On Wed, 2017-09-13 at 19:11 +0200, Oleg Nesterov wrote: > On 09/12, Jürg Billeter wrote: > > > > On Tue, 2017-09-12 at 19:05 +0200, Oleg Nesterov wrote: > > > On 09/09, Jürg Billeter wrote: > > > > Unlike > > > > PR_SET_PDEATHSIG, this is inherited across fork to allow killing a whole > > > > subtree without race conditions. > > > > > > but I am still not sure this is right... at least I can't understand the > > > "without race conditions" above. > > > > > > IOW, the child can do prctl(PR_SET_PDEATHSIG_PROC, SIGKILL) right after > > > fork(), > > > why this is not enough to kill a whole subtree without race conditions? > > > > What if the parent dies between fork() and prctl()? > > The child will be killed? Sorry, can't understand...
If PR_SET_PDEATHSIG_PROC was not inherited across fork and the parent died between fork() and prctl(PR_SET_PDEATHSIG_PROC, SIGKILL) in the child, the child would not be killed. It would be reparented to init(1) or a subreaper, i.e., you end up with a runaway process. It would be possible to safe guard against this race condition in other ways but inheriting the setting avoids it nicely, and makes it easy to apply/enforce PDEATHSIG_PROC for all descendants. > > > Say, CLONE_PARENT. Should it succeed if ->pdeath_signal_proc != 0 ? > > > > Yes, I don't see an issue with that. The new process will be a sibling > > and inheriting pdeath_signal_proc seems sensible to me for this. > > I meant, the process created by clone(CLONE_PARENT) won't be killed by > pdeath_signal if the creator process exits, exactly because it won't be > its child. Not that I think this is wrong. Right, creator and parent won't be the same. Jürg
