tree f79cfb0387174dac254ca4e846d03bcc5387c525
parent fd01164a8d3f350abf2ba26f6ff693d7ec4afa0e
author yusukes <yusukes@google.com> 1518575330 -0800
committer yusukes <yusukes@google.com> 1518631674 -0800

Prevent WaitForProperty() from using ~100% of CPU time on 32bit builds

Since 'struct timespec' members (time_t and long) are both 32bit on
32bit systems, and std::chrono::{seconds,nanoseconds}::rep are both
>32bit, timespec members assigned in DurationToTimeSpec() can have a
negative value, especially when WaitForProperty() is called with the
default timeout value which is std::chrono::milliseconds::max().

Regarding functionality, passing a negative value to
__system_property_wait() is okay because WaitForProperty() still
waits for the property value (so unit tests are passing), but while
WaitForProperty() does that, the function, to be more exact,
SystemProperties::Wait() in bionic/, consumes ~100% of CPU time. This
happens because SystemProperties::Wait() which implements
__system_property_wait() has a tight while-loop with a __futex_wait()
call, and the futex call immediately returns EINVAL when the timespec
passed in has a negative value.

With this CL, WaitForProperty() will never pass a negative timespec
to __system_property_wait(), and therefore the __futex_wait() call
in bionic works as expected without consuming too much CPU time even
on 32bit systems.

Bug: None
Test: libbase_test32 still passes
Test: strace no longer shows repeated EINVALs from __futex_wait
Change-Id: Id1834fac8cd2876b02dbe4479bf3d3eda2fa7da1