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Subjectkprobes broken since 0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()")
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Hello,

I'm currently seeing latest Linus' master being somewhat broken w.r.t
krpobes. In particular I have the following test-case:

#!/bin/bash

mkfs.btrfs -f /dev/vdc &> /dev/null
mount /dev/vdc /media/scratch/

bpftrace -e 'kprobe:btrfs_sync_file {printf("kprobe: %s\n", kstack());}'
&>bpf-output &
bpf_trace_pid=$!

# force btrfs_sync_file to be called
sleep 2
xfs_io -f -c "pwrite 0 4m" -c "fsync" /media/scratch/file5

kill $bpf_trace_pid
sleep 1

grep -q kprobe bpf-output
retval=$?
rm -f bpf-output
umount /media/scratch

exit $retval

It traces btrfs_sync_file which is called when fsync is executed on a
btrfs file, however I don't see the stacktrace being printed i.e the
kprobe doesn't fire at all. The following alternative program:

bpftrace -e 'tracepoint:btrfs:btrfs_sync_file {printf("tracepoint:
%s\n", kstack());} kprobe:btrfs_sync_file {printf("kprobe: %s\n",
kstack());}'

only prints the stack from the tracepoint and not from the kprobe, given
that the tracepoint is called from the btrfs_sync_file function.

I started bisecting this and arrived at the following commit:

0d00449c7a28 ("x86: Replace ist_enter() with nmi_enter()")

FWIW the following series is applied on the kernel I was testing:
https://lore.kernel.org/lkml/159870598914.1229682.15230803449082078353.stgit@devnote2/

but it's still broken.

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