RFC Making mount operation soft + intr

While implementing and testing autofsng, I found myself getting frustrated at NFS mounts that fail and stay hung.

A typical example is to try and mount from a bogus NFS port:

mount -o port=10000 host:/export /mnt

If the default is hard, the kernel will never time out trying to get initial contact with the server.

If the default is nointr, then there is no way for userspace to kill the mount process.

Both of these options make sense while the filesystem is mounted, but in the case where the server will never respond with a live nfs export, can easily hang processes and leave locked super_block's live in the system (impeding a reboot).

The solution I am suggesting is to force the initial rpc calls used in setting up a mount to be soft + intr, regardless of what the real options are. We can do this by passing RPC_TASK_SOFT to the rpc_calls for NFS3PROC_FSINFO, NFS3PROC_GETATTR, NFSPROC_GETATTR and NFSPROC_STATFS.

To force 'intr' regardless of whether clnt->cl_intr is set or not, we use a similar technique to the recently added RPC_TASK_NOINTR flag used for transaction integrity. We introduce RPC_TASK_FORCEINTR, which overrides RPC_TASK_NOINTR (the latter is dependent on clnt->cl_intr).

AFAICT, making these ops soft+intr is safe because there is no data to be lost in the event of a cancelled/timed-out operation.

The first patch following this message adds RPC_TASK_FORCEINTR and makes the rootfh ops use it and RPC_TASK_SOFT.

The second patch adds a parameter to rpc_clnt_sigmask that takes an optional set of task flags, of which only RPC_TASK_FORCEINTR is inspected for. This keeps the mount paths from blocking signals SIGINT and SIGQUIT.

mount(8) still needs to be taught not to block these signals if it doesn't care whether the mount operation succeeded or not (in the case of it not being a loop device and mtab isn't being updated).

(I haven't touched the nfsv4 stuff as I haven't yet wrapped my head around that code.)

Comments?

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