RFR: 8254877: GCLogPrecious::_lock rank constrains what locks you are allowed to have when crashing [v2]

[Stefan Karlsson]

> This is an alternative version of the fix proposed in 900:
> https://github.com/openjdk/jdk/pull/900
> 
> Erik's description:
>> Today, when you crash, the GCLogPrecious::_lock is taken. This effectively limits you to only get clean crash reports if you crash or assert without holding a lock of rank tty or lower. It is arguably difficult to know what locks you are going to have when crashing. Therefore, I don't think the precious GC log should constrain possible crashing contexts in that fashion.
> 
> As Erik mentioned in that PR, I'd like to retain the ability to easily dump the precious log when debugging. The proposed fix changes the Mutex to a Semaphore, and use trywait to safely access the buffer. In the unlikely event that another thread is holding the lock, the hs_err printer skips printing the log.
> 
> This also makes it possible to call precious logging from within the stack watermark processing code. I think there's a possibility that we might call the following error logging, when we fail to commit memory for a ZPage, when relocating, during stack watermark processing:
> `log_error_p(gc)("Failed to commit memory (%s)", err.to_string());`

Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:

  Review 1

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/903/files
  - new: https://git.openjdk.java.net/jdk/pull/903/files/fdadc38a..88e2b4e2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=903&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=903&range=00-01

  Stats: 81 lines in 4 files changed: 66 ins; 3 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/903.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/903/head:pull/903

PR: https://git.openjdk.java.net/jdk/pull/903