Re: [PATCH] f2fs: move f2fs to use reader-unfair rwsems

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 02/22, Christoph Hellwig wrote:
> It looks like this patch landed in linux-next despite all the perfectly
> reasonable objections.  Jaegeuk, please drop it again.

I've been waiting for a generic solution as suggested here. Until then, I'd like
to keep this in f2fs *only* in order to ship the fix in products. Once there's
a right fix, let me drop or revise this patch again.

> 
> On Wed, Jan 12, 2022 at 03:06:12PM +0100, Peter Zijlstra wrote:
> > On Mon, Jan 10, 2022 at 11:41:23AM -0800, Tim Murray wrote:
> > 
> > > 1. f2fs-ckpt thread is running f2fs_write_checkpoint(), holding the
> > > cp_rwsem write lock while doing so via f2fs_lock_all() in
> > > block_operations().
> > > 2. Random very-low-priority thread A makes some other f2fs call that
> > > tries to get the cp_rwsem read lock by atomically adding on the rwsem,
> > > fails and deschedules in uninterruptible sleep. cp_rwsem now has a
> > > non-zero reader count but is write-locked.
> > > 3. f2fs-ckpt thread releases the cp_rwsem write lock. cp_rwsem now has
> > > a non-zero reader count and is not write-locked, so is reader-locked.
> > > 4. Other threads call fsync(), which requests checkpoints from
> > > f2fs-ckpt, and block on a completion event that f2fs-ckpt dispatches.
> > > cp_rwsem still has a non-zero reader count because the low-prio thread
> > > A from (2) has not been scheduled again yet.
> > > 5. f2fs-ckpt wakes up to perform checkpoints, but it stalls on the
> > > write lock via cmpxchg in block_operations() until the low-prio thread
> > > A has run and released the cp_rwsem read lock. Because f2fs-ckpt can't
> > > run, all fsync() callers are also effectively blocked by the
> > > low-priority thread holding the read lock.
> > > 
> > > I think this is the rough shape of the problem (vs readers holding the
> > > lock for too long or something like that) because the low-priority
> > > thread is never run between when it is initially made runnable by
> > > f2fs-ckpt and when it runs tens/hundreds of milliseconds later then
> > > immediately unblocks f2fs-ckpt.
> > 
> > *urgh*... so you're making the worst case less likely but fundamentally
> > you don't change anything.
> > 
> > If one of those low prio threads manages to block while holding
> > cp_rwsem your checkpoint thread will still block for a very long time.
> > 
> > So while you improve the average case, the worst case doesn't improve
> > much I think.
> > 
> > Also, given that this is a system wide rwsem, would percpu-rwsem not be
> > 'better' ? Arguably with the same hack cgroups uses for it (see
> > cgroup_init()) to lower the cost of percpu_down_write().
> > 
> > Now, I'm not a filesystem developer and I'm not much familiar with the
> > problem space, but this locking reads like a fairly big problem. I'm not
> > sure optimizing the lock is the answer.
> > 
> > 
> ---end quoted text---