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btrfs: fix memory ordering between normal and ordered work functions

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Ordered work functions aren't guaranteed to be handled by the same thread
which executed the normal work functions. The only way execution between
normal/ordered functions is synchronized is via the WORK_DONE_BIT,
unfortunately the used bitops don't guarantee any ordering whatsoever.

This manifested as seemingly inexplicable crashes on ARM64, where
async_chunk::inode is seen as non-null in async_cow_submit which causes
submit_compressed_extents to be called and crash occurs because
async_chunk::inode suddenly became NULL. The call trace was similar to:

    pc : submit_compressed_extents+0x38/0x3d0
    lr : async_cow_submit+0x50/0xd0
    sp : ffff800015d4bc20

    <registers omitted for brevity>

    Call trace:
     submit_compressed_extents+0x38/0x3d0
     async_cow_submit+0x50/0xd0
     run_ordered_work+0xc8/0x280
     btrfs_work_helper+0x98/0x250
     process_one_work+0x1f0/0x4ac
     worker_thread+0x188/0x504
     kthread+0x110/0x114
     ret_from_fork+0x10/0x18

Fix this by adding respective barrier calls which ensure that all
accesses preceding setting of WORK_DONE_BIT are strictly ordered before
setting the flag. At the same time add a read barrier after reading of
WORK_DONE_BIT in run_ordered_work which ensures all subsequent loads
would be strictly ordered after reading the bit. This in turn ensures
are all accesses before WORK_DONE_BIT are going to be strictly ordered
before any access that can occur in ordered_func.

Reported-by: Chris Murphy <lists@colorremedies.com>
Fixes: 08a9ff3264 ("btrfs: Added btrfs_workqueue_struct implemented ordered execution based on kernel workqueue")
CC: stable@vger.kernel.org # 4.4+
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2011928
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Tested-by: Chris Murphy <chris@colorremedies.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Nikolay Borisov 2021-11-02 14:49:16 +02:00 committed by David Sterba
parent 6f019c0e01
commit 45da9c1767
1 changed files with 14 additions and 0 deletions
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14
fs/btrfs/async-thread.c
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@ -234,6 +234,13 @@ static void run_ordered_work(struct __btrfs_workqueue *wq,
ordered_list);
if (!test_bit(WORK_DONE_BIT, &work->flags))
break;
/*
* Orders all subsequent loads after reading WORK_DONE_BIT,
* paired with the smp_mb__before_atomic in btrfs_work_helper
* this guarantees that the ordered function will see all
* updates from ordinary work function.
*/
smp_rmb();
/*
* we are going to call the ordered done function, but
@ -317,6 +324,13 @@ static void btrfs_work_helper(struct work_struct *normal_work)
thresh_exec_hook(wq);
work->func(work);
if (need_order) {
/*
* Ensures all memory accesses done in the work function are
* ordered before setting the WORK_DONE_BIT. Ensuring the thread
* which is going to executed the ordered work sees them.
* Pairs with the smp_rmb in run_ordered_work.
*/
smp_mb__before_atomic();
set_bit(WORK_DONE_BIT, &work->flags);
run_ordered_work(wq, work);
} else {