# git rev-parse -q --verify 20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f^{commit}
# git fetch -q -n -f git://fs.ozlabs.ibm.com/kernel/linus master
# git rev-parse -q --verify 20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f^{commit}
20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f
# git checkout -q -f -B kisskb 20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f
# git clean -qxdf
# < git log -1
# commit 20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f
# Author: Christian Brauner <brauner@kernel.org>
# Date:   Mon Jul 24 17:00:49 2023 +0200
# 
#     file: always lock position for FMODE_ATOMIC_POS
#     
#     The pidfd_getfd() system call allows a caller with ptrace_may_access()
#     abilities on another process to steal a file descriptor from this
#     process. This system call is used by debuggers, container runtimes,
#     system call supervisors, networking proxies etc. So while it is a
#     special interest system call it is used in common tools.
#     
#     That ability ends up breaking our long-time optimization in fdget_pos(),
#     which "knew" that if we had exclusive access to the file descriptor
#     nobody else could access it, and we didn't need the lock for the file
#     position.
#     
#     That check for file_count(file) was always fairly subtle - it depended
#     on __fdget() not incrementing the file count for single-threaded
#     processes and thus included that as part of the rule - but it did mean
#     that we didn't need to take the lock in all those traditional unix
#     process contexts.
#     
#     So it's sad to see this go, and I'd love to have some way to re-instate
#     the optimization. At the same time, the lock obviously isn't ever
#     contended in the case we optimized, so all we were optimizing away is
#     the atomics and the cacheline dirtying. Let's see if anybody even
#     notices that the optimization is gone.
#     
#     Link: https://lore.kernel.org/linux-fsdevel/20230724-vfs-fdget_pos-v1-1-a4abfd7103f3@kernel.org/
#     Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
#     Cc: stable@kernel.org
#     Signed-off-by: Christian Brauner <brauner@kernel.org>
#     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/kisskb/korg/gcc-13.1.0-nolibc/mips-linux/bin/mips-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-13.1.0-nolibc/mips-linux/bin/mips-linux-ld --version
# < git log --format=%s --max-count=1 20ea1e7d13c1b544fe67c4a8dc3943bb1ab33e6f
# make -s -j 24 ARCH=mips O=/kisskb/build/linus-rand_allnoconfig_mips-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/mips-linux/bin/mips-linux-  allnoconfig
# < make -s -j 24 ARCH=mips O=/kisskb/build/linus-rand_allnoconfig_mips-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/mips-linux/bin/mips-linux-  help
# make -s -j 24 ARCH=mips O=/kisskb/build/linus-rand_allnoconfig_mips-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/mips-linux/bin/mips-linux-  olddefconfig
# make -s -j 24 ARCH=mips O=/kisskb/build/linus-rand_allnoconfig_mips-gcc13 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-13.1.0-nolibc/mips-linux/bin/mips-linux-  
Completed OK
# rm -rf /kisskb/build/linus-rand_allnoconfig_mips-gcc13
# Build took: 0:00:50.838165