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.Dd August 29, 2018 .Dt DEV_REFTHREAD 9 .Os .Sh NAME .Nm dev_refthread , .Nm devvn_refthread , .Nm dev_relthread .Nd safely access device methods .Sh SYNOPSIS n sys/param.h n sys/conf.h .Ft "struct cdevsw *" .Fn dev_refthread "struct cdev *dev" "int *ref" .Ft "struct cdevsw *" .Fn devvn_refthread "struct vnode *vp" "struct cdev **devp" "int *ref" .Ft void .Fn dev_relthread "struct cdev *dev" "int ref" .Sh DESCRIPTION The .Fn dev_refthread (or .Fn devvn_refthread ) and .Fn dev_relthread routines provide a safe way to access .Xr devfs 5 devices that may be concurrently destroyed by .Fn destroy_dev (e.g., removable media).
p If successful, .Fn dev_refthread and .Fn devvn_refthread acquire a "thread reference" to the associated .Vt "struct cdev" and return a non-NULL pointer to the cdev's .Vt "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.
p A reference cannot prevent media removal. It is an implementation detail of individual drivers how method calls from callers with .Fn dev_refthread references are handled when the device is pending destruction. A common behavior for disk devices is to return the .Er ENXIO status, but that is not required by this KPI.
p The .Fn devvn_refthread variant of .Fn dev_refthread extracts the .Vt "struct cdev" pointer out of the .Dv VCHR .Xr vnode 9 automatically before performing the same actions as .Fn dev_refthread . Additionally, a pointer to the .Vt "struct cdev" is returned to the caller via .Fa "*devp" . .Fn devvn_refthread correctly handles possible parallel reclamation of the vnode.
p .Fn dev_relthread is used to release a reference to a .Vt "struct cdev" . .Fn dev_relthread .Sy must only be invoked when the associated invocation of .Fn dev_refthread or .Fn devvn_refthread returned a non-NULL .Vt "struct cdevsw *" . .Sh CONTEXT .Vt struct cdev objects have two reference counts, .Va si_refcount and .Va si_threadcount . The .Fn dev_refthread , .Fn devvn_refthread , and .Fn dev_relthread functions manipulate the .Va si_threadcount . The .Va si_threadcount reference guarantees the liveness of the .Vt struct cdev object. The other .Va si_refcount reference provides only the weaker guarantee that the memory backing the .Vt struct cdev has not been freed. .Sh RETURN VALUES If .Fn dev_refthread or .Fn devvn_refthread are unsuccessful, they return .Dv NULL . f Em If these routines are unsuccessful, they do not increment the .Vt "struct cdev" .Va si_threadcount and do not initialize the value pointed to by the .Fa "*ref" parameter in any way. .Ef .Sh SEE ALSO .Xr destroy_dev 9 , .Xr devfs 5 .Sh CAVEATS Do not invoke .Fn dev_relthread unless the matching refthread routine succeeded!