From: Waiman Long <firstname.lastname@example.org> To: Yang Shi <email@example.com>, syzbot <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, "email@example.com" <firstname.lastname@example.org>, email@example.com Subject: Re: WARNING: locking bug in lock_downgrade Date: Thu, 13 Dec 2018 14:46:13 -0500 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> On 12/12/2018 08:14 PM, Yang Shi wrote: > Cc'ed Peter, Ingo and Waiman. > > > It took me a few days to look into this warning, but I got lost in > lockdep code. > > > The problem is the commit dd2283f2605e ("mm: mmap: zap pages with read > mmap_sem in munmap") does an optimization for munmap by downgrading > write mmap_sem to read before zapping pages. But, lockdep reports > downgrading a read lock. > > > I'm pretty sure mmap_sem is held as write before downgrade_write() is > called in the patch. And, there are 4 places which may downgrade a > mmap_sem: > > - munmap > > - mremap > > - brk > > - clear_refs_write (fs/proc/task_mmu.c) > > > The first three come from my patches, and they just do: > down_write_killable() -> .. -> downgrade_write(). > > But the last one is a little bit more complicated, it does down_read() > ->.. -> up_read() ->.. -> down_write_killable() ->.. -> > downgrade_write(). > > And, the last one may be called from any process to touch the other > processes' mmap_sem. > > > By looking into lockdep code, I'm not sure if lockdep may get confused > by such sequence or not? > > > Any hint is appreciated. > > > Regards, > > Yang The warning was printed because hlock->read was set when doing the downgrade_write(). So it is either downgrade_write() was called a second time or a read lock was held originally. It is hard to tell what is the root cause without a reproducer. Cheers, Longman
next prev parent reply other threads:[~2018-12-13 19:46 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-11-26 20:38 syzbot 2018-12-13 1:14 ` Yang Shi 2018-12-13 19:46 ` Waiman Long [this message] 2019-01-09 14:18 ` Tetsuo Handa 2019-01-09 22:46 ` Waiman Long 2019-01-10 2:45 ` Tetsuo Handa
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