block/mq: drop per ctx cpu_lock

While converting the get_cpu() to get_cpu_light() I added a cpu lock to
ensure the same code is not invoked twice on the same CPU. And now I run
into this:

| kernel BUG at kernel/locking/rtmutex.c:996!
| invalid opcode: 0000 [#1] PREEMPT SMP
| CPU0: 13 PID: 75 Comm: kworker/u258:0 Tainted: G          I    3.18.7-rt1.5+ #12
| Workqueue: writeback bdi_writeback_workfn (flush-8:0)
| task: ffff88023742a620 ti: ffff88023743c000 task.ti: ffff88023743c000
| RIP: 0010:[<ffffffff81523cc0>]  [<ffffffff81523cc0>] rt_spin_lock_slowlock+0x280/0x2d0
| Call Trace:
|  [<ffffffff815254e7>] rt_spin_lock+0x27/0x60
taking the same lock again
|
|  [<ffffffff8127c771>] blk_mq_insert_requests+0x51/0x130
|  [<ffffffff8127d4a9>] blk_mq_flush_plug_list+0x129/0x140
|  [<ffffffff81272461>] blk_flush_plug_list+0xd1/0x250
|  [<ffffffff81522075>] schedule+0x75/0xa0
|  [<ffffffff8152474d>] do_nanosleep+0xdd/0x180
|  [<ffffffff810c8312>] __hrtimer_nanosleep+0xd2/0x1c0
|  [<ffffffff810c8456>] cpu_chill+0x56/0x80
|  [<ffffffff8107c13d>] try_to_grab_pending+0x1bd/0x390
|  [<ffffffff8107c431>] cancel_delayed_work+0x21/0x170
|  [<ffffffff81279a98>] blk_mq_stop_hw_queue+0x18/0x40
|  [<ffffffffa000ac6f>] scsi_queue_rq+0x7f/0x830 [scsi_mod]
|  [<ffffffff8127b0de>] __blk_mq_run_hw_queue+0x1ee/0x360
|  [<ffffffff8127b528>] blk_mq_map_request+0x108/0x190
take the lock  ^^^
|
|  [<ffffffff8127c8d2>] blk_sq_make_request+0x82/0x350
|  [<ffffffff8126f6c0>] generic_make_request+0xd0/0x120
|  [<ffffffff8126f788>] submit_bio+0x78/0x190
|  [<ffffffff811bd537>] _submit_bh+0x117/0x180
|  [<ffffffff811bf528>] __block_write_full_page.constprop.38+0x138/0x3f0
|  [<ffffffff811bf880>] block_write_full_page+0xa0/0xe0
|  [<ffffffff811c02b3>] blkdev_writepage+0x13/0x20
|  [<ffffffff81127b25>] __writepage+0x15/0x40
|  [<ffffffff8112873b>] write_cache_pages+0x1fb/0x440
|  [<ffffffff811289be>] generic_writepages+0x3e/0x60
|  [<ffffffff8112a17c>] do_writepages+0x1c/0x30
|  [<ffffffff811b3603>] __writeback_single_inode+0x33/0x140
|  [<ffffffff811b462d>] writeback_sb_inodes+0x2bd/0x490
|  [<ffffffff811b4897>] __writeback_inodes_wb+0x97/0xd0
|  [<ffffffff811b4a9b>] wb_writeback+0x1cb/0x210
|  [<ffffffff811b505b>] bdi_writeback_workfn+0x25b/0x380
|  [<ffffffff8107b50b>] process_one_work+0x1bb/0x490
|  [<ffffffff8107c7ab>] worker_thread+0x6b/0x4f0
|  [<ffffffff81081863>] kthread+0xe3/0x100
|  [<ffffffff8152627c>] ret_from_fork+0x7c/0xb0

After looking at this for a while it seems that it is save if blk_mq_ctx is
used multiple times, the in struct lock protects the access.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d159d824d0a4955e95f529013368be5e8e2a16b6..af7764e932fa644b2b67c7344743d696e19afd6a 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1373,9 +1373,7 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
                        if (list_empty(&plug->mq_list))
                                trace_block_plug(q);
                        else if (request_count >= BLK_MAX_REQUEST_COUNT) {
-                               spin_unlock(&data.ctx->cpu_lock);
                                blk_flush_plug_list(plug, false);
-                               spin_lock(&data.ctx->cpu_lock);
                                trace_block_plug(q);
                        }
                        list_add_tail(&rq->queuelist, &plug->mq_list);
@@ -1570,7 +1568,6 @@ static int blk_mq_hctx_cpu_offline(struct blk_mq_hw_ctx *hctx, int cpu)
                blk_mq_hctx_clear_pending(hctx, ctx);
        }
        spin_unlock(&ctx->lock);
-       __blk_mq_put_ctx(ctx);
 
        if (list_empty(&tmp))
                return NOTIFY_OK;
@@ -1764,7 +1761,6 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
                memset(__ctx, 0, sizeof(*__ctx));
                __ctx->cpu = i;
                spin_lock_init(&__ctx->lock);
-               spin_lock_init(&__ctx->cpu_lock);
                INIT_LIST_HEAD(&__ctx->rq_list);
                __ctx->queue = q;
 

diff --git a/block/blk-mq.h b/block/blk-mq.h
index 2e90087710a18c8777d1f823ca94aa2bf279e801..4b7cbf0e6e822ee162ef68421411528d5634d246 100644 (file)
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -9,7 +9,6 @@ struct blk_mq_ctx {
                struct list_head        rq_list;
        }  ____cacheline_aligned_in_smp;
 
-       spinlock_t              cpu_lock;
        unsigned int            cpu;
        unsigned int            index_hw;
 
@@ -80,7 +79,6 @@ static inline struct blk_mq_ctx *__blk_mq_get_ctx(struct request_queue *q,
        struct blk_mq_ctx *ctx;
 
        ctx = per_cpu_ptr(q->queue_ctx, cpu);
-       spin_lock(&ctx->cpu_lock);
        return ctx;
 }
 
@@ -95,14 +93,8 @@ static inline struct blk_mq_ctx *blk_mq_get_ctx(struct request_queue *q)
        return __blk_mq_get_ctx(q, get_cpu_light());
 }
 
-static void __blk_mq_put_ctx(struct blk_mq_ctx *ctx)
-{
-       spin_unlock(&ctx->cpu_lock);
-}
-
 static inline void blk_mq_put_ctx(struct blk_mq_ctx *ctx)
 {
-       __blk_mq_put_ctx(ctx);
        put_cpu_light();
 }