linux-kernel - Re: [PATCH v2] erofs: fix atomic context detection when !CONFIG_DEBUG_LOCK

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Message-ID: <960a1491-47bc-464b-a955-37d62470c0ef@linux.alibaba.com>
Date: Mon, 4 Aug 2025 17:49:07 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Junli Liu <liujunli@...iang.com>, linux-erofs@...ts.ozlabs.org,
 linux-kernel@...r.kernel.org
Cc: xiang@...nel.org, chao@...nel.org, yangsonghua@...iang.com
Subject: Re: [PATCH v2] erofs: fix atomic context detection when
 !CONFIG_DEBUG_LOCK_ALLOC



... the patch format is still broken, is your mail server
or your mail client broken?

On 2025/8/4 17:38, 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
> 
> Link: https://lore.kernel.org/r/58b661d0-0ebb-4b45-a10d-c5927fb791cd@paulmck-laptop

I prefer to replace the "Link:" to
[2] https://lore.kernel.org/r/58b661d0-0ebb-4b45-a10d-c5927fb791cd@paulmck-laptop

Since "Link:" tag mainly refers to the your current patch
in the lore...

Thanks,
Gao Xiang

> Signed-off-by: Junli Liu
> ---
> fs/erofs/zdata.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> index 792f20888..2d7329700 100644
> --- a/fs/erofs/zdata.c
> +++ b/fs/erofs/zdata.c
> @@ -1432,6 +1432,16 @@ static void z_erofs_decompressqueue_kthread_work(struct kthread_work *work)
> }
> #endif
> 
> +/* Use (kthread_)work in atomic contexts to minimize scheduling overhead */
> +static inline bool z_erofs_in_atomic(void)
> +{
> + if (IS_ENABLED(CONFIG_PREEMPTION) && rcu_preempt_depth())
> + return true;
> + if (!IS_ENABLED(CONFIG_PREEMPT_COUNT))
> + return true;
> + return !preemptible();
> +}
> +
> static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
> int bios)
> {
> @@ -1446,8 +1456,7 @@ static void z_erofs_decompress_kickoff(struct z_erofs_decompressqueue *io,
> 
> if (atomic_add_return(bios, &io->pending_bios))
> return;
> - /* Use (kthread_)work and sync decompression for atomic contexts only */
> - if (!in_task() || irqs_disabled() || rcu_read_lock_any_held()) {
> + if (z_erofs_in_atomic()) {
> #ifdef CONFIG_EROFS_FS_PCPU_KTHREAD
> struct kthread_worker *worker;
> 
> -- 
> 2.25.1
> 
> 
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