On Fri, Feb 26, 2021 at 3:14 PM Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote: > > Cc: Michal > > On 2/26/21 2:44 PM, Shakeel Butt wrote: > > On Fri, Feb 26, 2021 at 2:09 PM syzbot > > <syzbot+506c8a2a115201881d45@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > <snip> > >> other info that might help us debug this: > >> > >> Possible interrupt unsafe locking scenario: > >> > >> CPU0 CPU1 > >> ---- ---- > >> lock(hugetlb_lock); > >> local_irq_disable(); > >> lock(slock-AF_INET); > >> lock(hugetlb_lock); > >> <Interrupt> > >> lock(slock-AF_INET); > >> > >> *** DEADLOCK *** > > > > This has been reproduced on 4.19 stable kernel as well [1] and there > > is a reproducer as well. > > > > It seems like sendmsg(MSG_ZEROCOPY) from a buffer backed by hugetlb. I > > wonder if we just need to make hugetlb_lock softirq-safe. > > > > [1] https://syzkaller.appspot.com/bug?extid=6383ce4b0b8ec575ad93 > > Thanks Shakeel, > > Commit c77c0a8ac4c5 ("mm/hugetlb: defer freeing of huge pages if in non-task > context") attempted to address this issue. It uses a work queue to > acquire hugetlb_lock if the caller is !in_task(). > > In another recent thread, there was the suggestion to change the > !in_task to in_atomic. > > I need to do some research on the subtle differences between in_task, > in_atomic, etc. TBH, I 'thought' !in_task would prevent the issue > reported here. But, that obviously is not the case. I think the freeing is happening in the process context in this report but it is creating the lock chain from softirq-safe slock to irq-unsafe hugetlb_lock. So, two solutions I can think of are: (1) always defer the freeing of hugetlb pages to a work queue or (2) make hugetlb_lock softirq-safe.
