blk-mq: avoid stall during boot due to synchronize_rcu_expedited

On the kernel 6.19-rc, I am experiencing 15-second boot stall in a
virtual machine when probing a virtio-scsi disk:
[    1.011641] SCSI subsystem initialized
[    1.013972] virtio_scsi virtio6: 16/0/0 default/read/poll queues
[    1.015983] scsi host0: Virtio SCSI HBA
[    1.019578] ACPI: \_SB_.GSIA: Enabled at IRQ 16
[    1.020225] ahci 0000:00:1f.2: AHCI vers 0001.0000, 32 command slots, 1.5 Gbps, SATA mode
[    1.020228] ahci 0000:00:1f.2: 6/6 ports implemented (port mask 0x3f)
[    1.020230] ahci 0000:00:1f.2: flags: 64bit ncq only
[    1.024688] scsi host1: ahci
[    1.025432] scsi host2: ahci
[    1.025966] scsi host3: ahci
[    1.026511] scsi host4: ahci
[    1.028371] scsi host5: ahci
[    1.028918] scsi host6: ahci
[    1.029266] ata1: SATA max UDMA/133 abar m4096@0xfea23000 port 0xfea23100 irq 16 lpm-pol 1
[    1.029305] ata2: SATA max UDMA/133 abar m4096@0xfea23000 port 0xfea23180 irq 16 lpm-pol 1
[    1.029316] ata3: SATA max UDMA/133 abar m4096@0xfea23000 port 0xfea23200 irq 16 lpm-pol 1
[    1.029327] ata4: SATA max UDMA/133 abar m4096@0xfea23000 port 0xfea23280 irq 16 lpm-pol 1
[    1.029341] ata5: SATA max UDMA/133 abar m4096@0xfea23000 port 0xfea23300 irq 16 lpm-pol 1
[    1.029356] ata6: SATA max UDMA/133 abar m4096@0xfea23000 port 0xfea23380 irq 16 lpm-pol 1
[    1.118111] scsi 0:0:0:0: Direct-Access     QEMU     QEMU HARDDISK 2.5+ PQ: 0 ANSI: 5
[    1.348916] ata1: SATA link down (SStatus 0 SControl 300)
[    1.350713] ata2: SATA link down (SStatus 0 SControl 300)
[    1.351025] ata6: SATA link down (SStatus 0 SControl 300)
[    1.351160] ata5: SATA link down (SStatus 0 SControl 300)
[    1.351326] ata3: SATA link down (SStatus 0 SControl 300)
[    1.351536] ata4: SATA link down (SStatus 0 SControl 300)
[    1.449153] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input2
[   16.483477] sd 0:0:0:0: Power-on or device reset occurred
[   16.483691] sd 0:0:0:0: [sda] 2097152 512-byte logical blocks: (1.07 GB/1.00 GiB)
[   16.483762] sd 0:0:0:0: [sda] Write Protect is off
[   16.483877] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[   16.569225] sd 0:0:0:0: [sda] Attached SCSI disk

I bisected it and it is caused by the commit 89e1fb7 which
introduces calls to synchronize_rcu_expedited.

This commit replaces synchronize_rcu_expedited and kfree with a call to
kfree_rcu_mightsleep, avoiding the 15-second delay.

Signed-off-by: Mikulas Patocka <[email protected]>
Fixes: 89e1fb7 ("blk-mq: fix potential uaf for 'queue_hw_ctx'")
Reviewed-by: Uladzislau Rezki (Sony) <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>

Author: Mikulas Patocka

block/blk-mq.c

@@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
 		 * Make sure reading the old queue_hw_ctx from other
 		 * context concurrently won't trigger uaf.
 		 */
-		synchronize_rcu_expedited();
-		kfree(hctxs);
+		kfree_rcu_mightsleep(hctxs);
 		hctxs = new_hctxs;
 	}