.660289] ? _raw_spin_unlock_irqrestore+0x30/0x60
>>> [ 322.660294] ? lockdep_hardirqs_on+0x7d/0x100
>>> [ 322.660301] ? __pfx_kunit_try_run_case+0x10/0x10 [kunit]
>>> [ 322.660310] ? __pfx_kunit_generic_run_threadfn_adapter+0x10/0x10
>>> [kunit]
>>> [ 322.660319] kunit_generic_run_threadfn_adapter+0x4a/0x90 [kunit]
>>> [ 322.660328] kthread+0x2e7/0x3c0
>>> [ 322.660334] ? __pfx_kthread+0x10/0x10
>>> [ 322.660339] ret_from_fork+0x2d/0x70
>>> [ 322.660345] ? __pfx_kthread+0x10/0x10
>>> [ 322.660349] ret_from_fork_asm+0x1b/0x30
>>> [ 322.660358] </TASK>
>>> [ 322.660818] ok 8 test_early_put
>>>
>>> Cc: Christian König <christian.koenig at amd.com>
>>> Cc: Boris Brezillon <boris.brezillon at collabora.com>
>>> Cc: Danilo Krummrich <dakr at redhat.com>
>>> Cc: dri-devel at lists.freedesktop.org
>>> Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>>> ---
>>> drivers/gpu/drm/drm_exec.c | 2 +-
>>> include/drm/drm_exec.h | 35 +++++++++++++++++++++++++++++++----
>>> 2 files changed, 32 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c
>>> index ff69cf0fb42a..5d2809de4517 100644
>>> --- a/drivers/gpu/drm/drm_exec.c
>>> +++ b/drivers/gpu/drm/drm_exec.c
>>> @@ -56,7 +56,7 @@ static void drm_exec_unlock_all(struct drm_exec
>>> *exec)
>>> struct drm_gem_object *obj;
>>> unsigned long index;
>>> - drm_exec_for_each_locked_object(exec, index, obj) {
>>> + drm_exec_for_each_locked_object_reverse(exec, index, obj) {
>>
>> Well that's a really good catch, just one more additional thought below.
>>
>>> dma_resv_unlock(obj->resv);
>>> drm_gem_object_put(obj);
>>> }
>>> diff --git a/include/drm/drm_exec.h b/include/drm/drm_exec.h
>>> index e0462361adf9..55764cf7c374 100644
>>> --- a/include/drm/drm_exec.h
>>> +++ b/include/drm/drm_exec.h
>>> @@ -51,6 +51,20 @@ struct drm_exec {
>>> struct drm_gem_object *prelocked;
>>> };
>>> +/**
>>> + * drm_exec_obj() - Return the object for a give drm_exec index
>>> + * @exec: Pointer to the drm_exec context
>>> + * @index: The index.
>>> + *
>>> + * Return: Pointer to the locked object corresponding to @index if
>>> + * index is within the number of locked objects. NULL otherwise.
>>> + */
>>> +static inline struct drm_gem_object *
>>> +drm_exec_obj(struct drm_exec *exec, unsigned long index)
>>> +{
>>> + return index < exec->num_objects ? exec->objects[index] : NULL;
>>> +}
>>> +
>>> /**
>>> * drm_exec_for_each_locked_object - iterate over all the locked
>>> objects
>>> * @exec: drm_exec object
>>> @@ -59,10 +73,23 @@ struct drm_exec {
>>> *
>>> * Iterate over all the locked GEM objects inside the drm_exec
>>> object.
>>> */
>>> -#define drm_exec_for_each_locked_object(exec, index, obj) \
>>> - for (index = 0, obj = (exec)->objects[0]; \
>>> - index < (exec)->num_objects; \
>>> - ++index, obj = (exec)->objects[index])
>>> +#define drm_exec_for_each_locked_object(exec, index, obj) \
>>> + for ((index) = 0; ((obj) = drm_exec_obj(exec, index)); ++(index))
>>
>> Mhm, that makes it possible to modify the number of objects while
>> inside the loop, doesn't it?
>
> Sorry, you lost me a bit there. Isn't that possible with the previous
> code as well?
Yeah, indeed. Reviewed-by: Christian König <christian.koenig at amd.com>
Regards,
Christian.
>
> /Thanks,
>
> Thomas
>
>
>
>>
>> I'm not sure if that's a good idea or not.
>>
>> Regards,
>> Christian.
>>
>>> +
>>> +/**
>>> + * drm_exec_for_each_locked_object_reverse - iterate over all the
>>> locked
>>> + * objects in reverse locking order
>>> + * @exec: drm_exec object
>>> + * @index: unsigned long index for the iteration
>>> + * @obj: the current GEM object
>>> + *
>>> + * Iterate over all the locked GEM objects inside the drm_exec
>>> object in
>>> + * reverse locking order. Note that @index may go below zero and wrap,
>>> + * but that will be caught by drm_exec_object(), returning a NULL
>>> object.
>>> + */
>>> +#define drm_exec_for_each_locked_object_reverse(exec, index, obj) \
>>> + for ((index) = (exec)->num_objects - 1; \
>>> + ((obj) = drm_exec_obj(exec, index)); --(index))
>>> /**
>>> * drm_exec_until_all_locked - loop until all GEM objects are locked
>>
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