(adding Luca for systemd POV) On Tue, Jul 14, 2026 at 07:03:56PM +0000, David Matlack wrote: > Remove the single-opener restriction for /dev/liveupdate by removing the > atomic in_use tracking and the exclusive open check in luo_open() that > returned -EBUSY. Protect luo_session_deserialize() with a mutex guard so > that concurrent open attempts by multiple processes safely executes > deserialization only once. Update liveupdate selftest to verify that > multiple concurrent openers succeed. > > LUO does not inherently require a single opener. There is some > documentation about it simplifying state management, but the only thing > it actually protects is the session deserialization during first open, > which can be easily handled with a mutex. > > Relaxing the single-opener requirement avoids the kernel forcing a > design pattern on userspace that it itself does not require, e.g. > allowing multiple userspace processes to create and manage sessions. > > Signed-off-by: David Matlack <[email protected]> > --- > Cc: Jason Gunthorpe <[email protected]> > Cc: Samiullah Khawaja <[email protected]> > > kernel/liveupdate/luo_core.c | 46 +++---------------- > kernel/liveupdate/luo_session.c | 8 +++- > .../testing/selftests/liveupdate/liveupdate.c | 14 +++--- > 3 files changed, 18 insertions(+), 50 deletions(-) > > diff --git a/kernel/liveupdate/luo_core.c b/kernel/liveupdate/luo_core.c > index 1b2bda22902d..931bb79da276 100644 > --- a/kernel/liveupdate/luo_core.c > +++ b/kernel/liveupdate/luo_core.c > @@ -254,19 +254,8 @@ bool liveupdate_enabled(void) > * which allows a userspace agent to manage the LUO state machine and its > * associated resources, such as preservable file descriptors. > * > - * To ensure that the state machine is controlled by a single entity, access > - * to this device is exclusive: only one process is permitted to have > - * ``/dev/liveupdate`` open at any given time. Subsequent open attempts will > - * fail with -EBUSY until the first process closes its file descriptor. > - * This singleton model simplifies state management by preventing conflicting > - * commands from multiple userspace agents. > */ > > -struct luo_device_state { > - struct miscdevice miscdev; > - atomic_t in_use; > -}; > - > static int luo_ioctl_create_session(struct luo_ucmd *ucmd) > { > struct liveupdate_ioctl_create_session *argp = ucmd->cmd; > @@ -329,28 +318,9 @@ static int luo_ioctl_retrieve_session(struct luo_ucmd > *ucmd) > > static int luo_open(struct inode *inodep, struct file *filep) > { > - struct luo_device_state *ldev = container_of(filep->private_data, > - struct luo_device_state, > - miscdev); > - > - if (atomic_cmpxchg(&ldev->in_use, 0, 1)) > - return -EBUSY; > - > /* Always return -EIO to user if deserialization fail */ > - if (luo_session_deserialize()) { > - atomic_set(&ldev->in_use, 0); > + if (luo_session_deserialize()) > return -EIO; > - } > - > - return 0; > -} > - > -static int luo_release(struct inode *inodep, struct file *filep) > -{ > - struct luo_device_state *ldev = container_of(filep->private_data, > - struct luo_device_state, > - miscdev); > - atomic_set(&ldev->in_use, 0); > > return 0; > } > @@ -419,17 +389,13 @@ static long luo_ioctl(struct file *filep, unsigned int > cmd, unsigned long arg) > static const struct file_operations luo_fops = { > .owner = THIS_MODULE, > .open = luo_open, > - .release = luo_release, > .unlocked_ioctl = luo_ioctl, > }; > > -static struct luo_device_state luo_dev = { > - .miscdev = { > - .minor = MISC_DYNAMIC_MINOR, > - .name = "liveupdate", > - .fops = &luo_fops, > - }, > - .in_use = ATOMIC_INIT(0), > +static struct miscdevice luo_dev = { > + .minor = MISC_DYNAMIC_MINOR, > + .name = "liveupdate", > + .fops = &luo_fops, > }; > > static int __init liveupdate_ioctl_init(void) > @@ -437,6 +403,6 @@ static int __init liveupdate_ioctl_init(void) > if (!liveupdate_enabled()) > return 0; > > - return misc_register(&luo_dev.miscdev); > + return misc_register(&luo_dev); > } > late_initcall(liveupdate_ioctl_init); > diff --git a/kernel/liveupdate/luo_session.c b/kernel/liveupdate/luo_session.c > index b79b2a488974..ca4d0639d39a 100644 > --- a/kernel/liveupdate/luo_session.c > +++ b/kernel/liveupdate/luo_session.c > @@ -584,13 +584,17 @@ static int luo_session_deserialize_one(struct > luo_session_header *sh, > > int luo_session_deserialize(void) > { > - struct luo_session_header *sh = &luo_session_global.incoming; > + static DEFINE_MUTEX(luo_session_deserialize_lock); > static bool is_deserialized; > + static int saved_err; > + > + struct luo_session_header *sh = &luo_session_global.incoming; > struct luo_session_ser *ser; > struct kho_block_set_it it; > - static int saved_err; > int err; > > + guard(mutex)(&luo_session_deserialize_lock); > + > /* If has been deserialized, always return the same error code */ > if (is_deserialized) > return saved_err; > diff --git a/tools/testing/selftests/liveupdate/liveupdate.c > b/tools/testing/selftests/liveupdate/liveupdate.c > index 502fb3567e38..a8e1a8911849 100644 > --- a/tools/testing/selftests/liveupdate/liveupdate.c > +++ b/tools/testing/selftests/liveupdate/liveupdate.c > @@ -11,7 +11,7 @@ > * /dev/liveupdate character device and its session management capabilities. > * > * Tests include: > - * - Device access: basic open/close, and enforcement of exclusive access. > + * - Device access: basic open/close, and support for multiple openers. > * - Session management: creation of unique sessions, and duplicate name > detection. > * - Resource preservation: successfully preserving individual and multiple > memfds, > * verifying contents remain accessible. > @@ -70,13 +70,12 @@ TEST_F(liveupdate_device, basic_open_close) > } > > /* > - * Test Case: Exclusive Open Enforcement > + * Test Case: Multiple Open Support > * > - * Verifies that the /dev/liveupdate device can only be opened by one process > - * at a time. It checks that a second attempt to open the device fails with > - * the EBUSY error code. > + * Verifies that the /dev/liveupdate device can be opened by multiple openers > + * concurrently without errors. > */ > -TEST_F(liveupdate_device, exclusive_open) > +TEST_F(liveupdate_device, multiple_open) > { > self->fd1 = open(LIVEUPDATE_DEV, O_RDWR); > > @@ -85,8 +84,7 @@ TEST_F(liveupdate_device, exclusive_open) > > ASSERT_GE(self->fd1, 0); > self->fd2 = open(LIVEUPDATE_DEV, O_RDWR); > - EXPECT_LT(self->fd2, 0); > - EXPECT_EQ(errno, EBUSY); > + ASSERT_GE(self->fd2, 0); > } > > /* Helper function to create a LUO session via ioctl. */ > > base-commit: 10fd52dc7bbd6013e949ff1833be0821b7553fb0 > -- > 2.55.0.141.g00534a21ce-goog >
-- Sincerely yours, Mike.
