RFR: 8328866: Add raw monitor rank support to the debug agent. [v4]

[Serguei Spitsyn]

On Wed, 8 May 2024 22:41:45 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> I just wanted to say that Alex's suggestion is in a similar direction.
>> I do not see a big advantage to save just on creation of a few monitors. It is not a scalability problem as their number is constant. Creation of the monitors at initialization time is a simplification as we do not care about synchronization at this phase. Also, it is easier to understand because there is no interaction with other threads.
>
> The init code would have to know the name of each monitor since it is no longer being passed in. Something like the following might work:
> 
> static DebugRawMonitor dbg_monitors[] = {
>   { NULL, "JDWP VM_DEATH Lock",  vmDeathLockForDebugLoop_Rank, NULL, 0 },
>   { NULL, "JDWP Callback Block", callbackBlock_Rank, NULL, 0 },
>   { NULL, "JDWP Callback Lock",  callbackLock_Rank, NULL, 0 },
>   ...
> };
> 
> And then the init() function can iterate over the array and allocate the monitor for each entry. Note that this array needs to be kept in sync with the DebugRawMonitorRank enum (same entries and in the same order). I can assert during the initialization that the rank field for each entry is equal to its array index and that the array size is NUM_DEBUG_RAW_MONITORS.

Can we do as below? :

static DebugRawMonitor dbg_monitors[]; // initialized with all zeros by default
. . .
static void
createDebugRawMonitor(DebugRawMonitorRank rank, const char* name) {
    dbg_monitors[rank] = debugMonitorCreate(rank, name);
}

static void
monitors_initialize() {
  createDebugRawMonitor( vmDeathLockForDebugLoop_Rank, "JDWP VM_DEATH Lock");
  createDebugRawMonitor( callbackBlock_Rank, "JDWP Callback Block");
  createDebugRawMonitor( callbackLock_Rank, "JDWP Callback Lock");
  . . .
}

The pair of  functions `createDebugRawMonitor()+debugMonitorCreate()` can be refactored into just one function.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19044#discussion_r1594823032