On Sat, Jan 18, 2020 at 02:20:32AM +0000, Al Viro wrote: > On Fri, Jan 17, 2020 at 05:17:34PM -0800, Omar Sandoval wrote: > > > No. This is completely wrong; just make it ->link_replace() and be done > > > with that; no extra arguments and *always* the same conditions wrt > > > positive/negative. One of the reasons why ->rename() tends to be > > > ugly (and a source of quite a few bugs over years) are those "if > > > target is positive/if target is negative" scattered over the instances. > > > > > > Make the choice conditional upon the positivity of target. > > > > Yup, you already convinced me that ->link_replace() is better in your > > last email. > > FWIW, that might be not so simple ;-/ Reason: NFS-like stuff. Client > sees a negative in cache; the problem is how to decide whether to > tell the server "OK, I want normal link()" vs. "if it turns out that > someone has created it by the time you see the request, give do > a replacing link". Sure, if could treat ->link() telling you -EEXIST > as "OK, repeat it with ->link_replace(), then", but that's an extra > roundtrip... So that's a point in favor of ->link(). But then if we overload ->link() instead of adding ->link_replace() and we want EOPNOTSUPP to fail fast, we need to add something like FMODE_SUPPORTS_AT_REPLACE. Some options I see are: 1. Go with ->link_replace() until network filesystem specs support AT_REPLACE. That would be a bit of a mess down the line, though. 2. Stick with ->link(), let the filesystem implementations deal with the positive targets, and add FMODE_SUPPORTS_AT_REPLACE so that feature detection remains easy for userspace. 3. Option 2, but don't bother with FMODE_SUPPORTS_AT_REPLACE. FWIW, there is precendent for option 3: RENAME_EXCHANGE. That has the same "files are the same" noop condition, and we don't know whether RENAME_EXCHANGE is supported until ->rename(). A cursory search shows that applications using RENAME_EXCHANGE try it and fall back to a non-atomic exchange on EINVAL. They could do the exact same thing for AT_REPLACE. None of it is elegant, but which approach would you hate the least?
