# git rev-parse -q --verify fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c^{commit}
fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c
already have revision, skipping fetch
# git checkout -q -f -B kisskb fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c
# git clean -qxdf
# < git log -1
# commit fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c
# Author: Linus Torvalds <torvalds@linux-foundation.org>
# Date:   Tue Aug 24 10:39:25 2021 -0700
# 
#     pipe: do FASYNC notifications for every pipe IO, not just state changes
#     
#     It turns out that the SIGIO/FASYNC situation is almost exactly the same
#     as the EPOLLET case was: user space really wants to be notified after
#     every operation.
#     
#     Now, in a perfect world it should be sufficient to only notify user
#     space on "state transitions" when the IO state changes (ie when a pipe
#     goes from unreadable to readable, or from unwritable to writable).  User
#     space should then do as much as possible - fully emptying the buffer or
#     what not - and we'll notify it again the next time the state changes.
#     
#     But as with EPOLLET, we have at least one case (stress-ng) where the
#     kernel sent SIGIO due to the pipe being marked for asynchronous
#     notification, but the user space signal handler then didn't actually
#     necessarily read it all before returning (it read more than what was
#     written, but since there could be multiple writes, it could leave data
#     pending).
#     
#     The user space code then expected to get another SIGIO for subsequent
#     writes - even though the pipe had been readable the whole time - and
#     would only then read more.
#     
#     This is arguably a user space bug - and Colin King already fixed the
#     stress-ng code in question - but the kernel regression rules are clear:
#     it doesn't matter if kernel people think that user space did something
#     silly and wrong.  What matters is that it used to work.
#     
#     So if user space depends on specific historical kernel behavior, it's a
#     regression when that behavior changes.  It's on us: we were silly to
#     have that non-optimal historical behavior, and our old kernel behavior
#     was what user space was tested against.
#     
#     Because of how the FASYNC notification was tied to wakeup behavior, this
#     was first broken by commits f467a6a66419 and 1b6b26ae7053 ("pipe: fix
#     and clarify pipe read/write wakeup logic"), but at the time it seems
#     nobody noticed.  Probably because the stress-ng problem case ends up
#     being timing-dependent too.
#     
#     It was then unwittingly fixed by commit 3a34b13a88ca ("pipe: make pipe
#     writes always wake up readers") only to be broken again when by commit
#     3b844826b6c6 ("pipe: avoid unnecessary EPOLLET wakeups under normal
#     loads").
#     
#     And at that point the kernel test robot noticed the performance
#     refression in the stress-ng.sigio.ops_per_sec case.  So the "Fixes" tag
#     below is somewhat ad hoc, but it matches when the issue was noticed.
#     
#     Fix it for good (knock wood) by simply making the kill_fasync() case
#     separate from the wakeup case.  FASYNC is quite rare, and we clearly
#     shouldn't even try to use the "avoid unnecessary wakeups" logic for it.
#     
#     Link: https://lore.kernel.org/lkml/20210824151337.GC27667@xsang-OptiPlex-9020/
#     Fixes: 3b844826b6c6 ("pipe: avoid unnecessary EPOLLET wakeups under normal loads")
#     Reported-by: kernel test robot <oliver.sang@intel.com>
#     Tested-by: Oliver Sang <oliver.sang@intel.com>
#     Cc: Eric Biederman <ebiederm@xmission.com>
#     Cc: Colin Ian King <colin.king@canonical.com>
#     Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
# < /opt/cross/kisskb/korg/gcc-8.1.0-nolibc/hppa-linux/bin/hppa-linux-gcc --version
# < /opt/cross/kisskb/korg/gcc-8.1.0-nolibc/hppa-linux/bin/hppa-linux-ld --version
# < git log --format=%s --max-count=1 fe67f4dd8daa252eb9aa7acb61555f3cc3c1ce4c
# < make -s -j 48 ARCH=parisc O=/kisskb/build/linus_parisc-allnoconfig_parisc-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/hppa-linux/bin/hppa-linux-  allnoconfig
# < make -s -j 48 ARCH=parisc O=/kisskb/build/linus_parisc-allnoconfig_parisc-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/hppa-linux/bin/hppa-linux-  help
# make -s -j 48 ARCH=parisc O=/kisskb/build/linus_parisc-allnoconfig_parisc-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/hppa-linux/bin/hppa-linux-  olddefconfig
# make -s -j 48 ARCH=parisc O=/kisskb/build/linus_parisc-allnoconfig_parisc-gcc8 CROSS_COMPILE=/opt/cross/kisskb/korg/gcc-8.1.0-nolibc/hppa-linux/bin/hppa-linux-  
/kisskb/src/arch/parisc/math-emu/fpudispatch.c: In function 'decode_0c':
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:312:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+2] = fpregs[r1+2];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:313:9: note: here
         case 1: /* double */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:314:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+1] = fpregs[r1+1];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:315:9: note: here
         case 0: /* single */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:327:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+2] = fpregs[r1+2];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:328:9: note: here
         case 1: /* double */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:329:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+1] = fpregs[r1+1];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:330:9: note: here
         case 0: /* single */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:343:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+2] = fpregs[r1+2];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:344:9: note: here
         case 1: /* double */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:345:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+1] = fpregs[r1+1];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:346:9: note: here
         case 0: /* single */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:359:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+2] = fpregs[r1+2];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:360:9: note: here
         case 1: /* double */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:361:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+1] = fpregs[r1+1];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:362:9: note: here
         case 0: /* single */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:305:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:319:4: note: here
    case 3: /* FABS */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:320:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:335:4: note: here
    case 6: /* FNEG */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:336:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:351:4: note: here
    case 7: /* FNEGABS */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:352:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:367:4: note: here
    case 4: /* FSQRT */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:368:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:379:4: note: here
    case 5: /* FRND */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:410:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:422:4: note: here
    case 1: /* FCNVXF */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:423:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:437:4: note: here
    case 2: /* FCNVFX */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:438:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:452:4: note: here
    case 3: /* FCNVFXT */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:453:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:467:4: note: here
    case 5: /* FCNVUF (PA2.0 only) */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:468:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:482:4: note: here
    case 6: /* FCNVFU (PA2.0 only) */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:483:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:497:4: note: here
    case 7: /* FCNVFUT (PA2.0 only) */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:498:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:512:4: note: here
    case 4: /* undefined */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:574:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:593:4: note: here
    case 1: /* FTEST */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:625:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:636:4: note: here
    case 1: /* FSUB */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:637:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:648:4: note: here
    case 2: /* FMPY */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:649:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:660:4: note: here
    case 3: /* FDIV */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:661:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:672:4: note: here
    case 4: /* FREM */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:300:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   switch (subop) {
   ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:393:2: note: here
  case 1: /* class 1 */
  ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:408:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   switch (subop) {
   ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:516:2: note: here
  case 2: /* class 2 */
  ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:521:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if  (fpu_type_flags & PA2_0_FPU_FLAG) {
       ^
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:614:2: note: here
  case 3: /* class 3 */
  ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c: In function 'decode_0e':
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:738:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+1] = fpregs[r1+1];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:739:9: note: here
         case 0: /* single */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:749:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+1] = fpregs[r1+1];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:750:9: note: here
         case 0: /* single */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:760:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+1] = fpregs[r1+1];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:761:9: note: here
         case 0: /* single */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:771:18: warning: this statement may fall through [-Wimplicit-fallthrough=]
      fpregs[t+1] = fpregs[r1+1];
      ~~~~~~~~~~~~^~~~~~~~~~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:772:9: note: here
         case 0: /* single */
         ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:733:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:743:4: note: here
    case 3: /* FABS */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:744:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:754:4: note: here
    case 6: /* FNEG */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:755:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:765:4: note: here
    case 7: /* FNEGABS */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:766:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:776:4: note: here
    case 4: /* FSQRT */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:777:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:788:4: note: here
    case 5: /* FRMD */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:817:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:829:4: note: here
    case 1: /* FCNVXF */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:830:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:844:4: note: here
    case 2: /* FCNVFX */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:845:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:859:4: note: here
    case 3: /* FCNVFXT */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:860:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:874:4: note: here
    case 5: /* FCNVUF (PA2.0 only) */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:875:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:889:4: note: here
    case 6: /* FCNVFU (PA2.0 only) */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:890:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:904:4: note: here
    case 7: /* FCNVFUT (PA2.0 only) */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:905:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch(fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:919:4: note: here
    case 4: /* undefined */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:1021:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:1029:4: note: here
    case 1: /* FSUB */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:1030:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:1038:4: note: here
    case 2: /* FMPY or XMPYU */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:1042:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
     if (extru(ir,fpxpos,1)) {
        ^
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:1074:4: note: here
    case 3: /* FDIV */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:1075:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     switch (fmt) {
     ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:1083:4: note: here
    case 4: /* FREM */
    ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:728:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   switch (subop) {
   ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:802:2: note: here
  case 1: /* class 1 */
  ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:815:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   switch (subop) {
   ^~~~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:922:2: note: here
  case 2: /* class 2 */
  ^~~~
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:938:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if  (fpu_type_flags & PA2_0_FPU_FLAG) {
       ^
/kisskb/src/arch/parisc/math-emu/fpudispatch.c:997:2: note: here
  case 3: /* class 3 */
  ^~~~
/kisskb/src/block/genhd.c: In function 'diskstats_show':
/kisskb/src/block/genhd.c:1181:1: warning: the frame size of 1680 bytes is larger than 1280 bytes [-Wframe-larger-than=]
 }
 ^
Completed OK
# rm -rf /kisskb/build/linus_parisc-allnoconfig_parisc-gcc8
# Build took: 0:00:21.727637