Re: [PATCH v3] erofs: fix atomic context detection when !CONFIG_DEBUG_LOCK_ALLOC

Re: [PATCH v3] erofs: fix atomic context detection when !CONFIG_DEBUG_LOCK_ALLOC

[Gao Xiang]

On 2025/8/5 09:19, Junli Liu wrote:
> Since EROFS handles decompression in non-atomic contexts due to
> uncontrollable decompression latencies and vmap() usage, it tries
> to detect atomic contexts and only kicks off a kworker on demand
> in order to reduce unnecessary scheduling overhead.
> 
> However, the current approach is insufficient and can lead to
> sleeping function calls in invalid contexts, causing kernel
> warnings and potential system instability. See the stacktrace [1]
> and previous discussion [2].
> 
> The current implementation only checks rcu_read_lock_any_held(),
> which behaves inconsistently across different kernel configurations:
> 
> - When CONFIG_DEBUG_LOCK_ALLOC is enabled: correctly detects
>    RCU critical sections by checking rcu_lock_map
> - When CONFIG_DEBUG_LOCK_ALLOC is disabled: compiles to
>    "!preemptible()", which only checks preempt_count and misses
>    RCU critical sections
> 
> This patch introduces z_erofs_in_atomic() to provide comprehensive
> atomic context detection:
> 
> 1. Check RCU preemption depth when CONFIG_PREEMPTION is enabled,
>     as RCU critical sections may not affect preempt_count but still
>     require atomic handling
> 
> 2. Always use async processing when CONFIG_PREEMPT_COUNT is disabled,
>     as preemption state cannot be reliably determined
> 
> 3. Fall back to standard preemptible() check for remaining cases
> 
> The function replaces the previous complex condition check and ensures
> that z_erofs always uses (kthread_)work in atomic contexts to minimize
> scheduling overhead and prevent sleeping in invalid contexts.
> 
> [1] Problem stacktrace
> BUG: sleeping function called from invalid context at
> kernel/locking/rtmutex_api.c:510
> in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 107,
> name: irq/54-ufshcd
> preempt_count: 0, expected: 0
> RCU nest depth: 2, expected: 0
> 
> [2] https://lore.kernel.org/r/58b661d0-0ebb-4b45-a10d-c5927fb791cd@paulmck-laptop
> 
> Signed-off-by: Junli Liu <liujunli@lixiang.com>

This version seems applicable to me:

Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Thanks,
Gao Xiang