Re: [PATCH v5 25/29] md/raid10: split reshape bios before bitmap accounting

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] Double-splitting a bio using the same mempool-backed bioset causes a deadlock under concurrent load.
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--- Patch [25]: [PATCH v5 25/29] md/raid10: split reshape bios before bitmap accounting ---

commit 6f3a3e2c85f7d5d7d630d6c3cb813006807edbdc
Author: Yu Kuai [off-list ref]

md/raid10: split reshape bios before bitmap accounting

Use the shared mddev_bio_split_at_reshape_offset() helper so RAID10
submits only one-side bios to llbitmap during reshape.

[Severity: High]
Does this introduce a mempool deadlock if we need to split the bio again later?

If this initial split succeeds, the thread holds a bio from the conf->bio_split
mempool. Later, the thread proceeds to __make_request() and then to
raid10_write_request().

If the target disks have bad blocks, max_sectors is further reduced, causing a
second split attempt from the exact same mempool:

raid10_write_request() {
    ...
    if (max_sectors < r10_bio->sectors) {
        ...
        bio = bio_submit_split_bioset(bio, r10_bio->sectors,
                                      &conf->bio_split);
    ...
}

Since the conf->bio_split bioset is initialized with BIO_POOL_SIZE (2), if two
concurrent I/O requests cross chunk boundaries and hit bad blocks
simultaneously, could both threads allocate one bio each, and then both
permanently block in mempool_alloc() waiting for a second bio?

+	if (!bio) {
+		if (rw == WRITE)
+			md_write_end(mddev);
+		return true;
+	}
+
 	if (!__make_request(mddev, bio, sectors))
 		md_write_end(mddev);

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Sashiko AI review · https://sashiko.dev/#/patchset/20260802195038.164272-1-yukuai@kernel.org?part=25