On 4/12/22 12:20, Thomas Gleixner wrote: > On Mon, Apr 11 2022 at 19:51, Nico Pache wrote: >> On 4/8/22 09:54, Thomas Gleixner wrote: >>> The below reproduces the problem nicely, i.e. the lock() in the parent >>> times out. So why would the OOM killer fail to cause the same problem >>> when it reaps the private anon mapping where the private futex sits? >>> >>> If you revert the lock order in the child the robust muck works. >> >> Thanks for the reproducer Thomas :) >> >> I think I need to re-up my knowledge around COW and how it effects >> that stack. There are increased oddities when you add the pthread >> library that I cant fully wrap my head around at the moment. > > The pthread library functions are just conveniance so I did not have to > hand code the futex and robust list handling. > >> My confusion lies in how the parent/child share a robust list here, but they >> obviously do. In my mind the mut_s would be different in the child/parent after >> the fork and pthread_mutex_init (and friends) are done in the child. > > They don't share a robust list, each thread has it's own. > > The shared mutex mut_s is initialized in the parent before fork and it's > the same address in the child and it's not COWed because the mapping is > MAP_SHARED. > > The child allocates private memory and initializes the private mutex in > that private mapping. > > So now child does: > > pthread_mutex_lock(mut_s); > > That's the mutex in the memory shared with the parent. After that the > childs robusts list head points to mut_s::robust_list. > > Now it does: > > pthread_mutex_lock(mut_p); > > after that the childs robust list head points to mut_p::robust_list and > mut_p::robust_list points to mut_s::robust_list. > > So now the child unmaps the private memory and exists. > > The kernel tries to walk the robust list pointer and faults when trying > to access mut_p. End of walk and mut_s stays locked. > > So now think about the OOM case. The killed process has a shared mapping > with some other unrelated process (file, shmem) where mut_p sits. > > It gets killed after: > pthread_mutex_lock(mut_s); > pthread_mutex_lock(mut_p); > > So the OOM reaper rips the VMA which contains mut_p and therefore breaks > the chain which is necessary to reach mut_p. > > See? Yes, thank you for the detailed explanation, the missing piece just clicked in my head :) Cheers, -- Nico > > Thanks, > > tglx > > >
