bcache: avoid journal no-space deadlock by reserving 1 journal bucket

In the Linux kernel, the following vulnerability has been resolved:

bcache: avoid journal no-space deadlock by reserving 1 journal bucket

The journal no-space deadlock was reported time to time. Such deadlock

can happen in the following situation.

When all journal buckets are fully filled by active jset with heavy

write I/O load, the cache set registration (after a reboot) will load

all active jsets and inserting them into the btree again (which is

called journal replay). If a journaled bkey is inserted into a btree

node and results btree node split, new journal request might be

triggered. For example, the btree grows one more level after the node

split, then the root node record in cache device super block will be

upgrade by bch_journal_meta() from bch_btree_set_root(). But there is no

space in journal buckets, the journal replay has to wait for new journal

bucket to be reclaimed after at least one journal bucket replayed. This

is one example that how the journal no-space deadlock happens.

The solution to avoid the deadlock is to reserve 1 journal bucket in

run time, and only permit the reserved journal bucket to be used during

cache set registration procedure for things like journal replay. Then

the journal space will never be fully filled, there is no chance for

journal no-space deadlock to happen anymore.

This patch adds a new member "bool do_reserve" in struct journal, it is

inititalized to 0 (false) when struct journal is allocated, and set to

1 (true) by bch_journal_space_reserve() when all initialization done in

run_cache_set(). In the run time when journal_reclaim() tries to

allocate a new journal bucket, free_journal_buckets() is called to check

whether there are enough free journal buckets to use. If there is only

1 free journal bucket and journal->do_reserve is 1 (true), the last

bucket is reserved and free_journal_buckets() will return 0 to indicate

no free journal bucket. Then journal_reclaim() will give up, and try

next time to see whetheer there is free journal bucket to allocate. By

this method, there is always 1 jouranl bucket reserved in run time.

During the cache set registration, journal->do_reserve is 0 (false), so

the reserved journal bucket can be used to avoid the no-space deadlock.