Messages in this thread | | | Subject | Re: [PATCH] fs: proc: use down_read_killable in proc_pid_cmdline_read() | From | Yang Shi <> | Date | Fri, 23 Feb 2018 12:08:20 -0800 |
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On 2/23/18 11:45 AM, Alexey Dobriyan wrote: > On Fri, Feb 23, 2018 at 11:42:34AM -0800, Yang Shi wrote: >> >> On 2/23/18 11:33 AM, Alexey Dobriyan wrote: >>> On Wed, Feb 21, 2018 at 03:13:10PM -0800, Yang Shi wrote: >>> >>>>>>> 2) access_remote_vm() et al will do the same ->mmap_sem, and >>>>>> Yes, it does. But, __access_remote_vm() is called by access_process_vm() >>>>>> too, which is used by much more places, i.e. ptrace, so I was not sure >>>>>> if it is preferred to convert to killable version. So, I leave it untouched. >>>>> Yeah, but ->mmap_sem is taken 3 times per /proc/*/cmdline read >>>>> and your scalability tests should trigger next backtrace right away. >>>> Yes, however, I didn't run into it if mmap_sem is acquired earlier. >>>> >>>> How about defining a killable version, like >>>> __access_remote_vm_killable() which use down_read_killable(), then the >>>> killable version can be used by proc/*/cmdline? There might be other >>>> users in the future. >>> It would be a disaster as interfaces multiply. >> Might be not that bad. > Maybe. > > But you need to explain why there is no backtrace several lines later: > > access_remote_vm > __access_remote_vm > down_read(&mm->mmap_sem)
I think it might be because:
CPU A CPU B read /proc/*/cmdline get_mm acquire mmap_sem munmap(300G) try to acquire mmap_sem --> go to sleep release mmap_sem got mmap_sem release mmap_sem
access_remote_vm put_mm
The munmap might happen right before access_remote_vm(), but I just didn't run into it for the time being. It may be hit on another machine or with some changes to the test cases.
BTW, even the hung I met happened occassionally, not very often. So, the access_remote_vm() hit sounds less often. But, I agree it is still possible in theory.
Regards, Yang
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