Re: lockdep: holding locks across syscall boundaries

[Peter Zijlstra]

On Fri, Oct 27, 2023 at 09:14:53AM -0600, Jens Axboe wrote:
> Hi,
> 
> Normally we'd expect locking state to be clean and consistent across
> syscall entry and exit, as that is always the case for sync syscalls.

> We currently have a work-around for holding a lock from aio, see
> kiocb_start_write(), which pretends to drop the lock from lockdeps
> perspective, as it's held from submission to until kiocb_end_write() is
> called at completion time.

I was not aware of this, the only such hack I knew about was the
filesystem freezer thing.

The problem with holding locks past the end of a syscall is that you'll
nest whatever random lock hierarchies possibly by every other syscall
under that lock.

> This is a bit of an ugly work-around, and defeats the purpose of
> lockdep.
> 
> Since I've now got another case where I want to hold a resource across
> syscalls, is there a better way to do this?
> 
> This is for inode_dio_start(), which increments an inode int count, and
> inode_dio_end() which decrements it. If a task is doing
> inode_dio_start() and then inode_dio_wait(), I want to trigger this. I
> have a hack that does this, but it disables lockdep_sys_exit() as
> otherwise I just get that warning rather than the more useful one.

Suppose syscall-a returns with your kiocb thing held, call it lock A
Suppose syscall-b returns with your inode thing held, call it lock B

Then userspace does:

	syscall-a
	syscall-b

while it also does:

	syscall-b
	syscall-a

and we're up a creek, no?