Re: [PATCH v4] blktrace: Fix potentail deadlock between delete & sysfs ops

[Waiman Long]

On 08/18/2017 04:18 PM, Bart Van Assche wrote:
> On Fri, 2017-08-18 at 16:01 -0400, Waiman Long wrote:
>> [ ... ]
>> Instead of using bd_mutex, a new global blktrace mutex is now used
>> to protect against concurrent access, creation and destruction of the
>> blk_trace structure that is used only in the blktrace.c file. As
>> blktrace files will not be frequently accessed, using a global mutex
>> should not cause any performance problem.
>> [ ... ]
>> +/*
>> + * The bd_mutex was used previously for protecting blk_trace structure.
>> + * That could lead to deadlock with concurrent block device deletion and
>> + * sysfs access. So a global blktrace_mutex is now used instead for
>> + * protecting the blk_trace structure.
>> + *
>> + * The references to the opened sysfs or device files should prevent the
>> + * underlying block device from being removed.
>> + */
>> +static DEFINE_MUTEX(blktrace_mutex);
> Hello Waiman,
>
> Thanks for having addressed my previous comment. Regarding this patch: sorry
> but I don't think it's a good idea to use a global mutex for serializing
> accesses to tracing data of a single block device. Global mutexes create
> unwanted lock dependencies between different block devices. Additionally, on
> multiprocessor systems global mutexes can cause cache line ping-pong between
> processors and hence can cause a severe slowdown. Please make blktrace_mutex
> per block device instead of global.

I fully understand the problem of a global lock. The main reason of
using a global lock here is that the blktrace APIs are not in a
performance critical path. In fact, I think it is used primarily for
debugging purpose. Activating it will certainly slow thing down no
matter what kind of lock is used.

I also don't believe that the blktrace APIs will be used in a high
enough frequency that it will cause a performance issue. Please let me
know if I am wrong in my assumptions.

Cheers,
Longman