Re: [PATCH] interconnect: avoid memory allocation when 'icc_bw_lock' is held

[Bryan O'Donoghue <bryan.odonoghue@linaro.org>]

On 29/05/2025 15:46, Gabor Juhos wrote:
> The 'icc_bw_lock' mutex is introduced in commit af42269c3523
> ("interconnect: Fix locking for runpm vs reclaim") in order
> to decouple serialization of bw aggregation from codepaths
> that require memory allocation.
> 
> However commit d30f83d278a9 ("interconnect: core: Add dynamic
> id allocation support") added a devm_kasprintf() call into a
> path protected by the 'icc_bw_lock' which causes this lockdep
> warning (at least on the IPQ9574 platform):

Missing a Fixes tag.

> 
>      ======================================================
>      WARNING: possible circular locking dependency detected
>      6.15.0-next-20250529 #0 Not tainted
>      ------------------------------------------------------
>      swapper/0/1 is trying to acquire lock:
>      ffffffc081df57d8 (icc_bw_lock){+.+.}-{4:4}, at: icc_init+0x8/0x108
> 
>      but task is already holding lock:
>      ffffffc081d7db10 (fs_reclaim){+.+.}-{0:0}, at: icc_init+0x28/0x108
> 
>      which lock already depends on the new lock.
> 
>      the existing dependency chain (in reverse order) is:
> 
>      -> #1 (fs_reclaim){+.+.}-{0:0}:
>             fs_reclaim_acquire+0x7c/0xb8
>             slab_alloc_node.isra.0+0x48/0x188
>             __kmalloc_node_track_caller_noprof+0xa4/0x2b8
>             devm_kmalloc+0x5c/0x138
>             devm_kvasprintf+0x6c/0xb8
>             devm_kasprintf+0x50/0x68
>             icc_node_add+0xbc/0x160
>             icc_clk_register+0x15c/0x230
>             devm_icc_clk_register+0x20/0x90
>             qcom_cc_really_probe+0x320/0x338
>             nss_cc_ipq9574_probe+0xac/0x1e8
>             platform_probe+0x70/0xd0
>             really_probe+0xdc/0x3b8
>             __driver_probe_device+0x94/0x178
>             driver_probe_device+0x48/0xf0
>             __driver_attach+0x13c/0x208
>             bus_for_each_dev+0x6c/0xb8
>             driver_attach+0x2c/0x40
>             bus_add_driver+0x100/0x250
>             driver_register+0x68/0x138
>             __platform_driver_register+0x2c/0x40
>             nss_cc_ipq9574_driver_init+0x24/0x38
>             do_one_initcall+0x88/0x340
>             kernel_init_freeable+0x2ac/0x4f8
>             kernel_init+0x28/0x1e8
>             ret_from_fork+0x10/0x20
> 
>      -> #0 (icc_bw_lock){+.+.}-{4:4}:
>             __lock_acquire+0x1348/0x2090
>             lock_acquire+0x108/0x2d8
>             icc_init+0x50/0x108
>             do_one_initcall+0x88/0x340
>             kernel_init_freeable+0x2ac/0x4f8
>             kernel_init+0x28/0x1e8
>             ret_from_fork+0x10/0x20
> 
>      other info that might help us debug this:
> 
>       Possible unsafe locking scenario:
> 
>             CPU0                    CPU1
>             ----                    ----
>        lock(fs_reclaim);
>                                     lock(icc_bw_lock);
>                                     lock(fs_reclaim);
>        lock(icc_bw_lock);
> 
>       *** DEADLOCK ***
> 
>      1 lock held by swapper/0/1:
>       #0: ffffffc081d7db10 (fs_reclaim){+.+.}-{0:0}, at: icc_init+0x28/0x108
> 
>      stack backtrace:
>      CPU: 3 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.15.0-next-20250529 #0 NONE
>      Hardware name: Qualcomm Technologies, Inc. IPQ9574/AP-AL02-C7 (DT)
>      Call trace:
>       show_stack+0x20/0x38 (C)
>       dump_stack_lvl+0x90/0xd0
>       dump_stack+0x18/0x28
>       print_circular_bug+0x334/0x448
>       check_noncircular+0x12c/0x140
>       __lock_acquire+0x1348/0x2090
>       lock_acquire+0x108/0x2d8
>       icc_init+0x50/0x108
>       do_one_initcall+0x88/0x340
>       kernel_init_freeable+0x2ac/0x4f8
>       kernel_init+0x28/0x1e8
>       ret_from_fork+0x10/0x20
> 
> Move the memory allocation part of the code outside of the protected
> path to eliminate the warning. Also add a note about why it is moved
> to there,
> 
> Fixes: d30f83d278a9 ("interconnect: core: Add dynamic id allocation support")
> Signed-off-by: Gabor Juhos <j4g8y7@gmail.com>
> ---
>   drivers/interconnect/core.c | 14 ++++++++++----
>   1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/interconnect/core.c b/drivers/interconnect/core.c
> index 1a41e59c77f85a811f78986e98401625f4cadfa3..acdb3b8f1e54942dbb1b71ec2b170b08ad709e6b 100644
> --- a/drivers/interconnect/core.c
> +++ b/drivers/interconnect/core.c
> @@ -1023,6 +1023,16 @@ void icc_node_add(struct icc_node *node, struct icc_provider *provider)
>   		return;
> 
>   	mutex_lock(&icc_lock);
> +
> +	if (node->id >= ICC_DYN_ID_START) {
> +		/*
> +		 * Memory allocation must be done outside of codepaths
> +		 * protected by icc_bw_lock.
> +		 */
> +		node->name = devm_kasprintf(provider->dev, GFP_KERNEL, "%s@%s",
> +					    node->name, dev_name(provider->dev));
> +	}
> +
>   	mutex_lock(&icc_bw_lock);
> 
>   	node->provider = provider;
> @@ -1038,10 +1048,6 @@ void icc_node_add(struct icc_node *node, struct icc_provider *provider)
>   	node->avg_bw = node->init_avg;
>   	node->peak_bw = node->init_peak;
> 
> -	if (node->id >= ICC_DYN_ID_START)
> -		node->name = devm_kasprintf(provider->dev, GFP_KERNEL, "%s@%s",
> -					    node->name, dev_name(provider->dev));
> -
>   	if (node->avg_bw || node->peak_bw) {
>   		if (provider->pre_aggregate)
>   			provider->pre_aggregate(node);
> 
> ---
> base-commit: 5fed7fe33c2cd7104fc87b7bc699a7be892befa2
> change-id: 20250529-icc-bw-lockdep-ed030d892a19
> 
> Best regards,
> --
> Gabor Juhos <j4g8y7@gmail.com>
> 
> 

The locking in this code is a mess.

Which data-structures does icc_lock protect node* pointers I think and 
which data-structures does icc_bw_lock protect - "bw" data structures ?

Hmm.

Looking at this code I'm not sure at all what icc_lock was introduced to do.

Can we not just drop it entirely ?

---
bod