Re: [PATCH -next v5 29/32] ext4: add tracepoints for DISKSIZE_GROW_PENDING set, clear, and wait

On 8/14/2026 6:18 PM, [email protected] wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] Sequence counter deadlock in tracepoint when called from softirq context on 32-bit SMP.
> --
> 
> --- Patch [29]: [PATCH -next v5 29/32] ext4: add tracepoints for DISKSIZE_GROW_PENDING set, clear, and wait ---
> commit f14aeb9661220b55cf0eeb4ff49da9fa6a2a18ca
> Author: Zhang Yi <[email protected]>
> 
> ext4: add tracepoints for DISKSIZE_GROW_PENDING set, clear, and wait
>      
> This commit adds trace events ext4_iomap_mark_disksize_pending(),
> ext4_iomap_clear_disksize_pending(), and ext4_iomap_wait_disksize_pending().
> These tracepoints track disksize-grow-pending state changes and waiting.
> 
>> diff --git a/include/trace/events/ext4.h b/include/trace/events/ext4.h
>> index 69596a216dcb1..4539ef8e5f86c 100644
>> --- a/include/trace/events/ext4.h
>> +++ b/include/trace/events/ext4.h
> [ ... ]
>> +DECLARE_EVENT_CLASS(ext4_iomap_disksize_pending,
>> +	TP_PROTO(struct inode *inode),
>> +	TP_ARGS(inode),
>> +	TP_STRUCT__entry(
>> +		__field(dev_t, dev)
>> +		__field(u64, ino)
>> +		__field(loff_t, i_size)
>> +		__field(loff_t, i_disksize)
>> +	),
>> +	TP_fast_assign(
>> +		__entry->dev = inode->i_sb->s_dev;
>> +		__entry->ino = inode->i_ino;
>> +		__entry->i_size = i_size_read(inode);
> 
> [Severity: High]
> Could this cause a sequence counter deadlock on 32-bit SMP architectures?
> 
> If a process executing i_size_write() is preempted by a block I/O completion
> hardware interrupt, and pending softirqs are executed on return from the
> interrupt, the softirq could call ext4_iomap_end_bio() in fs/ext4/page-io.c:
> 
> fs/ext4/page-io.c:ext4_iomap_end_bio() {
>          ...
>          if (io_mode == EXT4_IOMAP_IOEND_DISKSIZE_GROW_IO)
>                  ext4_iomap_clear_disksize_pending(ioend->io_inode);
>          ...
> }
> 
> This would trigger trace_ext4_iomap_clear_disksize_pending() which calls
> i_size_read() here in TP_fast_assign.
> 
> Since i_size_write() disables preemption but not softirqs, the seqcount
> would be odd. This would cause i_size_read() to spin indefinitely waiting
> for the interrupted i_size_write() to complete on the same CPU.
> 

Yeah, I'll remove the i_size check in the next version. It doesn't
look like we really need it.

Thanks,
Yi.

>> +		__entry->i_disksize = READ_ONCE(EXT4_I(inode)->i_disksize);
>> +	),
>> +	TP_printk("dev %d:%d ino %llu i_size %lld i_disksize %lld",
>> +		  MAJOR(__entry->dev), MINOR(__entry->dev),
>> +		  __entry->ino, __entry->i_size, __entry->i_disksize)
>> +);
>