Hi, On Thu, Aug 06, 2020 at 04:57:02PM +0800, Tom Yan wrote: > The problem/race I am trying to point out is, one process can > get()/read before another finishing its get()+put() pair (which is > required by volume setting/adjusting), so something like > get1()->get2()->put1()->put2() could easily happen (where each put() > relies on / is "configured" with volumes of their respective get()). > The lock will need to intentionally stall further get()/read as well. In my opinion, in the above case, it's possible to serialize each transaction which consists of get/put (read/write in userspace application) with lock/unlock mechanism. +-----------+-----------+ | process A | process B | +-----------+-----------+ | lock | | | get | | | |lock(EPERM)| reschedule lock/get/set/unlock actions | set | | | |lock(EPERM)| reschedule lock/get/set/unlock actions | unlock | | | | lock | | | get | | | set | | | unlock | +-----------+-----------+ (Of course, the above is achieved when the series of operations is done by userspace applications. For simplicity, I don't mention about in-kernel initiators of the get/set actions. In this point, I don't address to the message Pierre posted.) > If we for some reason want to avoid using locks, put() needs to be > atomic by design (like, "embed" get() in itself and use arrays for > volume values, instead of requiring those to be implemented in the > userspace manually / with a loop). Unfortunately that isn't the case > in ALSA. I get your intension is something like compare-and-swap[1]. At present, ALSA control core has no functionality like it, but it's worth to investigate. The ioctl request should includes a pair of 'struct snd_ctl_elem_value' in its argument. In a design of ALSA control core, the pair should be processed in each driver since ALSA control core has no 'cache' of the content of 'struct snd_ctl_elem_value' except for user-defined control element set. Here, would I ask your opinion to the lock/get/set/unlock actions from userspace? It can really not be one of solution for the issue? [1] https://en.wikipedia.org/wiki/Compare-and-swap Regards Takashi Sakamoto
