[AFS3-std] Revised Proposal: Extension of RXAFS_SetLock to support LockUpgrade and LockDowngrade

[Jeffrey Altman]

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The following is a revised proposal that incorporates the comments
received in February.

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The RXAFS_SetLock RPC currently supports four operations:

%#define LockRead               0
%#define LockWrite              1
%#define LockExtend             2
%#define LockRelease            3

This set of operations do not provide the functionality
necessary in order to safely support the upgrading of a read
lock to the write lock and the downgrading of a write lock to
a read lock.  Currently, upgrades and downgrades are implemented
in the afs client by dropping a lock and requesting the other
type.  Unfortunately, this creates a race condition which can
result in the loss of the lock in its entirety.

To remove the race condition two additional operations are
proposed for the existing RXAFS_SetLock RPC:

%#define LockUpgrade            4
%#define LockDowngrade          5

Extending the existing RPC is safe to do because the existing
implementations currently return EINVAL for these values.

The semantics of the new operations are as follows:

  LockUpgrade:

  if the client has been issued a "Read" lock and
  no other parties have been issued locks on the object,
  then change the lock type to "Write" and return success.

  If the client has already been issued a "Write" lock, then
  return EEXIST.

  If the client has not been issued any lock, then return
  ENOENT.

  If the client does not have permission to obtain a "Write"
  lock due to lack of PRSFS_WRITE or PRSFS_INSERT privileges,
  then return EACCES.

  Otherwise, return EAGAIN.

  LockDowngrade - if the client has been issued a "Write" lock,
  then change the lock type to "Read" and return success.

  If the client already has been issued a "Read" lock, then
  return EEXIST.

  If the client has not been issued any lock, then return
  ENOENT.

  If the client does not have permission to obtain a "Read"
  lock due to lack of PRSFS_LOCK privilege, then return EACCESS.

  Otherwise, return EAGAIN.

  Lock Lifetimes:

  Locks have a lifetime of five minutes in AFS.  Both of the
  LockUpgrade and LockDowngrade operations reset the lock
  expiration timer upon success.

In order to avoid the need to issue try the new operations and
fallback to the old release and lock approach, a Capability flag
VICED_CAPABILITY_LOCK_UPGRADES will be created using a number assigned
by registrar@grand.central.org.

Jeffrey Altman

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