On Tue, Feb 05, 2019 at 12:08:25PM -0700, Jens Axboe wrote: > Proof is in the pudding, here's the main commit introducing io_uring > and now wiring it up to the AF_UNIX garbage collection: > > http://git.kernel.dk/cgit/linux-block/commit/?h=io_uring&id=158e6f42b67d0abe9ee84886b96ca8c4b3d3dfd5 > > How does that look? In a word - wrong. Some theory: garbage collector assumes that there is a subset of file references such that * for all files with such references there's an associated unix_sock. * all such references are stored in SCM_RIGHTS datagrams that can be found by the garbage collector (currently: for data-bearing AF_UNIX sockets - queued SCM_RIGHTS datagrams, for listeners - SCM_RIGHTS datagrams sent via yet-to-be-accepted connections). * there is an efficient way to count those references for given file (->inflight of the corresponding unix_sock). * removal of those references would render the graph acyclic. * file can _NOT_ be subject to syscalls unless there are references to it outside of that subset. unix_inflight() moves a reference into the subset unix_notinflight() moves a reference out of the subset activity that might add such references ought to call wait_for_unix_gc() first (basically, to stall the massive insertions when gc is running). Note that unix_gc() does *NOT* work in terms of dropping file references - the primary effect is locating the SCM_RIGHTS datagrams that can be disposed of and taking them out. It simply won't do anything to your file references, no matter what. Add a printk into your ->release() and try to register io_uring descriptor into itself, then close it. And observe ->release() not being called for that object. Ever. PS: The algorithm used by unix_gc() is basically this - grab unix_gc_lock (giving exclusion with unix_inflight/unix_notinflight and stabilizing ->inflight counters) Candidates = {} for all unix_sock u such that u->inflight > 0 if file corresponding to u has no other references Candidates += u /* everything else already is reachable; due to unix_gc_lock these can't die or get syscall-visible references under us */ Might_Die = Candidates /* invariant to maintain: for u in Candidates u->inflight will be equal to the number of references from SCM_RIGHTS datagrams *except* those immediately reachable from elements of Might_Die */ for all u in Candidates for each file reference v in SCM_RIGHTS datagrams immediately reachable from u if v in Candidates v->inflight-- To_Scan = () // stuff reachable from those must live for all u in Might_Die if u->inflight > 0 queue u into To_Scan while To_Scan is non-empty u = dequeue(To_Scan) Might_Die -= u for each file reference v in SCM_RIGHTS datagrams immediately reachable from u if v in Candidates v->inflight++ // maintain the invariant if v in Might_Die queue v into To_Scan /* at that point nothing in Might_Die is reachable from the outside */ /* restore the original values of ->inflight */ for all u in Might_Die for each file reference v in SCM_RIGHTS datagrams immediately reachable from u if v in Candidates v->inflight++ hitlist = () for all u in Might_Die for each SCM_RIGHTS datagram D immediately reachable from u if D contains references to something in Candidates move D to hitlist /* all those datagrams would've never become reachable */ drop unix_gc_lock discard all datagrams in hitlist.
