James Carlson wrote: >Rich.Brown at Sun.COM writes: > > >> The first two new flags to be defined: >> #define CC_WOULDBLOCK 0x1 /* set upon return by monitor */ >> #define CC_DONTBLOCK 0x2 /* set by caller */ >> >> The caller sets CC_DONTBLOCK in cc_flags to direct the monitor not to >> perform an operation that might block. In the case where a monitor would >> perform a blocking operation and CC_DONTBLOCK is set, the monitor >> sets CC_WOULDBLOCK in the cc_flags and returns EAGAIN to the caller. >> >> > >I'm not sure I understand how this is intended to be used. > > It is intended to be used by callers that do not want to block inside a monitor while waiting for a delegation to be returned.
>What exactly is the caller supposed to do when he gets CC_WOULDBLOCK? > > I'm not certain that there is a general rule to follow here, however, I can give you examples of what NFS does. Each version of the NFS server will use the CC_DONTBLOCK flag on calls to VOP_READ, VOP_WRITE, VOP_SETATTR, and (in the case of v4) VOP_OPEN. When a monitor on a delegated file detects a conflict in one of the ops, it will issue a recall of the delegation and then check to see if the CC_DONTBLOCK flag is set. If it is set, it will then set the CC_WOULDBLOCK flag and return EAGAIN. When the NFSv2 server gets this, it will simply drop the response, causing the client to reissue the request. When the NFSv3 server gets the EAGAIN w/CC_WOULDBLOCK, it will return the error NFS3ERR_JUKEBOX to the client. The NFSv3 client knows to retry when it sees this error. The NFSv4 server will return NFS4ERR_DELAY when it gets this error from the monitor. >How long should he wait before trying again? Or is it always possible >to detect this case and defer the operation until at can be run >without CC_DONTBLOCK? > > > The way the NFSv2&v3 server currently work is to use cross calls into the NFSv4 server to check if a file is delegated. If so, the delegation is recalled and the v2 & v3 server do what is described above. The reason for this fast track is to allow us to remove those cross calls from the v2 & v3 server and have all conflict detection handled by the monitors. The point is, the only caller that will use these flags are ones that are built to handle it. That is, if they get EAGAIN, they have a mechanism for retrying. I hope this helped. Jim
