SRB-mode callers

z/OS UNIX supports programs that are running on service request block (SRB) dispatchable units, in addition to the more standard task control blocks (TCBs). This affects the PFS, as the resulting vnode operations are also running in SRB mode.

SRB mode is even more restrictive than cross-memory mode. Additional restrictions on the PFS include the following:

There are no MVS™ WAITs; instead you have to use SUSPEND/RESUME. This can impact some of the internal functions of the PFS that may not be easy to modify, including task switching, lock managers, and tracing.
Note: The osi_wait/osi_post services transparently support both TCB and SRB-mode callers.
No TCB is available (Psatold=0). The TCB address is used by some programs to build identifiers, or in other algorithms.
There is no EOT or ESTAE recovery, although you can use an FRR.
Note: vn_recovery support is still available from the LFS.
Because SRB callers do not receive POSIX signals, they cannot break out of extended waits, as they can in the EINTR cases.
Signal-enabled osi_waits should still be set up where they are set up now, because this also indicates that the osi_wait may be interrupted for process termination.

The following OSI services are enabled for SRB-mode callers:

osi_copyin
osi_copyout
osi_copy64
osi_getvnode
osi_mountstatus
osi_postosi_sched
osi_selpost
osi_uiomove
osi_upda
osi_wait
osi_wakeup

The PFS signifies that it supports SRB-mode callers on the pfsi_srb bit that is returned during PFS initialization. The LFS inhibits SRB-mode calls to PFSs that do not support them.

All sockets-related vnode operations are potentially callable from an SRB, and in the future this may be extended to file-related operations. Therefore, the PFS should be made completely SRB safe.

Refer to z/OS MVS Programming: Authorized Assembler Services Guide for more information about SRB-mode programs.