# git rev-parse -q --verify 3fa6fc04815a70a110445368c8741c178970c87d^{commit}
3fa6fc04815a70a110445368c8741c178970c87d
already have revision, skipping fetch
# git checkout -q -f -B kisskb 3fa6fc04815a70a110445368c8741c178970c87d
# git clean -qxdf
# < git log -1
# commit 3fa6fc04815a70a110445368c8741c178970c87d
# Author: Michael Ellerman <mpe@ellerman.id.au>
# Date:   Sun May 3 00:33:16 2020 +1000
# 
#     powerpc/64s: Fix unrecoverable SLB crashes due to preemption check
#     
#     Hugh reported that his trusty G5 crashed after a few hours under load
#     with an "Unrecoverable exception 380".
#     
#     The crash is in interrupt_return() where we check lazy_irq_pending(),
#     which calls get_paca() and with CONFIG_DEBUG_PREEMPT=y that goes to
#     check_preemption_disabled() via debug_smp_processor_id().
#     
#     As Nick explained on the list:
#     
#       Problem is MSR[RI] is cleared here, ready to do the last few things
#       for interrupt return where we're not allowed to take any other
#       interrupts.
#     
#       SLB interrupts can happen just about anywhere aside from kernel
#       text, global variables, and stack. When that hits, it appears to be
#       unrecoverable due to RI=0.
#     
#     The problematic access is in preempt_count() which is:
#     
#             return READ_ONCE(current_thread_info()->preempt_count);
#     
#     Because of THREAD_INFO_IN_TASK, current_thread_info() just points to
#     current, so the access is to somewhere in kernel memory, but not on
#     the stack or in .data, which means it can cause an SLB miss. If we
#     take an SLB miss with RI=0 it is fatal.
#     
#     The easiest solution is to add a version of lazy_irq_pending() that
#     doesn't do the preemption check and call it from the interrupt return
#     path.
#     
#     Fixes: 68b34588e202 ("powerpc/64/sycall: Implement syscall entry/exit logic in C")
#     Reported-by: Hugh Dickins <hughd@google.com>
#     Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
#     Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
#     Link: https://lore.kernel.org/r/20200502143316.929341-1-mpe@ellerman.id.au
# < /opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-ld --version
# < git log --format=%s --max-count=1 3fa6fc04815a70a110445368c8741c178970c87d
# < make -s -j 8 ARCH=powerpc O=/kisskb/build/powerpc-fixes_holly_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  holly_defconfig
# < make -s -j 8 ARCH=powerpc O=/kisskb/build/powerpc-fixes_holly_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  help
# make -s -j 8 ARCH=powerpc O=/kisskb/build/powerpc-fixes_holly_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  olddefconfig
# make -s -j 8 ARCH=powerpc O=/kisskb/build/powerpc-fixes_holly_defconfig_powerpc-gcc5 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-5.5.0-nolibc/powerpc64-linux/bin/powerpc64-linux-  
INFO: Uncompressed kernel (size 0x5a75d8) overlaps the address of the wrapper(0x400000)
INFO: Fixing the link_address of wrapper to (0x600000)
Completed OK
# rm -rf /kisskb/build/powerpc-fixes_holly_defconfig_powerpc-gcc5
# Build took: 0:01:05.555560