Subject: Re: [bug] userspace hitting sporadic SIGBUS on xfs (Power9, ppc64le), v4.19 and later

From: Jan Stancek <[email-masked]>
Date: Tue, 3 Dec 2019 09:35:28 -0500 (EST)

----- Original Message -----
> On Tue, Dec 03, 2019 at 07:50:39AM -0500, Jan Stancek wrote:
> > My theory is that there's a race in iomap. There appear to be
> > interleaved calls to iomap_set_range_uptodate() for same page
> > with varying offset and length. Each call sees bitmap as _not_
> > entirely "uptodate" and hence doesn't call SetPageUptodate().
> > Even though each bit in bitmap ends up uptodate by the time
> > all calls finish.
> 
> Weird.  That should be prevented by the page lock that all callers
> of iomap_set_range_uptodate.  But in case I miss something, does
> the patch below trigger?  If not it is not jut a race, but might
> be some weird ordering problem with the bitops, especially if it
> only triggers on ppc, which is very weakly ordered.
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index d33c7bc5ee92..25e942c71590 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -148,6 +148,8 @@ iomap_set_range_uptodate(struct page *page, unsigned off,
> unsigned len)
>  	unsigned int i;
>  	bool uptodate = true;
>  
> +	WARN_ON_ONCE(!PageLocked(page));
> +
>  	if (iop) {
>  		for (i = 0; i < PAGE_SIZE / i_blocksize(inode); i++) {
>  			if (i >= first && i <= last)
> 

Hit it pretty quick this time:

# uptime
 09:27:42 up 22 min,  2 users,  load average: 0.09, 13.38, 26.18

# /mnt/testarea/ltp/testcases/bin/genbessel                                                                                                                                     
Bus error (core dumped)

# dmesg | grep -i -e warn -e call                                                                                                                                               
[    0.000000] dt-cpu-ftrs: not enabling: system-call-vectored (disabled or unsupported by kernel)
[    0.000000] random: get_random_u64 called from cache_random_seq_create+0x98/0x1e0 with crng_init=0
[    0.000000] rcu:     Offload RCU callbacks from CPUs: (none).
[    5.312075] megaraid_sas 0031:01:00.0: megasas_disable_intr_fusion is called outbound_intr_mask:0x40000009
[    5.357307] megaraid_sas 0031:01:00.0: megasas_disable_intr_fusion is called outbound_intr_mask:0x40000009
[    5.485126] megaraid_sas 0031:01:00.0: megasas_enable_intr_fusion is called outbound_intr_mask:0x40000000

So, extra WARN_ON_ONCE applied on top of v5.4-8836-g81b6b96475ac
did not trigger.

Is it possible for iomap code to submit multiple bio-s for same
locked page and then receive callbacks in parallel?