[PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Mikulas Patocka]

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 89e1fb7ceffd 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 <mpatocka@redhat.com>
Fixes: 89e1fb7ceffd ("blk-mq: fix potential uaf for 'queue_hw_ctx'")

---
 block/blk-mq.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

Index: linux-2.6/block/blk-mq.c
===================================================================
--- linux-2.6.orig/block/blk-mq.c	2026-01-06 16:45:11.000000000 +0100
+++ linux-2.6/block/blk-mq.c	2026-01-06 16:48:00.000000000 +0100
@@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(str
 		 * 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;
 	}
 

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Uladzislau Rezki]

On Tue, Jan 06, 2026 at 04:56:07PM +0100, Mikulas Patocka wrote:
> 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 89e1fb7ceffd 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 <mpatocka@redhat.com>
> Fixes: 89e1fb7ceffd ("blk-mq: fix potential uaf for 'queue_hw_ctx'")
> 
> ---
>  block/blk-mq.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> Index: linux-2.6/block/blk-mq.c
> ===================================================================
> --- linux-2.6.orig/block/blk-mq.c	2026-01-06 16:45:11.000000000 +0100
> +++ linux-2.6/block/blk-mq.c	2026-01-06 16:48:00.000000000 +0100
> @@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(str
>  		 * 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);
>
I agree, doing freeing that way is not optimal. But kfree_rcu_mightsleep()
also might not work. It has a fallback, if we can not place an object into
"page" due to memory allocation failure, it inlines freeing:

<snip>
synchronize_rcu();
free().
<snip>

Please note, synchronize_rcu() can easily be converted into expedited
version. See rcu_gp_is_expedited().

Inlining is a corner case but it can happen. The best way is to add
rcu_head to the blk_mq_hw_ctx structure and use kfree_rcu(). It never
blocks.

--
Uladzislau Rezki

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Mikulas Patocka]

On Tue, 6 Jan 2026, Uladzislau Rezki wrote:

> On Tue, Jan 06, 2026 at 04:56:07PM +0100, Mikulas Patocka wrote:
> > 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 89e1fb7ceffd 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 <mpatocka@redhat.com>
> > Fixes: 89e1fb7ceffd ("blk-mq: fix potential uaf for 'queue_hw_ctx'")
> > 
> > ---
> >  block/blk-mq.c |    3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > Index: linux-2.6/block/blk-mq.c
> > ===================================================================
> > --- linux-2.6.orig/block/blk-mq.c	2026-01-06 16:45:11.000000000 +0100
> > +++ linux-2.6/block/blk-mq.c	2026-01-06 16:48:00.000000000 +0100
> > @@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(str
> >  		 * 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);
> >
> I agree, doing freeing that way is not optimal. But kfree_rcu_mightsleep()
> also might not work. It has a fallback, if we can not place an object into
> "page" due to memory allocation failure, it inlines freeing:
> 
> <snip>
> synchronize_rcu();
> free().
> <snip>
> 
> Please note, synchronize_rcu() can easily be converted into expedited
> version. See rcu_gp_is_expedited().
> 
> --
> Uladzislau Rezki

Would this patch be better? It does GFP_KERNEL allocation which dones't 
fail in practice.

> Inlining is a corner case but it can happen. The best way is to add
> rcu_head to the blk_mq_hw_ctx structure and use kfree_rcu(). It never
> blocks.

We are not protecting the blk_mq_hw_ctx structure with RCU, we are 
protecting the q->queue_hw_ctx array. So, rcu_head cannot be added to an 
array. We could cast the array to rcu_head (and make sure that the initial 
allocation is at least sizeof(struct rcu_head)), but that is hacky.

Mikulas

---
 block/blk-mq.c |   23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

Index: linux-2.6/block/blk-mq.c
===================================================================
--- linux-2.6.orig/block/blk-mq.c	2026-01-06 15:55:41.000000000 +0100
+++ linux-2.6/block/blk-mq.c	2026-01-06 16:22:40.000000000 +0100
@@ -4531,6 +4531,18 @@ static struct blk_mq_hw_ctx *blk_mq_allo
 	return NULL;
 }
 
+struct rcu_free_hctxs {
+	struct rcu_head head;
+	struct blk_mq_hw_ctx **hctxs;
+};
+
+static void rcu_free_hctxs(struct rcu_head *head)
+{
+	struct rcu_free_hctxs *r = container_of(head, struct rcu_free_hctxs, head);
+	kfree(r->hctxs);
+	kfree(r);
+}
+
 static void __blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
 				     struct request_queue *q)
 {
@@ -4539,6 +4551,7 @@ static void __blk_mq_realloc_hw_ctxs(str
 
 	if (q->nr_hw_queues < set->nr_hw_queues) {
 		struct blk_mq_hw_ctx **new_hctxs;
+		struct rcu_free_hctxs *r;
 
 		new_hctxs = kcalloc_node(set->nr_hw_queues,
 				       sizeof(*new_hctxs), GFP_KERNEL,
@@ -4553,8 +4566,14 @@ static void __blk_mq_realloc_hw_ctxs(str
 		 * Make sure reading the old queue_hw_ctx from other
 		 * context concurrently won't trigger uaf.
 		 */
-		synchronize_rcu_expedited();
-		kfree(hctxs);
+		r = kmalloc(sizeof(struct rcu_free_hctxs), GFP_KERNEL);
+		if (!r) {
+			synchronize_rcu_expedited();
+			kfree(hctxs);
+		} else {
+			r->hctxs = hctxs;
+			call_rcu(&r->head, rcu_free_hctxs);
+		}
 		hctxs = new_hctxs;
 	}
 
> 

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Uladzislau Rezki]

On Tue, Jan 06, 2026 at 05:59:16PM +0100, Mikulas Patocka wrote:
> 
> 
> On Tue, 6 Jan 2026, Uladzislau Rezki wrote:
> 
> > On Tue, Jan 06, 2026 at 04:56:07PM +0100, Mikulas Patocka wrote:
> > > 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 89e1fb7ceffd 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 <mpatocka@redhat.com>
> > > Fixes: 89e1fb7ceffd ("blk-mq: fix potential uaf for 'queue_hw_ctx'")
> > > 
> > > ---
> > >  block/blk-mq.c |    3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > 
> > > Index: linux-2.6/block/blk-mq.c
> > > ===================================================================
> > > --- linux-2.6.orig/block/blk-mq.c	2026-01-06 16:45:11.000000000 +0100
> > > +++ linux-2.6/block/blk-mq.c	2026-01-06 16:48:00.000000000 +0100
> > > @@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(str
> > >  		 * 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);
> > >
> > I agree, doing freeing that way is not optimal. But kfree_rcu_mightsleep()
> > also might not work. It has a fallback, if we can not place an object into
> > "page" due to memory allocation failure, it inlines freeing:
> > 
> > <snip>
> > synchronize_rcu();
> > free().
> > <snip>
> > 
> > Please note, synchronize_rcu() can easily be converted into expedited
> > version. See rcu_gp_is_expedited().
> > 
> > --
> > Uladzislau Rezki
> 
> Would this patch be better? It does GFP_KERNEL allocation which dones't 
> fail in practice.
> 
> > Inlining is a corner case but it can happen. The best way is to add
> > rcu_head to the blk_mq_hw_ctx structure and use kfree_rcu(). It never
> > blocks.
> 
> We are not protecting the blk_mq_hw_ctx structure with RCU, we are 
> protecting the q->queue_hw_ctx array. So, rcu_head cannot be added to an 
> array. We could cast the array to rcu_head (and make sure that the initial 
> allocation is at least sizeof(struct rcu_head)), but that is hacky.
> 
> Mikulas
> 
> ---
>  block/blk-mq.c |   23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
> 
> Index: linux-2.6/block/blk-mq.c
> ===================================================================
> --- linux-2.6.orig/block/blk-mq.c	2026-01-06 15:55:41.000000000 +0100
> +++ linux-2.6/block/blk-mq.c	2026-01-06 16:22:40.000000000 +0100
> @@ -4531,6 +4531,18 @@ static struct blk_mq_hw_ctx *blk_mq_allo
>  	return NULL;
>  }
>  
> +struct rcu_free_hctxs {
> +	struct rcu_head head;
> +	struct blk_mq_hw_ctx **hctxs;
> +};
> +
> +static void rcu_free_hctxs(struct rcu_head *head)
> +{
> +	struct rcu_free_hctxs *r = container_of(head, struct rcu_free_hctxs, head);
> +	kfree(r->hctxs);
> +	kfree(r);
> +}
> +
>  static void __blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
>  				     struct request_queue *q)
>  {
> @@ -4539,6 +4551,7 @@ static void __blk_mq_realloc_hw_ctxs(str
>  
>  	if (q->nr_hw_queues < set->nr_hw_queues) {
>  		struct blk_mq_hw_ctx **new_hctxs;
> +		struct rcu_free_hctxs *r;
>  
>  		new_hctxs = kcalloc_node(set->nr_hw_queues,
>  				       sizeof(*new_hctxs), GFP_KERNEL,
> @@ -4553,8 +4566,14 @@ static void __blk_mq_realloc_hw_ctxs(str
>  		 * Make sure reading the old queue_hw_ctx from other
>  		 * context concurrently won't trigger uaf.
>  		 */
> -		synchronize_rcu_expedited();
> -		kfree(hctxs);
> +		r = kmalloc(sizeof(struct rcu_free_hctxs), GFP_KERNEL);
> +		if (!r) {
> +			synchronize_rcu_expedited();
> +			kfree(hctxs);
> +		} else {
> +			r->hctxs = hctxs;
> +			call_rcu(&r->head, rcu_free_hctxs);
> +		}
>  		hctxs = new_hctxs;
>  	}
>  
> > 
> 
I see. That will work but this looks like a temporary fix. It would be
great to understand why synchronize_rcu_expedited() is blocked for so long.
16 seconds is a way too long.

Is that easy to reproduce?

--
Uladzislau Rezki

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Mikulas Patocka]

On Wed, 7 Jan 2026, Uladzislau Rezki wrote:

> On Tue, Jan 06, 2026 at 05:59:16PM +0100, Mikulas Patocka wrote:
> > 
> > 
> > On Tue, 6 Jan 2026, Uladzislau Rezki wrote:
> > 
> > > On Tue, Jan 06, 2026 at 04:56:07PM +0100, Mikulas Patocka wrote:
> > > > 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 89e1fb7ceffd 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 <mpatocka@redhat.com>
> > > > Fixes: 89e1fb7ceffd ("blk-mq: fix potential uaf for 'queue_hw_ctx'")
> > > > 
> > > > ---
> > > >  block/blk-mq.c |    3 +--
> > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > > 
> > > > Index: linux-2.6/block/blk-mq.c
> > > > ===================================================================
> > > > --- linux-2.6.orig/block/blk-mq.c	2026-01-06 16:45:11.000000000 +0100
> > > > +++ linux-2.6/block/blk-mq.c	2026-01-06 16:48:00.000000000 +0100
> > > > @@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(str
> > > >  		 * 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);
> > > >
> > > I agree, doing freeing that way is not optimal. But kfree_rcu_mightsleep()
> > > also might not work. It has a fallback, if we can not place an object into
> > > "page" due to memory allocation failure, it inlines freeing:
> > > 
> > > <snip>
> > > synchronize_rcu();
> > > free().
> > > <snip>
> > > 
> > > Please note, synchronize_rcu() can easily be converted into expedited
> > > version. See rcu_gp_is_expedited().
> > > 
> > > --
> > > Uladzislau Rezki
> > 
> > Would this patch be better? It does GFP_KERNEL allocation which dones't 
> > fail in practice.
> > 
> > > Inlining is a corner case but it can happen. The best way is to add
> > > rcu_head to the blk_mq_hw_ctx structure and use kfree_rcu(). It never
> > > blocks.
> > 
> > We are not protecting the blk_mq_hw_ctx structure with RCU, we are 
> > protecting the q->queue_hw_ctx array. So, rcu_head cannot be added to an 
> > array. We could cast the array to rcu_head (and make sure that the initial 
> > allocation is at least sizeof(struct rcu_head)), but that is hacky.
> > 
> > Mikulas
> > 
> > ---
> >  block/blk-mq.c |   23 +++++++++++++++++++++--
> >  1 file changed, 21 insertions(+), 2 deletions(-)
> > 
> > Index: linux-2.6/block/blk-mq.c
> > ===================================================================
> > --- linux-2.6.orig/block/blk-mq.c	2026-01-06 15:55:41.000000000 +0100
> > +++ linux-2.6/block/blk-mq.c	2026-01-06 16:22:40.000000000 +0100
> > @@ -4531,6 +4531,18 @@ static struct blk_mq_hw_ctx *blk_mq_allo
> >  	return NULL;
> >  }
> >  
> > +struct rcu_free_hctxs {
> > +	struct rcu_head head;
> > +	struct blk_mq_hw_ctx **hctxs;
> > +};
> > +
> > +static void rcu_free_hctxs(struct rcu_head *head)
> > +{
> > +	struct rcu_free_hctxs *r = container_of(head, struct rcu_free_hctxs, head);
> > +	kfree(r->hctxs);
> > +	kfree(r);
> > +}
> > +
> >  static void __blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
> >  				     struct request_queue *q)
> >  {
> > @@ -4539,6 +4551,7 @@ static void __blk_mq_realloc_hw_ctxs(str
> >  
> >  	if (q->nr_hw_queues < set->nr_hw_queues) {
> >  		struct blk_mq_hw_ctx **new_hctxs;
> > +		struct rcu_free_hctxs *r;
> >  
> >  		new_hctxs = kcalloc_node(set->nr_hw_queues,
> >  				       sizeof(*new_hctxs), GFP_KERNEL,
> > @@ -4553,8 +4566,14 @@ static void __blk_mq_realloc_hw_ctxs(str
> >  		 * Make sure reading the old queue_hw_ctx from other
> >  		 * context concurrently won't trigger uaf.
> >  		 */
> > -		synchronize_rcu_expedited();
> > -		kfree(hctxs);
> > +		r = kmalloc(sizeof(struct rcu_free_hctxs), GFP_KERNEL);
> > +		if (!r) {
> > +			synchronize_rcu_expedited();
> > +			kfree(hctxs);
> > +		} else {
> > +			r->hctxs = hctxs;
> > +			call_rcu(&r->head, rcu_free_hctxs);
> > +		}
> >  		hctxs = new_hctxs;
> >  	}
> >  
> > > 
> > 
> I see. That will work but this looks like a temporary fix. It would be
> great to understand why synchronize_rcu_expedited() is blocked for so long.
> 16 seconds is a way too long.

synchronize_rcu_expedited is called 257 times from the block layer. One 
call is approximately 50ms.

This is one of the stacktraces:
[    3.087639] CPU: 12 UID: 0 PID: 260 Comm: (udev-worker) Not tainted 6.19.0-rc4 #26 PREEMPT_{RT,(full)}
[    3.087642] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
[    3.087643] Call Trace:
[    3.087644]  <TASK>
[    3.087644]  dump_stack_lvl+0x47/0x60
[    3.087649]  __blk_mq_realloc_hw_ctxs+0x14e/0x170
[    3.087652]  blk_mq_init_allocated_queue+0xf5/0x3b0
[    3.087654]  blk_mq_alloc_queue+0x62/0xa0
[    3.087655]  scsi_alloc_sdev+0x1e2/0x300 [scsi_mod]
[    3.087658]  scsi_probe_and_add_lun+0x1de/0x280 [scsi_mod]
[    3.087660]  ? preempt_count_add+0x48/0xc0
[    3.087663]  ? rt_spin_unlock+0x47/0xa0
[    3.087665]  ? rt_spin_unlock+0x2d/0xa0
[    3.087666]  __scsi_scan_target+0xd3/0x1d0 [scsi_mod]
[    3.087667]  scsi_scan_channel+0x4f/0x80 [scsi_mod]
[    3.087668]  scsi_scan_host_selected+0xc0/0xf0 [scsi_mod]
[    3.087670]  scsi_scan_host+0x181/0x1a0 [scsi_mod]
[    3.087671]  virtscsi_probe+0x333/0x341 [virtio_scsi]
[    3.087674]  virtio_dev_probe+0x1e5/0x300
[    3.087676]  really_probe+0xb9/0x240
[    3.087678]  __driver_probe_device+0x6e/0x100
[    3.087679]  driver_probe_device+0x1a/0x70
[    3.087680]  ? __device_attach_driver+0xa0/0xa0
[    3.087681]  __driver_attach+0x84/0x140
[    3.087682]  bus_for_each_dev+0x5e/0xa0
[    3.087683]  bus_add_driver+0xd8/0x1c0
[    3.087684]  ? libata_transport_exit+0x930/0x930 [libata]
[    3.087686]  driver_register+0x6c/0xd0
[    3.087687]  virtio_scsi_init+0xa1/0x1000 [virtio_scsi]
[    3.087688]  do_one_initcall+0x35/0x160
[    3.087690]  ? do_init_module+0x1f/0x250
[    3.087691]  ? __kmalloc_cache_noprof+0x183/0x330
[    3.087694]  do_init_module+0x5d/0x250
[    3.087695]  ? init_module_from_file+0x9e/0xc0
[    3.087695]  init_module_from_file+0x9e/0xc0
[    3.087696]  idempotent_init_module+0xee/0x2d0
[    3.087697]  __x64_sys_finit_module+0x56/0xa0
[    3.087698]  do_syscall_64+0x31e/0x370
[    3.087701]  entry_SYSCALL_64_after_hwframe+0x4b/0x53


> Is that easy to reproduce?

I uploaded my config here:
http://www.jikos.cz/~mikulas/testcases/config/.config-6.19-rc4

Mikulas

> --
> Uladzislau Rezki
> 

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Uladzislau Rezki]

On Wed, Jan 07, 2026 at 01:05:14PM +0100, Mikulas Patocka wrote:
> 
> 
> On Wed, 7 Jan 2026, Uladzislau Rezki wrote:
> 
> > On Tue, Jan 06, 2026 at 05:59:16PM +0100, Mikulas Patocka wrote:
> > > 
> > > 
> > > On Tue, 6 Jan 2026, Uladzislau Rezki wrote:
> > > 
> > > > On Tue, Jan 06, 2026 at 04:56:07PM +0100, Mikulas Patocka wrote:
> > > > > 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 89e1fb7ceffd 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 <mpatocka@redhat.com>
> > > > > Fixes: 89e1fb7ceffd ("blk-mq: fix potential uaf for 'queue_hw_ctx'")
> > > > > 
> > > > > ---
> > > > >  block/blk-mq.c |    3 +--
> > > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > > > 
> > > > > Index: linux-2.6/block/blk-mq.c
> > > > > ===================================================================
> > > > > --- linux-2.6.orig/block/blk-mq.c	2026-01-06 16:45:11.000000000 +0100
> > > > > +++ linux-2.6/block/blk-mq.c	2026-01-06 16:48:00.000000000 +0100
> > > > > @@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(str
> > > > >  		 * 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);
> > > > >
> > > > I agree, doing freeing that way is not optimal. But kfree_rcu_mightsleep()
> > > > also might not work. It has a fallback, if we can not place an object into
> > > > "page" due to memory allocation failure, it inlines freeing:
> > > > 
> > > > <snip>
> > > > synchronize_rcu();
> > > > free().
> > > > <snip>
> > > > 
> > > > Please note, synchronize_rcu() can easily be converted into expedited
> > > > version. See rcu_gp_is_expedited().
> > > > 
> > > > --
> > > > Uladzislau Rezki
> > > 
> > > Would this patch be better? It does GFP_KERNEL allocation which dones't 
> > > fail in practice.
> > > 
> > > > Inlining is a corner case but it can happen. The best way is to add
> > > > rcu_head to the blk_mq_hw_ctx structure and use kfree_rcu(). It never
> > > > blocks.
> > > 
> > > We are not protecting the blk_mq_hw_ctx structure with RCU, we are 
> > > protecting the q->queue_hw_ctx array. So, rcu_head cannot be added to an 
> > > array. We could cast the array to rcu_head (and make sure that the initial 
> > > allocation is at least sizeof(struct rcu_head)), but that is hacky.
> > > 
> > > Mikulas
> > > 
> > > ---
> > >  block/blk-mq.c |   23 +++++++++++++++++++++--
> > >  1 file changed, 21 insertions(+), 2 deletions(-)
> > > 
> > > Index: linux-2.6/block/blk-mq.c
> > > ===================================================================
> > > --- linux-2.6.orig/block/blk-mq.c	2026-01-06 15:55:41.000000000 +0100
> > > +++ linux-2.6/block/blk-mq.c	2026-01-06 16:22:40.000000000 +0100
> > > @@ -4531,6 +4531,18 @@ static struct blk_mq_hw_ctx *blk_mq_allo
> > >  	return NULL;
> > >  }
> > >  
> > > +struct rcu_free_hctxs {
> > > +	struct rcu_head head;
> > > +	struct blk_mq_hw_ctx **hctxs;
> > > +};
> > > +
> > > +static void rcu_free_hctxs(struct rcu_head *head)
> > > +{
> > > +	struct rcu_free_hctxs *r = container_of(head, struct rcu_free_hctxs, head);
> > > +	kfree(r->hctxs);
> > > +	kfree(r);
> > > +}
> > > +
> > >  static void __blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
> > >  				     struct request_queue *q)
> > >  {
> > > @@ -4539,6 +4551,7 @@ static void __blk_mq_realloc_hw_ctxs(str
> > >  
> > >  	if (q->nr_hw_queues < set->nr_hw_queues) {
> > >  		struct blk_mq_hw_ctx **new_hctxs;
> > > +		struct rcu_free_hctxs *r;
> > >  
> > >  		new_hctxs = kcalloc_node(set->nr_hw_queues,
> > >  				       sizeof(*new_hctxs), GFP_KERNEL,
> > > @@ -4553,8 +4566,14 @@ static void __blk_mq_realloc_hw_ctxs(str
> > >  		 * Make sure reading the old queue_hw_ctx from other
> > >  		 * context concurrently won't trigger uaf.
> > >  		 */
> > > -		synchronize_rcu_expedited();
> > > -		kfree(hctxs);
> > > +		r = kmalloc(sizeof(struct rcu_free_hctxs), GFP_KERNEL);
> > > +		if (!r) {
> > > +			synchronize_rcu_expedited();
> > > +			kfree(hctxs);
> > > +		} else {
> > > +			r->hctxs = hctxs;
> > > +			call_rcu(&r->head, rcu_free_hctxs);
> > > +		}
> > >  		hctxs = new_hctxs;
> > >  	}
> > >  
> > > > 
> > > 
> > I see. That will work but this looks like a temporary fix. It would be
> > great to understand why synchronize_rcu_expedited() is blocked for so long.
> > 16 seconds is a way too long.
> 
> synchronize_rcu_expedited is called 257 times from the block layer. One 
> call is approximately 50ms.
> 
OK. I thought the _one_ call of synchronize_rcu_expedited() was stuck for
~15 seconds. Whereas you just have many of them.

Therefore you can easily just go back to your original patch and use
kfree_rcu_mightsleep(hctxs)!

--
Uladzislau Rezki

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Jens Axboe]

On 1/6/26 9:59 AM, Mikulas Patocka wrote:
> @@ -4553,8 +4566,14 @@ static void __blk_mq_realloc_hw_ctxs(str
>  		 * Make sure reading the old queue_hw_ctx from other
>  		 * context concurrently won't trigger uaf.
>  		 */
> -		synchronize_rcu_expedited();
> -		kfree(hctxs);
> +		r = kmalloc(sizeof(struct rcu_free_hctxs), GFP_KERNEL);
> +		if (!r) {
> +			synchronize_rcu_expedited();
> +			kfree(hctxs);
> +		} else {
> +			r->hctxs = hctxs;
> +			call_rcu(&r->head, rcu_free_hctxs);
> +		}
>  		hctxs = new_hctxs;
>  	}

This is worse in every conceivable way, imho. The proper way to do this
would be to embed the rcu_head in whatever is allocated for the hctxs at
alloc time, if youre doing an alloc here you may as well just use
kfree_rcu_mightsleep() in the first place. There's nothing gained from
open coding that.

Since kfree_rcu_mightsleep() will only run into trouble under strained
conditions anyway, I think the original patch is fine for this.

-- 
Jens Axboe

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Uladzislau Rezki]

On Tue, Jan 06, 2026 at 04:56:07PM +0100, Mikulas Patocka wrote:
> 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 89e1fb7ceffd 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 <mpatocka@redhat.com>
> Fixes: 89e1fb7ceffd ("blk-mq: fix potential uaf for 'queue_hw_ctx'")
> 
> ---
>  block/blk-mq.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> Index: linux-2.6/block/blk-mq.c
> ===================================================================
> --- linux-2.6.orig/block/blk-mq.c	2026-01-06 16:45:11.000000000 +0100
> +++ linux-2.6/block/blk-mq.c	2026-01-06 16:48:00.000000000 +0100
> @@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(str
>  		 * 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;
>  	}
>  
> 
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

--
Uladzislau Rezki

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Bart Van Assche]

On 1/6/26 8:56 AM, Mikulas Patocka wrote:
> --- linux-2.6.orig/block/blk-mq.c	2026-01-06 16:45:11.000000000 +0100
> +++ linux-2.6/block/blk-mq.c	2026-01-06 16:48:00.000000000 +0100
> @@ -4553,8 +4553,7 @@ static void __blk_mq_realloc_hw_ctxs(str
>   		 * 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;
>   	}
Reviewed-by: Bart Van Assche <bvanassche@acm.org>

Re: [PATCH] blk-mq: avoid stall during boot due to synchronize_rcu_expedited

[Jens Axboe]

On Tue, 06 Jan 2026 16:56:07 +0100, Mikulas Patocka wrote:
> 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
> 
> [...]

Applied, thanks!

[1/1] blk-mq: avoid stall during boot due to synchronize_rcu_expedited
      commit: 9670db22e7ab4aefe2b2619589a47fef9d3e0c7e

Best regards,
-- 
Jens Axboe