linux-kernel - Re: [PATCH 2/2] block: hold queue lock while iterating in diskstats

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Date:   Thu, 20 Jan 2022 15:06:29 +0100
From:   Daniel Wagner <dwagner@...e.de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>, Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH 2/2] block: hold queue lock while iterating in
 diskstats_show

On Thu, Jan 20, 2022 at 03:01:27PM +0100, Daniel Wagner wrote:
> On Thu, Jan 20, 2022 at 09:51:18PM +0800, Ming Lei wrote:
> > Then Can you figure out where blk_mq_queue_tag_busy_iter+0x1e4 points to
> > in source code? And what is NULL pointer?

Yes %rax is NULL

> Here we go:
> 
> /usr/src/debug/kernel-default-5.3.18-59.27.1.x86_64/linux-5.3/linux-obj/../include/linux/sbitmap.h: 249
> 0xffffffffb244a254 <blk_mq_queue_tag_busy_iter+484>:    mov    (%rax),%rdx
> 
> 234static inline void __sbitmap_for_each_set(struct sbitmap *sb,
> 235                                          unsigned int start,
> 236                                          sb_for_each_fn fn, void *data)
> 237{
> 238        unsigned int index;
> 239        unsigned int nr;
> 240        unsigned int scanned = 0;
> 241
> 242        if (start >= sb->depth)
> 243                start = 0;
> 244        index = SB_NR_TO_INDEX(sb, start);
> 245        nr = SB_NR_TO_BIT(sb, start);
> 246
> 247        while (scanned < sb->depth) {
> 248                unsigned long word;
> 249                unsigned int depth = min_t(unsigned int,
> 250                                           sb->map[index].depth - nr,
> 251                                           sb->depth - scanned);


forgot to add this

crash> bt
PID: 17640  TASK: ffff956f4a468000  CPU: 13  COMMAND: "iostat"
 #0 [ffffb701aefb7980] machine_kexec at ffffffffba66fb91
 #1 [ffffb701aefb79d8] __crash_kexec at ffffffffba75927d
 #2 [ffffb701aefb7aa0] crash_kexec at ffffffffba75a13d
 #3 [ffffb701aefb7ab8] oops_end at ffffffffba636cdf
 #4 [ffffb701aefb7ad8] no_context at ffffffffba682baf
 #5 [ffffb701aefb7b40] do_page_fault at ffffffffba683e30
 #6 [ffffb701aefb7b70] page_fault at ffffffffbb0012fe
    [exception RIP: blk_mq_queue_tag_busy_iter+484]
    RIP: ffffffffbaa4a254  RSP: ffffb701aefb7c20  RFLAGS: 00010246
    RAX: 0000000000000000  RBX: 0000000000000001  RCX: 0000000000000000
    RDX: ffff954f43c14c00  RSI: ffffffffbaa442c0  RDI: ffff954fb3749010
    RBP: 0000000000000000   R8: 0000000800000000   R9: 00000008ffffffff
    R10: 0000000000000000  R11: 0000000000000000  R12: 0000000000000000
    R13: ffff9567b2200000  R14: ffff954798c09bc0  R15: ffff954798c09c20
    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
 #7 [ffffb701aefb7cb0] blk_mq_in_flight at ffffffffbaa461e5
 #8 [ffffb701aefb7cd0] diskstats_show at ffffffffbaa4f00d
 #9 [ffffb701aefb7e50] seq_read at ffffffffba90df16
#10 [ffffb701aefb7eb0] proc_reg_read at ffffffffba96d789
#11 [ffffb701aefb7ec8] vfs_read at ffffffffba8e4c39
#12 [ffffb701aefb7ef8] ksys_read at ffffffffba8e4fc1
#13 [ffffb701aefb7f38] do_syscall_64 at ffffffffba60538b
#14 [ffffb701aefb7f50] entry_SYSCALL_64_after_hwframe at ffffffffbb00008c
    RIP: 00007f6031f4eb5e  RSP: 00007ffd187a7e88  RFLAGS: 00000246
    RAX: ffffffffffffffda  RBX: 00005577a698c2c0  RCX: 00007f6031f4eb5e
    RDX: 0000000000000400  RSI: 00005577a698f480  RDI: 0000000000000003
    RBP: 00007f603222e300   R8: 0000000000000000   R9: 0000000000000000
    R10: 0000000000000000  R11: 0000000000000246  R12: 000000000000000a
    R13: 0000000000000d68  R14: 00007f603222d700  R15: 0000000000000d68
    ORIG_RAX: 0000000000000000  CS: 0033  SS: 002b


I've tried to figure out the request_pointer from the registers and I
think the pointer is still in %rdi

struct request_queue {
  last_merge = 0x0, 
  elevator = 0x0, 
  stats = 0xffff956f45a9bec0, 
  rq_qos = 0xffff954f54c57558, 
  mq_ops = 0xffffffffc0c27140 <nvme_tcp_mq_ops>, 
  queue_ctx = 0x4151cf2266c0, 
  queue_depth = 0x0, 
  queue_hw_ctx = 0xffff954f43c14c00, 
  nr_hw_queues = 0x50, 
  backing_dev_info = 0xffff953fae3ae800, 
  queuedata = 0xffff953622282800, 
  queue_flags = 0x5041d0, 


View attachment "request_queue.log" of type "text/plain" (15850 bytes)