Re: Understanding locking for buf

On 20.03.23 19:07, Konstantin Belousov wrote:
> On Mon, Mar 20, 2023 at 11:25:30AM +0100, Alexander Lochmann wrote:
>>
>>
>> On 16.03.23 12:24, Konstantin Belousov wrote:
>>>> Is the following correct?
>>>> The aforementioned accesses by 100033 in g_vfs_done() are no violations with
>>>> respect to the locking rule because from a global perspective the buf is
>>>> locked. It is the only concurrent access at that moment.
>>> I would formulate it differently:
>>>     No other thread might legitimately get access to the buffer using
>>>     either bread() or getblk() until current io operation finishes.
>>>     The io operation is handled in two contexts: top-level, where a thread
>>>     used getblk() as usual to claim buffer ownership, and completion
>>>     thread context (geom up thread). The completion code legitimately
>>>     manipulates the buffer, because the top-level code expects that after
>>>     the buffer strategy routine is called, effectively moving the ownership
>>>     to the geom up thread.
>> Thx. But the top-level thread, using getblk(), legitimately releases the
>> lock. Am I right?
> No.  It does not, please re-read what I wrote.
Yeah, but that's what I meant a few mails ago.
The lock is acquired *and* released by the top-level thread. Although 
some accesses happen from the geom thread.

(The numbers are the logged thread ids.)
In our log, I see the following:
- Kernel tries to mount the rootfs via readsuper(). The thread id is 100002.
- 100002 allocates an instance of struct buf.
- The b_lock is acquired by 100002 in buf_alloc().
- Various accesses to buf by 100002.
- Various accesses to buf by 100033 during g_vfs_done().
- Again various accesses to buf by 100002.
- The instances is unlocked and freed by 100002. (readsuper() ->
ffs_use_bread() -> brelse() -> buf_free()[ -> BUF_UNLOCK()])

-- 
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