[Cc Oleg] On Mon 02-03-20 17:34:49, Coly Li wrote: > When cache device and cached device are registered simuteneously and > register_cache() firstly acquires bch_register_lock. register_bdev() > has to wait before register_cache() finished, it might be a very long > time. > > If the registration is from udev rules in system boot up time, and > registration is not completed before udev timeout (default 180s), the > registration process will be killed by udevd. Then the following calls > to kthread_run() or kthread_create() will fail due to the pending > signal (they are implemented this way at this moment). > > For boot time, this is not good, because it means a cache device with > huge cached data will always fail in boot time, just because it > spends too much time to check its internal meta data (btree and dirty > sectors). > > The failure for cache device registration is solved by previous > patches, but failure due to timeout also exists in cached device > registration. As the above text explains, cached device registration > may also be timeout if it is blocked by a timeout cache device > registration process. Then in the following code path, > bioset_init() <= bcache_device_init() <= cached_dev_init() <= > register_bdev() <= register_bcache() > bioset_init() will fail because internally kthread_create() will fail > for pending signal in the following code path, > bioset_init() => alloc_workqueue() => init_rescuer() => > kthread_create() > > Maybe fix kthread_create() and kthread_run() is better method, but at > this moment a fast workaroudn is to flush pending signals before > calling bioset_init() in bcache_device_init(). I cannot really comment on the bcache part because I am not familiar with the code. It is quite surprising to see an initialization taking that long though. Anyway > This patch calls flush_signals() in bcache_device_init() if there is > pending signal for current process. It avoids bcache registration > failure in system boot up time due to bcache udev rule timeout. this sounds like a wrong way to address the issue. Killing the udev worker is a userspace policy and the kernel shouldn't simply ignore it. Is there any problem to simply increase the timeout on the system which uses a large bcache? Btw. Oleg, I have noticed quite a lot of flush_signals usage in the drivers land and I have really hard time to understand their purpose. What is the actual valid usage of this function? Should we somehow document it? > Signed-off-by: Coly Li <colyli@xxxxxxx> > --- > drivers/md/bcache/super.c | 12 ++++++++++++ > 1 file changed, 12 insertions(+) > > diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c > index 0c3c5419c52b..e8bbd4f171ca 100644 > --- a/drivers/md/bcache/super.c > +++ b/drivers/md/bcache/super.c > @@ -850,6 +850,18 @@ static int bcache_device_init(struct bcache_device *d, unsigned int block_size, > if (idx < 0) > return idx; > > + /* > + * There is a timeout in udevd, if the bcache device is registering > + * by udev rules, and not completed in time, the udevd may kill the > + * registration process. In this condition, there will be pending > + * signal here and cause bioset_init() failed for internally creating > + * its kthread. Here the registration should ignore the timeout and > + * continue, it is safe to ignore the pending signal and avoid to > + * fail bcache registration in boot up time. > + */ > + if (signal_pending(current)) > + flush_signals(current); > + > if (bioset_init(&d->bio_split, 4, offsetof(struct bbio, bio), > BIOSET_NEED_BVECS|BIOSET_NEED_RESCUER)) > goto err; > -- > 2.16.4 -- Michal Hocko SUSE Labs
