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SubjectRe: [PATCH v2] mmap_lock: Change trace and locking order
Date
* Steven Rostedt <rostedt@goodmis.org> [210907 12:53]:
> On Mon, 6 Sep 2021 17:00:18 +0200
> Vlastimil Babka <vbabka@suse.cz> wrote:
>
> > On 9/3/21 04:21, Liam Howlett wrote:
> > > Print to the trace log before releasing the lock to avoid racing with
> > > other trace log printers of the same lock type.
> >
> > That description could use more details maybe?
>
> Agreed, perhaps add something like this:
>
> Due to the tracing of taking the mmap_lock happened outside the lock
> itself, the trace can become confusing, making it look like more than one
> task has the same lock:
>
> task-749 [006] .... 14437980: mmap_lock_acquire_returned: mm=00000000c94d28b8 memcg_path= write=true success=true
> task-750 [007] .... 14437981: mmap_lock_acquire_returned: mm=00000000c94d28b8 memcg_path= write=true success=true
> task-749 [006] .... 14437983: mmap_lock_released: mm=00000000c94d28b8 memcg_path= write=true
>
> When in actuality the following occurred:
>
> task-749 [006] - take mmap_lock
> task-749 [006] - trace taking of mmap_lock
> task-749 [006] - release mmap_lock
>
> task-750 [007] - take mmap_lock
> task-750 [007] - trace taking of mmap_lock
>
> task-749 [006] - trace releasing of mmap_lock
>
> Although the mmap_lock was taken and released then taken again by another
> process, the lack of protection around the tracing of the activity caused
> it to show the events out of order. If the tracing of the taking and
> releasing of the mmap_lock is done inside the lock, it will protect this
> from happening.
>
> Andrew, I see you took this into your tree. Not sure if you sent it to
> Linus yet. Perhaps add the above to the patch if you have not yet sent it
> (with Liam's permission of course).

Yes, I agree this change is useful.

Let me know if you want a respin of the patch instead.

Thanks,
Liam

>
> Seeing that the patch has this as a link in the commit, at the very least,
> the above explanation will be archived.
>
> -- Steve
>
>
>
> >
> > > Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> > > Suggested-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> >
> > Acked-by: Vlastimil Babka <vbabka@suse.cz>
>
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