On Mon, Oct 28, 2019 at 12:46:57PM +0100, Greg KH wrote:
On Mon, Oct 28, 2019 at 05:39:28AM -0400, Sasha Levin wrote:On Sun, Oct 27, 2019 at 04:37:28PM +0100, gregkh@xxxxxxxxxxxxxxxxxxx wrote: > > The patch below does not apply to the 5.3-stable tree. > If someone wants it applied there, or to any other stable or longterm > tree, then please email the backport, including the original git commit > id to <stable@xxxxxxxxxxxxxxx>. > > thanks, > > greg k-h > > ------------------ original commit in Linus's tree ------------------ > > > From b21555786f18cd77f2311ad89074533109ae3ffa Mon Sep 17 00:00:00 2001 > From: Mikulas Patocka <mpatocka@xxxxxxxxxx> > Date: Wed, 2 Oct 2019 06:15:53 -0400 > Subject: [PATCH] dm snapshot: rework COW throttling to fix deadlock > > Commit 721b1d98fb517a ("dm snapshot: Fix excessive memory usage and > workqueue stalls") introduced a semaphore to limit the maximum number of > in-flight kcopyd (COW) jobs. > > The implementation of this throttling mechanism is prone to a deadlock: > > 1. One or more threads write to the origin device causing COW, which is > performed by kcopyd. > > 2. At some point some of these threads might reach the s->cow_count > semaphore limit and block in down(&s->cow_count), holding a read lock > on _origins_lock. > > 3. Someone tries to acquire a write lock on _origins_lock, e.g., > snapshot_ctr(), which blocks because the threads at step (2) already > hold a read lock on it. > > 4. A COW operation completes and kcopyd runs dm-snapshot's completion > callback, which ends up calling pending_complete(). > pending_complete() tries to resubmit any deferred origin bios. This > requires acquiring a read lock on _origins_lock, which blocks. > > This happens because the read-write semaphore implementation gives > priority to writers, meaning that as soon as a writer tries to enter > the critical section, no readers will be allowed in, until all > writers have completed their work. > > So, pending_complete() waits for the writer at step (3) to acquire > and release the lock. This writer waits for the readers at step (2) > to release the read lock and those readers wait for > pending_complete() (the kcopyd thread) to signal the s->cow_count > semaphore: DEADLOCK. > > The above was thoroughly analyzed and documented by Nikos Tsironis as > part of his initial proposal for fixing this deadlock, see: > https://www.redhat.com/archives/dm-devel/2019-October/msg00001.html > > Fix this deadlock by reworking COW throttling so that it waits without > holding any locks. Add a variable 'in_progress' that counts how many > kcopyd jobs are running. A function wait_for_in_progress() will sleep if > 'in_progress' is over the limit. It drops _origins_lock in order to > avoid the deadlock. > > Reported-by: Guruswamy Basavaiah <guru2018@xxxxxxxxx> > Reported-by: Nikos Tsironis <ntsironis@xxxxxxxxxxx> > Reviewed-by: Nikos Tsironis <ntsironis@xxxxxxxxxxx> > Tested-by: Nikos Tsironis <ntsironis@xxxxxxxxxxx> > Fixes: 721b1d98fb51 ("dm snapshot: Fix excessive memory usage and workqueue stalls") > Cc: stable@xxxxxxxxxxxxxxx # v5.0+ > Depends-on: 4a3f111a73a8c ("dm snapshot: introduce account_start_copy() and account_end_copy()") > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> > Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> Grabbing the listed dependency solved it for 5.3-4.19. For 4.14 and older I've also grabbed the semaphore->mutex conversion.Ugh, I missed that it said that there. I'll do this for 4.19, unless you have these ready to go for when the tree "opens up" again.
I'll have all of these ready to go when the tree opens up. -- Thanks, Sasha
