Re: Understanding locking for buf

On 03.03.23 01:19, Konstantin Belousov wrote:
> All it means it that the statistic output by mount -v would be somewhat off
> in the split of io ops by sync/async.  The total count is still correct.
Ic. Thx.

So. The b_lock is subject to the ownership switch even if a synchronous 
read via breadn_flags() and bufwait() is performed. Am I right?

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()])

If that instance's ownership were changed during the IO op, the b_lock 
would have been unlocked by the wrong context.
Is that correct? Am I missing something?

- Alex


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