X ptrace when client is under pagefault or syscall won't work

The kernel sends SIGSTOP to interrupt tracee, but the signal will be
processed only when the process returns from kernel space, e.g. here

	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/entry/common.c?id=v4.19-rc8-151-g23469de647c4#n160

This way the tracer won't receive obligatory information that tracee
stopped (via wait...) and even though ptrace(ATTACH) succeeds, all other
ptrace commands will fail:

	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/ptrace.c?id=v4.19-rc8-151-g23469de647c4#n1140
	https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/ptrace.c?id=v4.19-rc8-151-g23469de647c4#n207

My original idea was to use ptrace to run code in process to change it's
memory mappings, while the triggering process is under pagefault/read
to wcfs, and the above shows it won't work - trying to ptrace the
client from under wcfs will just block forever (the kernel will be
waiting for read operation to finish for ptrace, and read will be first
waiting on ptrace stopping to complete = deadlock)

wcfs/wcfs.go

@@ -323,6 +323,7 @@ package main
 //
 //	remmapping is done synchronously via ptrace.
 //	XXX via running wcfs-trusted code wcfs injects into clients.
+//	FIXME ptrace won't work when client thread is blocked under pagefault or syscall.
 //
 //	in order to support remmapping for each file/head/data
 //