[PATCH] bcache: fix a race between register and allocator thread

[Junhui Tang <tang.junhui.linux@xxxxxxxxx>]

>From d75d1b51aeedcc9349571be084c5f51d8f7e7612 Mon Sep 17 00:00:00 2001
From: Tang Junhui <tang.junhui.linux@xxxxxxxxx>
Date: Thu, 24 Jan 2019 16:01:35 +0800
Subject: [PATCH] bcache: fix a race between register and allocator thread

when register a new cache device, allocator get stucked,
the call trace is bellow:
Sangfor:EDS/eds-fe5192de /sf # cat /proc/3642/task/*/stack
[<ffffffffa02b01c8>] bch_bucket_alloc+0x178/0x2b0 [bcache]
[<ffffffffa02c737d>] bch_prio_write+0x18d/0x320 [bcache]
[<ffffffffa02b0042>] bch_allocator_thread+0x432/0x440 [bcache]
[<ffffffff810a1430>] kthread+0xc0/0xd0
[<ffffffff8173def7>] ret_from_fork+0x77/0xb0
[<ffffffffffffffff>] 0xffffffffffffffff

There is a race between register and allocator thread:
register thread                       allocator thread
run_cache_set
=>bch_initial_gc_finish
  alloc prio buckets
=>bch_cache_allocator_start
  start allocator thread             bch_allocator_thread
=>bch_prio_write                     =>invalidate_buckets
  write prio using prio buckets        fill bucket into ca->free_inc
=>SET_CACHE_SYNC
  set cache sync mode                 =>bch_prio_write
                                        since cache sync mode is set,
                                        write prio to bucket, and the
                                        prio bucket has been used in
                                        register thread, no bucket can
                                        used. so the allocator thread
                                        stuck here.
In this patch, register thread wait for the allocator thread to finish
work, then write prio to buckets and set cache to sync mode.

Signed-off-by: Tang Junhui <tang.junhui.linux@xxxxxxxxx>
---
 drivers/md/bcache/super.c | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index fa4058e..818d2f1 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1852,13 +1852,28 @@ static void run_cache_set(struct cache_set *c)
  bch_initial_gc_finish(c);

  err = "error starting allocator thread";
- for_each_cache(ca, c, i)
- if (bch_cache_allocator_start(ca))
- goto err;
-
  mutex_lock(&c->bucket_lock);
- for_each_cache(ca, c, i)
+ for_each_cache(ca, c, i) {
+ DEFINE_WAIT(wait);
+
+ if (bch_cache_allocator_start(ca)) {
+ mutex_unlock(&c->bucket_lock);
+ goto err;
+ }
+
+ for (;;) {
+ prepare_to_wait(&c->bucket_wait, &wait,
+ TASK_UNINTERRUPTIBLE);
+ if (!fifo_empty(&ca->free_inc))
+ break;
+ mutex_unlock(&c->bucket_lock);
+ schedule();
+ mutex_lock(&c->bucket_lock);
+ }
+
+ finish_wait(&c->bucket_wait, &wait);
  bch_prio_write(ca);
+ }
  mutex_unlock(&c->bucket_lock);

  err = "cannot allocate new UUID bucket";
-- 
1.8.3.1