DEV_REFTHREAD(9)       FreeBSD Kernel Developer's Manual      DEV_REFTHREAD(9)

NAME
     dev_refthread, devvn_refthread, dev_relthread - safely access device
     methods

SYNOPSIS
     #include <sys/param.h>
     #include <sys/conf.h>

     struct cdevsw *
     dev_refthread(struct cdev *dev, int *ref);

     struct cdevsw *
     devvn_refthread(struct vnode *vp, struct cdev **devp, int *ref);

     void
     dev_relthread(struct cdev *dev, int ref);

DESCRIPTION
     The dev_refthread() (or devvn_refthread()) and dev_relthread() routines
     provide a safe way to access devfs(5) devices that may be concurrently
     destroyed by destroy_dev() (e.g., removable media).

     If successful, dev_refthread() and devvn_refthread() acquire a "thread
     reference" to the associated struct cdev and return a non-NULL pointer to
     the cdev's struct cdevsw method table.  For the duration of that
     reference, the cdev's associated private data and method table object are
     valid.  Destruction of the cdev sleeps until the thread reference is
     released.

     A reference cannot prevent media removal.  It is an implementation detail
     of individual drivers how method calls from callers with dev_refthread()
     references are handled when the device is pending destruction.  A common
     behavior for disk devices is to return the ENXIO status, but that is not
     required by this KPI.

     The devvn_refthread() variant of dev_refthread() extracts the struct cdev
     pointer out of the VCHR vnode(9) automatically before performing the same
     actions as dev_refthread().  Additionally, a pointer to the struct cdev
     is returned to the caller via *devp.  devvn_refthread() correctly handles
     possible parallel reclamation of the vnode.

     dev_relthread() is used to release a reference to a struct cdev.
     dev_relthread() must only be invoked when the associated invocation of
     dev_refthread() or devvn_refthread() returned a non-NULL struct cdevsw *.

CONTEXT
     struct cdev objects have two reference counts, si_refcount and
     si_threadcount.  The dev_refthread(), devvn_refthread(), and
     dev_relthread() functions manipulate the si_threadcount.  The
     si_threadcount reference guarantees the liveness of the struct cdev
     object.  The other si_refcount reference provides only the weaker
     guarantee that the memory backing the struct cdev has not been freed.

RETURN VALUES
     If dev_refthread() or devvn_refthread() are unsuccessful, they return
     NULL.  If these routines are unsuccessful, they do not increment the
     struct cdev si_threadcount and do not initialize the value pointed to by
     the *ref parameter in any way.

SEE ALSO
     destroy_dev(9), devfs(5)

CAVEATS
     Do not invoke dev_relthread() unless the matching refthread routine
     succeeded!

FreeBSD 13.1-RELEASE-p6         August 29, 2018        FreeBSD 13.1-RELEASE-p6