[Bug ports/15119] New: unnecessary busy loop in __lll_timedlock_wait on ARM

["katsuki.uwatoko at toshiba dot co.jp" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15119

             Bug #: 15119
           Summary: unnecessary busy loop in __lll_timedlock_wait on ARM
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ports
        AssignedTo: unassigned@sourceware.org
        ReportedBy: katsuki.uwatoko@toshiba.co.jp
                CC: carlos@redhat.com, roland@gnu.org
    Classification: Unclassified


Created attachment 6856
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6856
patch for Avoid unneecssary busy loop in __lll_timedlock_wait on ARM.

The following sequence causes unnecessary busy loop. 

"A thread" gets the lock. (futex = 1)
"B thread" tries to get the lock, and has not called futex syscall yet. (futex
= 2)
"A thread" releases the lock (futex = 0)
"C thread" gets the lock, and does not call futex syscall because of futex=0
(futex = 1)
"B thread" calls futex syscall, and returns with an error.
           Because futex syscall in Linux Kernel checks if the val is changed 
           from 2, which is the 3rd arg of the syscall at futex_wait_setup(),
           but in this case futex is 1.
"B thread" tries to get the lock in userspace but cannot get it 
           because futex is not 0.
           After all the thread keeps calling futex syscall 
           until "C thread" will release it (futex = 0) or timeout.

Therefore I think that the value should be set 2 in every loop 
the same as __lll_lock_wait_private, and attached a patch for this issue.

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