On 2023/10/27 17:30, Qu Wenruo wrote:
On 2023/10/27 17:25, Holger Hoffstätte wrote:
On 2023-10-26 23:01, Qu Wenruo wrote:
On 2023/10/27 00:30, Holger Hoffstätte wrote:
On 2023-10-26 15:31, Sam James wrote:
'btrfs: scrub: fix grouping of read IO' seems to intorduce a
-Wmaybe-uninitialized warning (which becomes fatal with the kernel's
passed -Werror=...) with 6.5.9:
```
/var/tmp/portage/sys-kernel/gentoo-kernel-6.5.9/work/linux-6.5/fs/btrfs/scrub.c: In function ‘scrub_simple_mirror.isra’:
/var/tmp/portage/sys-kernel/gentoo-kernel-6.5.9/work/linux-6.5/fs/btrfs/scrub.c:2075:29: error: ‘found_logical’ may be used uninitialized [-Werror=maybe-uninitialized[https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmaybe-uninitialized]]
2075 | cur_logical = found_logical +
BTRFS_STRIPE_LEN;
/var/tmp/portage/sys-kernel/gentoo-kernel-6.5.9/work/linux-6.5/fs/btrfs/scrub.c:2040:21: note: ‘found_logical’ was declared here
2040 | u64 found_logical;
| ^~~~~~~~~~~~~
```
Good find! found_logical is passed by reference to
queue_scrub_stripe(..) (inlined)
where it is used without ever being set:
...
/* Either >0 as no more extents or <0 for error. */
if (ret)
return ret;
if (found_logical_ret)
*found_logical_ret = stripe->logical;
sctx->cur_stripe++;
...
Something is missing here, and somehow I don't think it's just the
top-level
initialisation of found_logical.
This looks like a false alert for me.
@found_logical is intentionally uninitialized to catch any
uninitialized usage by compiler.
It would be set by queue_scrub_stripe() when there is any stripe found.
Can you show me where the reference is set before the quoted if block?
Sure.
Firstly inside queue_scrub_stripe():
```
ret = scrub_find_fill_first_stripe(bg, &sctx->extent_path,
&sctx->csum_path, dev,
physical,
mirror_num, logical, length,
stripe);
/* Either >0 as no more extents or <0 for error. */
if (ret)
return ret;
if (found_logical_ret)
*found_logical_ret = stripe->logical;
```
In this case, we would only set @found_logical_ret to the found stripe's
logical.
Either we got ret > 0, meaning no more extent in the chunk, or we got
some critical error.
Sorry, missing the latter part of the sentence:
In either case, the caller would handle it.
Then back to scrub_simple_mirrors():
```
ret = queue_scrub_stripe(sctx, bg, device, mirror_num,
cur_logical, logical_end -
cur_logical,
cur_physical, &found_logical);
if (ret > 0) {
/* No more extent, just update the accounting */
sctx->stat.last_physical = physical +
logical_length;
ret = 0;
break;
}
if (ret < 0)
break;
cur_logical = found_logical + BTRFS_STRIPE_LEN;
```
We got to the if block I mentioned.
Either we got ret != 0, then we would break out the whole loop of
scrub_simple_mirror().
Or we got ret == 0, which would initialize @found_logical.
Thanks,
Qu
The warning happens because according to the compiler this is exactly
not what's
happening, and I did not see any initializing write either.
thanks,
Holger
