On Sat, 13 Jan 2018 12:06:26 +0800
Coly Li <i@xxxxxxx> wrote:
> On 12/01/2018 11:24 PM, Pavel Vazharov wrote:
> > There was a possibility for infinite do-while loop inside the GC thread
> > function in case of total failure of the caching device. I was able to
> > reproduce it 3 times simulating disappearing of the caching device via
> > 'echo 1 > /sys/block/<dev>/device/delete'. In that case the btree_root
> > starts to return non zero and non -EAGAIN result, 'gc failed' message
> > start to fill the kernel log and the do-while becomes infinite loop
> > occupying single CPU core at 100%.
> > There is already a logic which unregisters the cache_set (or panics) in
> > case of io errors and thus we exit the loop here if the unregistering
> > procedure has already started.
> >
> > Signed-off-by: Pavel Vazharov <freakpv@xxxxxxxxx>
> > ---
> > drivers/md/bcache/btree.c | 8 ++++++--
> > 1 file changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
> > index 81e8dc3..a672081 100644
> > --- a/drivers/md/bcache/btree.c
> > +++ b/drivers/md/bcache/btree.c
> > @@ -1748,8 +1748,12 @@ static void bch_btree_gc(struct cache_set *c)
> > closure_sync(&writes);
> > cond_resched();
> >
> > - if (ret && ret != -EAGAIN)
> > - pr_warn("gc failed!");
> > + if (ret && ret != -EAGAIN) {
> > + if (test_bit(CACHE_SET_UNREGISTERING, &c->flags))
> > + break;
> > + else
> > + pr_warn("gc failed!");
> > + }
> > } while (ret);
> >
> > bch_btree_gc_finish(c);
> >
>
> Hi Pavel,
>
> I see the point here. But there are 2 code paths to call
> cache_set_flush(), one is from bch_cache_set_error(), one is from sysfs
> interface (echo 1 > /sys/fs/bcache/<UUID>/stop).
>
> CACHE_SET_UNREGISTERING is set in the first code path, the another code
> path from sysfs does not set CACHE_SET_UNREGISTERING. In this case maybe
> the above while-loop can not be stopped.
>
> In my device failure cache set, I add an io_disable (in v2 it is
> CACHE_SET_IO_DISABLE flag) to disable all cache set I/O, maybe it can be
> used to check the condition and break the while-loop.
>
> Thanks for the hint, I will also try to fix it in my patch set. If you
> don't mind, I am glad to have your "Reviewed-by:" after I post the v2
> patch set.
>
> Thanks.
>
> --
> Coly Li
Hi Coly,
CACHE_SET_IO_DISABLE looks like more general solution to the problem.
Thanks for the review invitation. I'll do my best.
--
Pavel Vazharov <freakpv@xxxxxxxxx>
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