Buildresult: linus/um-allmodconfig/um-x86_64 built on Jul 17 2021, 12:53

kisskb

Revisions | Branches | Compilers | Configs | Build Results | Build Failures |

Status:	OK
Date/Time:	Jul 17 2021, 12:53
Duration:	0:07:56.483017
Builder:	ka1
Revision:	Revert "signal: Allow tasks to cache one sigqueue struct" (`b4b27b9eed8ebdbf9f3046197d29d733c8c944f3)`
Target:	linus/um-allmodconfig/um-x86_64
Branch:	linus
Compiler:	um-x86_64 (x86_64-linux-gcc.br_real (Buildroot 2017.05) 5.4.0 / GNU ld (GNU Binutils) 2.27)
Config:	allmodconfig (download)
Log:	Download original Possible warnings (6) .config:4553:warning: override: reassigning to symbol GCC_PLUGIN_CYC_COMPLEXITY .config:4555:warning: override: reassigning to symbol GCC_PLUGIN_LATENT_ENTROPY .config:4559:warning: override: reassigning to symbol UML_NET_ETHERTAP .config:4561:warning: override: reassigning to symbol UML_NET_SLIP .config:4564:warning: override: reassigning to symbol UML_NET_MCAST .config:4566:warning: override: reassigning to symbol UML_NET_SLIRP Full Log # git rev-parse -q --verify b4b27b9eed8ebdbf9f3046197d29d733c8c944f3^{commit} b4b27b9eed8ebdbf9f3046197d29d733c8c944f3 already have revision, skipping fetch # git checkout -q -f -B kisskb b4b27b9eed8ebdbf9f3046197d29d733c8c944f3 # git clean -qxdf # < git log -1 # commit b4b27b9eed8ebdbf9f3046197d29d733c8c944f3 # Author: Linus Torvalds <torvalds@linux-foundation.org> # Date: Sun Jun 27 13:32:54 2021 -0700 # # Revert "signal: Allow tasks to cache one sigqueue struct" # # This reverts commits 4bad58ebc8bc4f20d89cff95417c9b4674769709 (and # 399f8dd9a866e107639eabd3c1979cd526ca3a98, which tried to fix it). # # I do not believe these are correct, and I'm about to release 5.13, so am # reverting them out of an abundance of caution. # # The locking is odd, and appears broken. # # On the allocation side (in __sigqueue_alloc()), the locking is somewhat # straightforward: it depends on sighand->siglock. Since one caller # doesn't hold that lock, it further then tests 'sigqueue_flags' to avoid # the case with no locks held. # # On the freeing side (in sigqueue_cache_or_free()), there is no locking # at all, and the logic instead depends on 'current' being a single # thread, and not able to race with itself. # # To make things more exciting, there's also the data race between freeing # a signal and allocating one, which is handled by using WRITE_ONCE() and # READ_ONCE(), and being mutually exclusive wrt the initial state (ie # freeing will only free if the old state was NULL, while allocating will # obviously only use the value if it was non-NULL, so only one or the # other will actually act on the value). # # However, while the free->alloc paths do seem mutually exclusive thanks # to just the data value dependency, it's not clear what the memory # ordering constraints are on it. Could writes from the previous # allocation possibly be delayed and seen by the new allocation later, # causing logical inconsistencies? # # So it's all very exciting and unusual. # # And in particular, it seems that the freeing side is incorrect in # depending on "current" being single-threaded. Yes, 'current' is a # single thread, but in the presense of asynchronous events even a single # thread can have data races. # # And such asynchronous events can and do happen, with interrupts causing # signals to be flushed and thus free'd (for example - sending a # SIGCONT/SIGSTOP can happen from interrupt context, and can flush # previously queued process control signals). # # So regardless of all the other questions about the memory ordering and # locking for this new cached allocation, the sigqueue_cache_or_free() # assumptions seem to be fundamentally incorrect. # # It may be that people will show me the errors of my ways, and tell me # why this is all safe after all. We can reinstate it if so. But my # current belief is that the WRITE_ONCE() that sets the cached entry needs # to be a smp_store_release(), and the READ_ONCE() that finds a cached # entry needs to be a smp_load_acquire() to handle memory ordering # correctly. # # And the sequence in sigqueue_cache_or_free() would need to either use a # lock or at least be interrupt-safe some way (perhaps by using something # like the percpu 'cmpxchg': it doesn't need to be SMP-safe, but like the # percpu operations it needs to be interrupt-safe). # # Fixes: 399f8dd9a866 ("signal: Prevent sigqueue caching after task got released") # Fixes: 4bad58ebc8bc ("signal: Allow tasks to cache one sigqueue struct") # Cc: Thomas Gleixner <tglx@linutronix.de> # Cc: Peter Zijlstra <peterz@infradead.org> # Cc: Oleg Nesterov <oleg@redhat.com> # Cc: Christian Brauner <christian.brauner@ubuntu.com> # Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> # < /opt/cross/kisskb/fe-x86-64-core-i7-2017.05/bin/x86_64-linux-gcc --version # < /opt/cross/kisskb/fe-x86-64-core-i7-2017.05/bin/x86_64-linux-ld --version # < git log --format=%s --max-count=1 b4b27b9eed8ebdbf9f3046197d29d733c8c944f3 # < make -s -j 48 ARCH=um O=/kisskb/build/linus_um-allmodconfig_um-x86_64 CROSS_COMPILE=/opt/cross/kisskb/fe-x86-64-core-i7-2017.05/bin/x86_64-linux- SUBARCH=x86_64 allmodconfig # Added to kconfig CONFIG_STANDALONE=y # Added to kconfig CONFIG_KCOV=n # Added to kconfig CONFIG_GCC_PLUGINS=n # Added to kconfig CONFIG_GCC_PLUGIN_CYC_COMPLEXITY=n # Added to kconfig CONFIG_GCC_PLUGIN_SANCOV=n # Added to kconfig CONFIG_GCC_PLUGIN_LATENT_ENTROPY=n # Added to kconfig CONFIG_GCC_PLUGIN_STRUCTLEAK=n # Added to kconfig CONFIG_GCC_PLUGIN_RANDSTRUCT=n # Added to kconfig CONFIG_UML_NET=n # Added to kconfig CONFIG_UML_NET_ETHERTAP=n # Added to kconfig CONFIG_UML_NET_TUNTAP=n # Added to kconfig CONFIG_UML_NET_SLIP=n # Added to kconfig CONFIG_UML_NET_DAEMON=n # Added to kconfig CONFIG_UML_NET_VDE=n # Added to kconfig CONFIG_UML_NET_MCAST=n # Added to kconfig CONFIG_UML_NET_PCAP=n # Added to kconfig CONFIG_UML_NET_SLIRP=n # Added to kconfig CONFIG_GCOV_KERNEL=n # Added to kconfig CONFIG_DEBUG_INFO_BTF=n # Added to kconfig CONFIG_BPF_PRELOAD=n # < make -s -j 48 ARCH=um O=/kisskb/build/linus_um-allmodconfig_um-x86_64 CROSS_COMPILE=/opt/cross/kisskb/fe-x86-64-core-i7-2017.05/bin/x86_64-linux- SUBARCH=x86_64 help # make -s -j 48 ARCH=um O=/kisskb/build/linus_um-allmodconfig_um-x86_64 CROSS_COMPILE=/opt/cross/kisskb/fe-x86-64-core-i7-2017.05/bin/x86_64-linux- SUBARCH=x86_64 olddefconfig .config:4553:warning: override: reassigning to symbol GCC_PLUGIN_CYC_COMPLEXITY .config:4555:warning: override: reassigning to symbol GCC_PLUGIN_LATENT_ENTROPY .config:4559:warning: override: reassigning to symbol UML_NET_ETHERTAP .config:4561:warning: override: reassigning to symbol UML_NET_SLIP .config:4564:warning: override: reassigning to symbol UML_NET_MCAST .config:4566:warning: override: reassigning to symbol UML_NET_SLIRP # make -s -j 48 ARCH=um O=/kisskb/build/linus_um-allmodconfig_um-x86_64 CROSS_COMPILE=/opt/cross/kisskb/fe-x86-64-core-i7-2017.05/bin/x86_64-linux- SUBARCH=x86_64 LINK linux Completed OK # rm -rf /kisskb/build/linus_um-allmodconfig_um-x86_64 # Build took: 0:07:56.483017

© Michael Ellerman 2006-2018.