# git rev-parse -q --verify 3a34b13a88caeb2800ab44a4918f230041b37dd9^{commit} 3a34b13a88caeb2800ab44a4918f230041b37dd9 already have revision, skipping fetch # git checkout -q -f -B kisskb 3a34b13a88caeb2800ab44a4918f230041b37dd9 # git clean -qxdf # < git log -1 # commit 3a34b13a88caeb2800ab44a4918f230041b37dd9 # Author: Linus Torvalds # Date: Fri Jul 30 15:42:34 2021 -0700 # # pipe: make pipe writes always wake up readers # # Since commit 1b6b26ae7053 ("pipe: fix and clarify pipe write wakeup # logic") we have sanitized the pipe write logic, and would only try to # wake up readers if they needed it. # # In particular, if the pipe already had data in it before the write, # there was no point in trying to wake up a reader, since any existing # readers must have been aware of the pre-existing data already. Doing # extraneous wakeups will only cause potential thundering herd problems. # # However, it turns out that some Android libraries have misused the EPOLL # interface, and expected "edge triggered" be to "any new write will # trigger it". Even if there was no edge in sight. # # Quoting Sandeep Patil: # "The commit 1b6b26ae7053 ('pipe: fix and clarify pipe write wakeup # logic') changed pipe write logic to wakeup readers only if the pipe # was empty at the time of write. However, there are libraries that # relied upon the older behavior for notification scheme similar to # what's described in [1] # # One such library 'realm-core'[2] is used by numerous Android # applications. The library uses a similar notification mechanism as GNU # Make but it never drains the pipe until it is full. When Android moved # to v5.10 kernel, all applications using this library stopped working. # # The library has since been fixed[3] but it will be a while before all # applications incorporate the updated library" # # Our regression rule for the kernel is that if applications break from # new behavior, it's a regression, even if it was because the application # did something patently wrong. Also note the original report [4] by # Michal Kerrisk about a test for this epoll behavior - but at that point # we didn't know of any actual broken use case. # # So add the extraneous wakeup, to approximate the old behavior. # # [ I say "approximate", because the exact old behavior was to do a wakeup # not for each write(), but for each pipe buffer chunk that was filled # in. The behavior introduced by this change is not that - this is just # "every write will cause a wakeup, whether necessary or not", which # seems to be sufficient for the broken library use. ] # # It's worth noting that this adds the extraneous wakeup only for the # write side, while the read side still considers the "edge" to be purely # about reading enough from the pipe to allow further writes. # # See commit f467a6a66419 ("pipe: fix and clarify pipe read wakeup logic") # for the pipe read case, which remains that "only wake up if the pipe was # full, and we read something from it". # # Link: https://lore.kernel.org/lkml/CAHk-=wjeG0q1vgzu4iJhW5juPkTsjTYmiqiMUYAebWW+0bam6w@mail.gmail.com/ [1] # Link: https://github.com/realm/realm-core [2] # Link: https://github.com/realm/realm-core/issues/4666 [3] # Link: https://lore.kernel.org/lkml/CAKgNAkjMBGeAwF=2MKK758BhxvW58wYTgYKB2V-gY1PwXxrH+Q@mail.gmail.com/ [4] # Link: https://lore.kernel.org/lkml/20210729222635.2937453-1-sspatil@android.com/ # Reported-by: Sandeep Patil # Cc: Michael Kerrisk # Signed-off-by: Linus Torvalds # < /opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux-gcc --version # < /opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux-ld --version # < git log --format=%s --max-count=1 3a34b13a88caeb2800ab44a4918f230041b37dd9 # < make -s -j 80 ARCH=mips O=/kisskb/build/linus_mips-allmodconfig_mips-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux- allmodconfig # Added to kconfig CONFIG_BUILD_DOCSRC=n # Added to kconfig CONFIG_MODULE_SIG=n # Added to kconfig CONFIG_SAMPLES=n # Added to kconfig CONFIG_MIPS_CPS_NS16550_BASE=0x1b0003f8 # Added to kconfig CONFIG_MIPS_CPS_NS16550_SHIFT=0 # < make -s -j 80 ARCH=mips O=/kisskb/build/linus_mips-allmodconfig_mips-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux- help # make -s -j 80 ARCH=mips O=/kisskb/build/linus_mips-allmodconfig_mips-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux- olddefconfig .config:13081:warning: override: reassigning to symbol MIPS_CPS_NS16550_SHIFT # make -s -j 80 ARCH=mips O=/kisskb/build/linus_mips-allmodconfig_mips-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/mips-linux/bin/mips-linux- /kisskb/src/drivers/input/joystick/analog.c:160:2: warning: #warning Precise timer not defined for this architecture. [-Wcpp] #warning Precise timer not defined for this architecture. ^~~~~~~ /kisskb/src/lib/test_scanf.c: In function 'numbers_list_field_width_val_width': /kisskb/src/lib/test_scanf.c:530:1: warning: the frame size of 2144 bytes is larger than 2048 bytes [-Wframe-larger-than=] } ^ /kisskb/src/lib/test_scanf.c: In function 'numbers_list_field_width_typemax': /kisskb/src/lib/test_scanf.c:488:1: warning: the frame size of 2640 bytes is larger than 2048 bytes [-Wframe-larger-than=] } ^ /kisskb/src/lib/test_scanf.c: In function 'numbers_list': /kisskb/src/lib/test_scanf.c:437:1: warning: the frame size of 2144 bytes is larger than 2048 bytes [-Wframe-larger-than=] } ^ In file included from /kisskb/src/arch/mips/include/asm/sibyte/sb1250.h:28, from /kisskb/src/drivers/watchdog/sb_wdog.c:58: /kisskb/src/arch/mips/include/asm/sibyte/bcm1480_scd.h:261: warning: "M_SPC_CFG_CLEAR" redefined #define M_SPC_CFG_CLEAR M_BCM1480_SPC_CFG_CLEAR In file included from /kisskb/src/arch/mips/include/asm/sibyte/sb1250.h:27, from /kisskb/src/drivers/watchdog/sb_wdog.c:58: /kisskb/src/arch/mips/include/asm/sibyte/sb1250_scd.h:392: note: this is the location of the previous definition #define M_SPC_CFG_CLEAR _SB_MAKEMASK1(32) In file included from /kisskb/src/arch/mips/include/asm/sibyte/sb1250.h:28, from /kisskb/src/drivers/watchdog/sb_wdog.c:58: /kisskb/src/arch/mips/include/asm/sibyte/bcm1480_scd.h:262: warning: "M_SPC_CFG_ENABLE" redefined #define M_SPC_CFG_ENABLE M_BCM1480_SPC_CFG_ENABLE In file included from /kisskb/src/arch/mips/include/asm/sibyte/sb1250.h:27, from /kisskb/src/drivers/watchdog/sb_wdog.c:58: /kisskb/src/arch/mips/include/asm/sibyte/sb1250_scd.h:393: note: this is the location of the previous definition #define M_SPC_CFG_ENABLE _SB_MAKEMASK1(33) Completed OK # rm -rf /kisskb/build/linus_mips-allmodconfig_mips-gcc8 # Build took: 0:27:46.549112