When a task that is allocating metadata needs to wait for the async
reclaim job to process its ticket and gets a signal (because it was killed
for example) before doing the wait, the task ends up erroring out but
with space reserved for its ticket, which never gets released, resulting
in a metadata space leak (more specifically a leak in the bytes_may_use
counter of the metadata space_info object).

Here's the sequence of steps leading to the space leak:

1) A task tries to create a file for example, so it ends up trying to
   start a transaction at btrfs_create();

2) The filesystem is currently in a state where there is not enough
   metadata free space to satisfy the transaction's needs. So at
   space-info.c:__reserve_metadata_bytes() we create a ticket and
   add it to the list of tickets of the space info object. Also,
   because the metadata async reclaim job is not running, we queue
   a job ro run metadata reclaim;

3) In the meanwhile the task receives a signal (like SIGTERM from
   a kill command for example);

4) After queing the async reclaim job, at __reserve_metadata_bytes(),
   we unlock the metadata space info and call handle_reserve_ticket();

5) That last function calls wait_reserve_ticket(), which acquires the
   lock from the metadata space info. Then in the first iteration of
   its while loop, it calls prepare_to_wait_event(), which returns
   -ERESTARTSYS because the task has a pending signal. As a result,
   we set the error field of the ticket to -EINTR and exit the while
   loop without deleting the ticket from the list of tickets (in the
   space info object). After exiting the loop we unlock the space info;

6) The async reclaim job is able to release enough metadata, acquires
   the metadata space info's lock and then reserves space for the ticket,
   since the ticket is still in the list of (non-priority) tickets. The
   space reservation happens at btrfs_try_granting_tickets(), called from
   maybe_fail_all_tickets(). This increments the bytes_may_use counter
   from the metadata space info object, sets the ticket's bytes field to
   zero (meaning success, that space was reserved) and removes it from
   the list of tickets;

7) wait_reserve_ticket() returns, with the error field of the ticket
   set to -EINTR. Then handle_reserve_ticket() just propagates that error
   to the caller. Because an error was returned, the caller does not
   release the reserved space, since the expectation is that any error
   means no space was reserved.

Fix this by removing the ticket from the list, while holding the space
info lock, at wait_reserve_ticket() when prepare_to_wait_event() returns
an error.

Also add some comments and an assertion to guarantee we never end up with
a ticket that has an error set and a bytes counter field set to zero, to
more easily detect regressions in the future.

This issue could be triggered sporadically by some test cases from fstests
such as generic/269 for example, which tries to fill a filesystem and then
kills fsstress processes running in the background.

When this issue happens, we get a warning in syslog/dmesg when unmounting
the filesystem, like the following:

  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 13240 at fs/btrfs/block-group.c:3186 btrfs_free_block_groups+0x314/0x470 [btrfs]
  (...)
  CPU: 0 PID: 13240 Comm: umount Tainted: G        W    L    5.3.0-rc8-btrfs-next-48+ #1
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
  RIP: 0010:btrfs_free_block_groups+0x314/0x470 [btrfs]
  (...)
  RSP: 0018:ffff9910c14cfdb8 EFLAGS: 00010286
  RAX: 0000000000000024 RBX: ffff89cd8a4d55f0 RCX: 0000000000000000
  RDX: 0000000000000000 RSI: ffff89cdf6a178a8 RDI: ffff89cdf6a178a8
  RBP: ffff9910c14cfde8 R08: 0000000000000000 R09: 0000000000000001
  R10: ffff89cd4d618040 R11: 0000000000000000 R12: ffff89cd8a4d5508
  R13: ffff89cde7c4a600 R14: dead000000000122 R15: dead000000000100
  FS:  00007f42754432c0(0000) GS:ffff89cdf6a00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007fd25a47f730 CR3: 000000021f8d6006 CR4: 00000000003606f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   close_ctree+0x1ad/0x390 [btrfs]
   generic_shutdown_super+0x6c/0x110
   kill_anon_super+0xe/0x30
   btrfs_kill_super+0x12/0xa0 [btrfs]
   deactivate_locked_super+0x3a/0x70
   cleanup_mnt+0xb4/0x160
   task_work_run+0x7e/0xc0
   exit_to_usermode_loop+0xfa/0x100
   do_syscall_64+0x1cb/0x220
   entry_SYSCALL_64_after_hwframe+0x49/0xbe
  RIP: 0033:0x7f4274d2cb37
  (...)
  RSP: 002b:00007ffcff701d38 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
  RAX: 0000000000000000 RBX: 0000557ebde2f060 RCX: 00007f4274d2cb37
  RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000557ebde2f240
  RBP: 0000557ebde2f240 R08: 0000557ebde2f270 R09: 0000000000000015
  R10: 00000000000006b4 R11: 0000000000000246 R12: 00007f427522ee64
  R13: 0000000000000000 R14: 0000000000000000 R15: 00007ffcff701fc0
  irq event stamp: 0
  hardirqs last  enabled at (0): [<0000000000000000>] 0x0
  hardirqs last disabled at (0): [<ffffffffb12b561e>] copy_process+0x75e/0x1fd0
  softirqs last  enabled at (0): [<ffffffffb12b561e>] copy_process+0x75e/0x1fd0
  softirqs last disabled at (0): [<0000000000000000>] 0x0
  ---[ end trace bcf4b235461b26f6 ]---
  BTRFS info (device sdb): space_info 4 has 19116032 free, is full
  BTRFS info (device sdb): space_info total=33554432, used=14176256, pinned=0, reserved=0, may_use=196608, readonly=65536
  BTRFS info (device sdb): global_block_rsv: size 0 reserved 0
  BTRFS info (device sdb): trans_block_rsv: size 0 reserved 0
  BTRFS info (device sdb): chunk_block_rsv: size 0 reserved 0
  BTRFS info (device sdb): delayed_block_rsv: size 0 reserved 0
  BTRFS info (device sdb): delayed_refs_rsv: size 0 reserved 0

Fixes: 374bf9c5 ("btrfs: unify error handling for ticket flushing")
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>