Re: [PATCH] cxl/hdm: fix a warning in devm_remove_action()

[Sungwoo Kim]

Hello,

I'd like to gently follow up on this patch.
Are there any concerns or issues about the patch that I should address
before it can be merged?

Best,
Sungwoo.

On Mon, Mar 9, 2026 at 4:21 PM Dave Jiang <dave.jiang@xxxxxxxxx> wrote:
>
>
>
> On 3/8/26 5:08 PM, Sungwoo Kim wrote:
> > In the following race scenario, devm_remove_action() can be called
> > before devm_add_action(), triggering a warning because there is no
> > action to remove.
> >
> > To fix this, extend a critical section to embrace both
> > __cxl_dpa_reserve() and devm_add_action().
> >
> > task 1:
> > cxl_dpa_alloc()
> >   __cxl_dpa_reserve()    ...(1)
> >   devm_add_action()      ...(4)
> >
> > task 2:
> > cxl_dpa_free()
> >   guard()
> >   if (!cxled->dpa_res)   ...(2) pass, due to (1)
> >     return 0;
> >   devm_cxl_dpa_release()
> >     devm_remove_action()  ...(3) warning, no action is added yet
> >
> > Warning log:
> >
> > WARNING: ./include/linux/device/devres.h:160 at devm_remove_action include/linux/device/devres.h:160 [inline], CPU#0: syz.1.6464/25993
> > WARNING: ./include/linux/device/devres.h:160 at devm_cxl_dpa_release drivers/cxl/core/hdm.c:290 [inline], CPU#0: syz.1.6464/25993
> > WARNING: ./include/linux/device/devres.h:160 at cxl_dpa_free+0x2a4/0x320 drivers/cxl/core/hdm.c:572, CPU#0: syz.1.6464/25993
> > Modules linked in:
> > CPU: 0 UID: 0 PID: 25993 Comm: syz.1.6464 Not tainted 6.19.0-g15a37b05f387 #10 PREEMPT(lazy)
> > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
> > RIP: 0010:devm_remove_action include/linux/device/devres.h:160 [inline]
> > RIP: 0010:devm_cxl_dpa_release drivers/cxl/core/hdm.c:290 [inline]
> > RIP: 0010:cxl_dpa_free+0x2a4/0x320 drivers/cxl/core/hdm.c:572
> > Code: 01 31 ff 41 89 c4 89 c6 e8 a9 91 1d fe 45 85 e4 0f 85 55 ff ff ff e8 4b 96 1d fe 90 0f 0b 90 e9 47 ff ff ff e8 3d 96 1d fe 90 <0f> 0b 90 e9 63 ff ff ff bb f0 ff ff ff e9 68 ff ff ff 48 c7 c7 f4
> > RSP: 0018:ffff888129f77bb8 EFLAGS: 00010282
> > RAX: 00000000000000f3 RBX: ffff888102f64000 RCX: ffffffff837301d3
> > RDX: 0000000000080000 RSI: ffffc90001c5a000 RDI: 0000000000000005
> > RBP: 00000000fffffffe R08: 0000000000000001 R09: ffff888123f98000
> > R10: 00000000fffffffe R11: 0000000000000000 R12: 0000000000000001
> > R13: ffff888102f64040 R14: dffffc0000000000 R15: ffff888102f64000
> > FS:  00007fae0488d6c0(0000) GS:ffff88827086d000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 0000000000000000 CR3: 000000012f7d6003 CR4: 0000000000770ef0
> > PKRU: 80000000
> > Call Trace:
> >  <TASK>
> >  dpa_size_store+0xfb/0x230 drivers/cxl/core/port.c:269
> >  dev_attr_store+0x58/0x80 drivers/base/core.c:2437
> >  sysfs_kf_write+0xf2/0x150 fs/sysfs/file.c:142
> >  kernfs_fop_write_iter+0x3d6/0x5d0 fs/kernfs/file.c:352
> >  new_sync_write fs/read_write.c:595 [inline]
> >  vfs_write+0x68e/0x1050 fs/read_write.c:688
> >  ksys_write+0x12a/0x250 fs/read_write.c:740
> >  do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> >  do_syscall_64+0xfc/0x670 arch/x86/entry/syscall_64.c:94
> >  entry_SYSCALL_64_after_hwframe+0x77/0x7f
> > RIP: 0033:0x7fae0399c669
> > Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
> > RSP: 002b:00007fae0488d028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> > RAX: ffffffffffffffda RBX: 00007fae03c15fa0 RCX: 00007fae0399c669
> > RDX: 000000000000000a RSI: 00002000000001c0 RDI: 0000000000000009
> > RBP: 00007fae03a32c71 R08: 0000000000000000 R09: 0000000000000000
> > R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> > R13: 00007fae03c16038 R14: 00007fae03c15fa0 R15: 00007ffe53519fb8
> >  </TASK>
> > irq event stamp: 1021
> > hardirqs last  enabled at (1029): [<ffffffffb7d04c4b>] __up_console_sem+0x7b/0x90 kernel/printk/printk.c:347
> > hardirqs last disabled at (1038): [<ffffffffb7d04c30>] __up_console_sem+0x60/0x90 kernel/printk/printk.c:345
> > softirqs last  enabled at (1016): [<ffffffffb7b73b93>] __do_softirq kernel/softirq.c:656 [inline]
> > softirqs last  enabled at (1016): [<ffffffffb7b73b93>] invoke_softirq kernel/softirq.c:496 [inline]
> > softirqs last  enabled at (1016): [<ffffffffb7b73b93>] __irq_exit_rcu+0xb3/0xe0 kernel/softirq.c:723
> > softirqs last disabled at (1005): [<ffffffffb7b73b93>] __do_softirq kernel/softirq.c:656 [inline]
> > softirqs last disabled at (1005): [<ffffffffb7b73b93>] invoke_softirq kernel/softirq.c:496 [inline]
> > softirqs last disabled at (1005): [<ffffffffb7b73b93>] __irq_exit_rcu+0xb3/0xe0 kernel/softirq.c:723
> >
> > Fixes: cf880423b6a0 ("cxl/hdm: Add support for allocating DPA to an endpoint decoder")
> > Fixes: 9c57cde0dcbd ("cxl/hdm: Enumerate allocated DPA")
> > Signed-off-by: Sungwoo Kim <iam@xxxxxxxxxxxx>
>
> Reviewed-by: Dave Jiang <dave.jiang@xxxxxxxxx>
>
>
> > ---
> >  drivers/cxl/core/hdm.c | 34 +++++++++++++++++++++++++++++-----
> >  1 file changed, 29 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/cxl/core/hdm.c b/drivers/cxl/core/hdm.c
> > index e3f0c39e68124..86239b1790a31 100644
> > --- a/drivers/cxl/core/hdm.c
> > +++ b/drivers/cxl/core/hdm.c
> > @@ -509,13 +509,20 @@ int devm_cxl_dpa_reserve(struct cxl_endpoint_decoder *cxled,
> >       struct cxl_port *port = cxled_to_port(cxled);
> >       int rc;
> >
> > -     scoped_guard(rwsem_write, &cxl_rwsem.dpa)
> > -             rc = __cxl_dpa_reserve(cxled, base, len, skipped);
> > +     guard(rwsem_write)(&cxl_rwsem.dpa);
> > +     rc = __cxl_dpa_reserve(cxled, base, len, skipped);
> >
> >       if (rc)
> >               return rc;
> >
> > -     return devm_add_action_or_reset(&port->dev, cxl_dpa_release, cxled);
> > +     /* See comments in cxl_dpa_alloc()*/
> > +     rc = devm_add_action(&port->dev, cxl_dpa_release, cxled);
> > +     if (rc) {
> > +             __cxl_dpa_release(cxled);
> > +             return rc;
> > +     }
> > +
> > +     return 0;
> >  }
> >  EXPORT_SYMBOL_NS_GPL(devm_cxl_dpa_reserve, "CXL");
> >
> > @@ -613,7 +620,7 @@ static int __cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, u64 size)
> >       struct resource *p, *last;
> >       int part;
> >
> > -     guard(rwsem_write)(&cxl_rwsem.dpa);
> > +     lockdep_assert_held_write(&cxl_rwsem.dpa);
> >       if (cxled->cxld.region) {
> >               dev_dbg(dev, "decoder attached to %s\n",
> >                       dev_name(&cxled->cxld.region->dev));
> > @@ -679,11 +686,28 @@ int cxl_dpa_alloc(struct cxl_endpoint_decoder *cxled, u64 size)
> >       struct cxl_port *port = cxled_to_port(cxled);
> >       int rc;
> >
> > +     guard(rwsem_write)(&cxl_rwsem.dpa);
> >       rc = __cxl_dpa_alloc(cxled, size);
> >       if (rc)
> >               return rc;
> >
> > -     return devm_add_action_or_reset(&port->dev, cxl_dpa_release, cxled);
> > +     /*
> > +      * Add the devres action while still holding cxl_rwsem.dpa to prevent
> > +      * a race with cxl_dpa_free(). Without this, a concurrent cxl_dpa_free()
> > +      * can observe dpa_res set (by __cxl_dpa_reserve()) and attempt
> > +      * devm_remove_action() before devm_add_action() has been called,
> > +      * triggering a WARN_ON in devm_remove_action().
> > +      * Also, devm_add_action_or_reset() cannot be used here because
> > +      * cxl_dpa_release() tries to hold the dpa lock that is already held,
> > +      * causing a self deadlock.
> > +      */
> > +     rc = devm_add_action(&port->dev, cxl_dpa_release, cxled);
> > +     if (rc) {
> > +             __cxl_dpa_release(cxled);
> > +             return rc;
> > +     }
> > +
> > +     return 0;
> >  }
> >
> >  static void cxld_set_interleave(struct cxl_decoder *cxld, u32 *ctrl)
>
>