Why need a lock in memory/capability map

[Yuxin Ren]

Hi All,

I note that when a thread does memory or capability mapping, both over
system call and IPC, it needs take a lock, which is the existence_lock of
target task. I cannot understand it needs take a per task, as cap/page
table are per thread.
What data does this lock to protect?

Thank you.
Yuixn
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